The astonishing diversity of great tit song

As I was coming home from work yesterday there was an unmistakable feeling of spring in the air. After what feels like a very long and harsh winter (it dropped to an incredible -16°c for a few days) the sunshine and warmth is certainly welcome.

It’s not just me that’s enjoying the sunshine either. At this time of year the breeding season for great tits (Parus major) really gets going and when the sun shines the males sing as they attempt to attract females to their territories.

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Great tits sing from around February to early June

For some people the first snowdrops mark the start of spring, for others the melting snow or the lengthening days. For me, great tit song signals that spring is on its way and is always a welcome sound.

Great tit song is one of the most distinctive sounds in nature. From February until early June these little birds can be heard singing in fields, parks, woodlands and even in the middle of cities from Ireland all the way to China.

The most common great tit song contains two repeating notes, one high and one low which are said to sound a bit like “tea-cher, tea-tea-cher”. Here is an example I recorded in Derby (UK) a few years ago with a spectrogram below so you can see how it looks.

great-tit-spectrogram-derbyRemarkably however, this is just one of over 70 different calls and songs great tits are known to produce with individual birds having repertoires of up to eight song types. In fact, the great tit’s musical repertoire is so vast that if you hear a bird song you don’t recognise there is a good chance it’s a great tit!

Here a few different variants on the “classic” great tit call I have recorded in the past with spectrograms so you can see the difference.

Recorded in Starnberg (Germany)

great-tit-spectrogram-starnberg

Recorded in Bath (UK)

great-tit-spectrogram-bath

Recorded in Durham (UK)

great-tit-spectrogram-durham

Recorded in Leamington Spa (UK)

great-tit-spectrogram-leamington

In the past I travelled all over the UK recording these birds as part of my research. I have recorded so many great tit songs that I have an almost Pavlovian response whenever I hear one as I feel the need to grab my microphone and start recording. This year however, all I need to do when I hear their song is look forward to longer days and better weather.

My research poster from an animal behaviour conference in Bonn

I have spent the last few days in Bonn, the home of Haribo and former capital city of West Germany, for the annual meeting of the German Animal Behaviour Society (Ethologische Gesellschaft).

I think all who attended the meeting agreed it was a huge success. It was fantastic to spend a few days talking with other behavioural ecologists about their work and also to be able to discuss my own work with other like-minded people. Whenever I attend meetings like this I always come away with a renewed enthusiasm for what I do and with new ideas buzzing around my head for potential future research projects.

This time I did not give a talk myself, but I did present a poster showing the results of my recent work on the Lombard effect in canaries. I will write a full blog post about this soon but for those who might be interested in the poster I thought I would post it here. If you have any questions please don’t hesitate to ask!

etho2017-poster2
My poster from Etho 2017. A meeting of the Ethologische Gesellschaft in Bonn.

A high-definition PDF version is available here

Urbanisation is changing the way birds sing

ResearchBlogging.orgIn 1800 only 3% of the world’s population lived in urban areas, yet as the industrial revolution picked up pace in the early 18th and 19th centuries the number of people moving from the countryside to work in the newly industrialised cities soared. By 1950 29% of the world’s population were living in cities and by 1985 this had grown to 42% while in 2025 it is estimated to that it will be over 60%1. That is just 10 years from now. In highly developed nations such as those in Western Europe and North America the 50% threshold has already been surpassed which means that if you live in a western nation and do not live in a city you are in a minority.

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Across the globe cities are expanding.

It’s not just where people choose to live that is rapidly changing either, but how many of us there are. The human population is currently expanding more rapidly than ever before and has grown from just a few hundred million people less than a thousand years ago to over 7 billion today, and this number is still rising2. As the human population grows cities are expanding quickly to meet the need for additional housing while the surrounding countryside is farmed and developed ever more intensely to provide us with the food, water and other resources we need to live.

Human population growth from around 5 million people in 8000 BCE to over 7 billion today. (figure from Keinen and Clark 2012).
Human population growth from around 5 million people in 8000 BCE to over 7 billion today. (figure from Keinen and Clark 2012).

Naturally, many people are concerned about how we are going to continue to feed and clothe ourselves as populations continue to expand, but there is also a growing concern among many about how such huge numbers of people are affecting the environment and the animals that inhabit it.

For animals which depend on natural habitats such as forests, meadows, or wetlands to survive, the growth of urban areas often spells bad news as these habitats are removed and paved over to make way for new suburbs, factories and roads. In the UK numerous species have declined for just this reason. For example, the bittern, a close relative of herons, was once widespread in reedbeds and wetlands across the UK but is now confined to a tiny area of the south-east after its habitats were drained to make way for agriculture and urban developments.

While many species cannot survive in urbanised areas, others are able to tolerate moderate levels of urbanisation and may continue living within cities despite drastic changes to their habitats. Life in the city is not without its challenges however, even for the most adaptable and resilient of species. Cities typically contain different threats to rural areas such as an abundance of cats which are responsible for killing huge numbers of birds and small mammals4, and high levels of chemical5, light6 and noise pollution7 which all have negative impacts.

This all sounds bad, and it really is, but while many species suffer badly from the effects of increasing urbanisation and habitat loss, there are a few species that have been able to adjust remarkably well to life in urban areas. One group that has been particularly well-studied in this regard are the songbirds and over the past 15 years or so, biologists have discovered some fascinating behavioural adaptations which have allowed some species to become successful city dwellers.

Great tits are one species which are adapting to city life,
Great tits are one species which are adapting to city life.

One of the most notable features of cities across the world is that they are incredibly noisy places. With heavy traffic, building sites, aircraft flying overhead and all manner of other sounds and distractions it’s a miracle anyone can hear anything at all. For songbirds however, all this noise is more than just a distraction, it can seriously affect their chances of finding mates and successfully reproducing and for males it is likely to affect how well they can defend their territories against rivals.

While we may find bird song pleasant to listen to (or annoying depending on how early in the morning it is), for songbirds it has a serious purpose. Males sing during the breeding season to attract females8 and to signal to other males that their territory is occupied and should not be entered9.

City noise can overlap and interfere with these signals making communication among birds difficult and unreliable. The background noise of a city is typically continuous low rumble concentrated at around 2kHz in frequency. Unfortunately for many birds this overlaps neatly with the frequency of their songs and this can make it difficult for other birds to hear them as they do not stand out from the irrelevant background noise.

Clearly this is a problem for birds which rely on song to communicate, yet research has revealed that birds have ways of overcoming this problem and one of them is to increase the frequency at which they sing so that their songs literally rise above the background noise and can be clearly heard.

Great tits sing at a higher frequency in noisy cities than in quieter rural areas (from Mockford and Marshall, 2009).
Sonograms of rural (a) and urban (c) great tit song compared to the background noise of a city (b). You can clearly see that the urban song is a higher frequency than rural song and above the frequency of the city noise. (from Mockford and Marshall, 2009).

Evidence that birds sing at higher frequencies in noisy cities than they do in quieter rural sites has now been found in numerous species including great tits (Parus major)7, blackbirds (Turdus merula)10, European robins (Erithacus rubecula)11 and song sparrows (Melospiza melodia)12.

In great tits the difference in song frequency between urban and rural sites has been measured at 478Hz13 and tests have shown that this is enough to substantially improve the distance over which song can travel in urban environments before it degrades and becomes inaudible14.

Birds may also face challenging noisy conditions in natural environments too such as where running water or wind creates high levels of low-frequency noise and these naturally noisy sites have allowed scientists to confirm that it really is the noise in cities and not some other factor which is causing city birds to sing at high frequencies. Biologists Henrik Brumm and Hans Slabbekoorn recorded the songs of white-throated dippers (Cinclus cinclus) living around noisy fast flowing streams in Scotland and found that they call at frequencies well above that of the background noise and higher than usual for this species suggesting that dippers in this area have adapted their calls to suit their noisy habitat15.

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Dippers close to noisy streams sing at a higher frequency than the background noise so their songs can be heard (from Brumm and Slabbekoorn, 2005).

The effect of natural background noise on song frequency has also been shown in African little greenbuls (Andropadus virens) which sing at a higher frequency in areas where the rainforest is merging with open grasslands (known as ecotone forests) than they do deep within the rainforest itself16. Analysis of these two habitats revealed that the background noise in the rainforest is largely concentrated at higher frequencies while in ecotone forests there is more low-frequency noise. By singing at a lower frequency little greenbuls within the rainforest can ensure that their song does not overlap with the higher frequency background noise found in rainforests, while by singing at a lower frequency little greenbuls in ecotone forests avoid the lower frequency background noise in their habitat.

The evidence that birds change the frequency of their songs as an adaptation to noisy conditions may seem quite conclusive but not everyone agrees. An alternative explanation for the observed frequency shifts is that higher frequency song is actually just an unavoidable and possibly unimportant side-effect of singing more loudly, and it is higher volume, not frequency, which allows birds in noisy environments to overcome the background noise17.

In support of this argument Erwin Nemeth and Henrik Brumm of the Max Planck Institute for Ornithology in Germany found that the typical increases in song frequency found in great tits and blackbirds may be too low to substantially improve signal transmission whereas small increases in song amplitude were found to increase the distance over which a bird’s song could be detected much more effectively18.

But why should song frequency increase when birds sing more loudly? Nemeth and Brumm suggest two possibilities. Firstly, the increase in frequency observed in songs in noisy environments could be a side-effect of what is known as the Lombard effect (named after the French scientist Étienne Lombard) in which animals unconsciously increase the volume and frequency of their calls when the level of the background noise rises.

Nachtigall_%28Luscinia_megarhynchos%29-2[1]
Nightingales in Berlin sing loudly to ensure that they are heard.

The Lombard effect is known to occur in humans (this is why it might feel like you have to shout to be heard at loud parties) and has also been shown in both lab and field studies of songbirds. Lab experiments on elegant crested tinamous (Eudromia elegans)19, and budgerigars (Melopsittacus undulates)20, have shown that these species both sing more loudly and at a higher frequency when background noise increases and the same result has been shown in the field in a study of Nightingales (Luscinia megarhynchos) in Berlin21.

A second reason why song frequency may increase when birds sing more loudly is that both the volume and frequency of bird songs depend on the same song producing organ which could limit how well birds can independently control frequency and volume. In birds this organ is the syrinx which is the bird equivalent of the mammalian larynx or voice box and is located at the base of the windpipe connected to the lungs. Birds produce song by forcing air at high pressure from the lungs through the syrinx causing membranes to vibrate creating sound. This sound can then be modified using numerous tiny muscles which alter the shape and tension of the sound producing membranes.

However, past studies of the avian vocal system have shown that without these tiny muscles altering the structure of the sound, both the frequency and amplitude of bird song unavoidably increase together. In other words, when birds sing louder they cannot help but also sing at a higher frequency22.

Of course, this may be totally irrelevant if birds are able to use muscles to independently control the frequency and volume of their songs but there is evidence to suggest that the frequency and volume of bird songs really are closely intertwined. One of the clearest examples of this comes from a study by Nemeth and his colleagues at the Max Planck institute who recorded blackbirds singing in sound-proof chambers and showed that volume and frequency really were strongly correlated17. When blackbirds sing more loudly they also sing at a higher frequency and this may be totally involuntary. Similar results have been found in other species including zebra finches (Taeniopygia guttata)23 and song doves (Streptopelia risoria)24 suggesting that this pattern may be widespread in birds as a whole.

1280px-Turdus_merula_-Gran_Canaria%2C_Canary_Islands%2C_Spain-8_%282%29[1]
When blackbirds increase the volume of the song they also sing at a higher frequency. (image credit Juan Emilio).

It has become very clear over the past few years that urban noise is causing bird song to change however, opinion is still divided on whether it is the frequency or amplitude changes that are most important to improving song transmission in noisy environments. It is possible that both have important roles to play in helping birds to adapt to noisy urban areas and hopefully future research will provide an answer to this question.

The study of how urban noise affects bird song is a very active area of research and there are many unresolved questions which are likely to be answered in the next few years. Most importantly we need to find out what the long-term impacts of urban noise are on bird populations. Although many species of birds do seem to be able to adapt to noise we do not know how the dramatic changes we are causing to their environments will affect them in the long-term. Furthermore, many species are not able to adapt to urban areas for numerous possible reasons. They may not possess the behavioural flexibility to cope with new environments or not they might not be physiologically capable of changes their songs or behaviour. That is why studies those discussed here matter, we are changing the planet in ways which have never been seen before and we know that many species are suffering as a result. The first step to protecting animals from these changes is to understand how they are affected and that is just what these studies aim to do.


References

1. Kegel, B (2014).Tiere in der Stadt: Eine Naturgeschichte. Köln: DuMont Buchverlag. (In German).

2. Keinan, A., & Clark, A. (2012). Recent Explosive Human Population Growth Has Resulted in an Excess of Rare Genetic Variants. Science, 336 (6082), 740-743 DOI: 10.1126/science.1217283

3. Barnosky AD, Matzke N, Tomiya S, Wogan GO, Swartz B, Quental TB, Marshall C, McGuire JL, Lindsey EL, Maguire KC, Mersey B, & Ferrer EA (2011). Has the Earth’s sixth mass extinction already arrived?. Nature, 471 (7336), 51-7 PMID: 21368823

4. van Heezik, Y., Smyth, A., Adams, A., & Gordon, J. (2010). Do domestic cats impose an unsustainable harvest on urban bird populations?. Biological Conservation, 143 (1), 121-130 DOI: 10.1016/j.biocon.2009.09.013

5. Liker A, Papp Z, Bókony V, & Lendvai AZ (2008). Lean birds in the city: body size and condition of house sparrows along the urbanization gradient. The Journal of animal ecology, 77 (4), 789-95 PMID: 18479344

6. Miller, M. (2006). Apparent Effects of Light Pollution on Singing Behavior of American Robins. The Condor, 108 (1) DOI: 10.1650/0010-5422(2006)108[0130:AEOLPO]2.0.CO;2

7. Slabbekoorn, H., & Peet, M. (2003). Ecology: Birds sing at a higher pitch in urban noise. Nature, 424 (6946), 267-267 DOI: 10.1038/424267a

8. Baker, M., Bjerke, T., Lampe, H., & Espmark, Y. (1986). Sexual Response of Female Great Tits to Variation in Size of Males’ Song Repertoires. The American Naturalist, 128 (4) DOI: 10.1086/284582

9. Krebs, J., Ashcroft, R., & Webber, M. (1978). Song repertoires and territory defence in the great tit. Nature, 271 (5645), 539-542 DOI: 10.1038/271539a0

10. Nemeth, E., & Brumm, H. (2009). Blackbirds sing higher-pitched songs in cities: adaptation to habitat acoustics or side-effect of urbanization? Animal Behaviour, 78 (3), 637-641 DOI: 10.1016/j.anbehav.2009.06.016

11. McLaughlin, K., & Kunc, H. (2012). Experimentally increased noise levels change spatial and singing behaviour. Biology Letters DOI: 10.1098/rsbl.2012.0771

12. Wood, W., & Yezerinac, S. (2006). Song sparrow (Melospiza melodia) song varies with urban noise. The Auk, 123 (3) DOI: 10.1642/0004-8038(2006)123[650:SSMMSV]2.0.CO;2

13. Mockford, E., & Marshall, R. (2009). Effects of urban noise on song and response behaviour in great tits. Proceedings of the Royal Society B: Biological Sciences, 276 (1669), 2979-2985 DOI: 10.1098/rspb.2009.0586

14. Mockford, E., Marshall, R., & Dabelsteen, T. (2011). Degradation of Rural and Urban Great Tit Song: Testing Transmission Efficiency. PLoS ONE, 6 (12) DOI: 10.1371/journal.pone.0028242

15. Brumm, H., & Slabbekoorn, H. (2005). Acoustic communication in noise. Advances in the Study of Behavior, 35, 151-209 DOI: 10.1016/S0065-3454(05)35004-2

16. Slabbekoorn H, & Smith TB (2002). Habitat-dependent song divergence in the little greenbul: an analysis of environmental selection pressures on acoustic signals. Evolution; international journal of organic evolution, 56 (9), 1849-58 PMID: 12389730

17. Nemeth, E., Pieretti, N., Zollinger, S., Geberzahn, N., Partecke, J., Miranda, A., & Brumm, H. (2013). Bird song and anthropogenic noise: vocal constraints may explain why birds sing higher-frequency songs in cities. Proceedings of the Royal Society B: Biological Sciences, 280 (1754), 20122798-20122798 DOI: 10.1098/rspb.2012.2798

18. Nemeth, E., & Brumm, H. (2010). Birds and Anthropogenic Noise: Are Urban Songs Adaptive?. The American Naturalist, 176 (4), 465-475 DOI: 10.1086/656275

19. Schuster, S., Zollinger, S., Lesku, J., & Brumm, H. (2012). On the evolution of noise-dependent vocal plasticity in birds. Biology Letters, 8 (6), 913-916 DOI: 10.1098/rsbl.2012.0676

20. Osmanski, M., & Dooling, R. (2009). The effect of altered auditory feedback on control of vocal production in budgerigars (Melopsittacus undulatus). The Journal of the Acoustical Society of America, 126 (2) DOI: 10.1121/1.3158928

21. Brumm, H. (2004). The impact of environmental noise on song amplitude in a territorial bird. Journal of Animal Ecology, 73 (3), 434-440 DOI: 10.1111/j.0021-8790.2004.00814.x

22. Titze, I. R. (1994). Principles of voice production (pp. 279-306). Englewood Cliffs: Prentice Hall.

23. Cynx J, Lewis R, Tavel B, & Tse H (1998). Amplitude regulation of vocalizations in noise by a songbird, Taeniopygia guttata. Animal behaviour, 56 (1), 107-13 PMID: 9710467

24. Elemans, C., Zaccarelli, R., & Herzel, H. (2008). Biomechanics and control of vocalization in a non-songbird Journal of The Royal Society Interface, 5 (24), 691-703 DOI: 10.1098/rsif.2007.1237

My study species in four facts

ResearchBlogging.org
The last few months have been a blur of conferences, workshops. I attended a fantastic two week course on sensory ecology in Sweden (read about that here and here), I went to London and presented a poster of my work at the annual meeting of the Association for the Study of Animal Behaviour and have recently returned from the Joint meeting of the British Ecological Society and the Société Française d’Écologie in Lille. Time has been flying by and now Christmas is upon us but I wanted to write at least one more post for 2014 so here it is. These are four facts about my wonderful study species the great tit (Parus major).

Great tits have personalities.
My study species.

Every great tit has a unique personality

It’s true, you might think that a bird is just a bird and they are all the same but research has shown that this is not the case. In fact, many species of birds including great tits are now known to have distinct personalities and behave differently to one another, just like humans do. Researchers at Groningen University in the Netherlands found that when they exposed adult great tits to a completely new environment which they had never experienced before some birds were confident and bold and explored their new environments quite happily, while others were much more cautious and did not adapt quickly to the unfamiliar surroundings. Further tests have shown that birds which are bold or shy the first time they are tested tend to stay that way in repeated experiments. These results suggest that personality types are probably fixed characteristics which do not easily change. So, for anyone who’s ever said that animals have personalities, you were right. And it’s not just great tits either; research over the past few years has identified distinct personalities in a huge number of species including mammals, birds, insects and even anemones!

Different personalities. Great tits are not all the same. Image credit: Per Tillman.
Different personalities. Great tits are not all the same. Image credit: Per Tillmann.

City great tits just aren’t as colourful as their countryside cousins

With their bright yellow breast feathers and marked and glossy black plumage great tits are hard to miss at the best of times and are particularly conspicuous during the breeding season when the males are singing their hearts out trying to attract mates. The purpose of the great tits striking colouration is not fully understood but it is likely that females are most attracted to those males which have the brightest and boldest feathers. It is unfortunate then for city birds that their feathers just aren’t as bright as their rural counterparts. A likely explanation for this is that the same carotenoid compound which is used to create the bright yellow colour in great tits feathers is also an anti-oxidant which reduces the physiological stress caused by oxidising pollutants in the environment. As urban birds are known to be exposed to much higher levels of pollutants than rural birds it may be that urban birds are forced to divert carotenoids away from their feathers and use them instead to reduce the levels of stress and cell damage that oxidising compounds can cause. Evidence of this comes from a study at Göteborg University in Sweden which found that the increased oxidative stress faced by urban birds correlated well with reduced levels of carotenoid pigments in their feathers.

Great tits can be incredibly aggressive

They might look cute and friendly but in reality great tits can be incredibly violent, and particularly so during the spring and summer when males are competing for the best breeding territories. During this period male great tits mark their territories by singing at regular intervals and this signal tells other males to move on and find an empty territory of their own. This works most of the time but not always. Sometimes males deliberately invade territories and may attempt to mate with the resident female or even try to force the current territory holder out and take over completely. If this happens the first thing the resident male will do is to sing frequently and loudly in an attempt to show their strength and to drive the intruder away. Should this not work things typically escalate quickly as the resident male flies back and forth across the intruder before coming into full-on physical confrontation. It is not common to see fights in the wild as invaders will usually leave before it comes to actual violence. It does happen sometimes though and you can see the result in these incredible videos.

Great tits sing at higher frequencies in cities than in rural areas

Cities are notoriously noisy places full of cars, people and factories, with aircraft flying overhead and noise from building sites, road repair crews or similar urban development projects. This level of noise is something which wild animals have only had to face in the last century or so, and for species which communicate using sound it could make life in cities very difficult as their vocal signals are easily lost amongst the clatter and din of urban life. Great tits are a good example of a species which might be expected to fare badly in urban habitats as the males rely on communicating by song to defend their territories, show aggression towards rivals and attract mates. To make matters worse, the typical song of a great tit lies in exactly the same frequency range as the background noise of most cities and this means that when a great tit sings its song is quickly swamped by urban noise and cannot be heard by other birds. Yet despite this, great tits are one of the most common song birds seen in city parks and at garden bird feeders so how do they manage to cope with the noise problem? The answer is simple but ingenious. Urban great tits have adjusted their typical song frequency so that they now sing around 350Hz higher. That might not sound like much but experiments have shown that this small adjustments means that the song of urban birds travels above the low-frequency rumble of urban noise and can still be heard by other birds, even at long distances. Genius!

When there are high background noise levels great tits adapt by singing at a higher pitch. Figure from Slabbekoorn and Peet (2003).
When there are high background noise levels great tits adapt by singing at a higher pitch. Figure from Slabbekoorn and Peet (2003).

References

Every great tit has a unique personality

Dingemanse, N. (2002). Repeatability and heritability of exploratory behaviour in great tits from the wild Animal Behaviour, 64 (6), 929-938 DOI: 10.1006/anbe.2002.2006

City great tits just aren’t as colourful as their countryside cousins

Isaksson, C., Örnborg, J., Stephensen, E., & Andersson, S. (2005). Plasma Glutathione and Carotenoid Coloration as Potential Biomarkers of Environmental Stress in Great Tits EcoHealth, 2 (2), 138-146 DOI: 10.1007/s10393-005-3869-5

Great tits sing at higher frequencies in cities than in rural areas

Slabbekoorn, H., & Peet, M. (2003). Ecology: Birds sing at a higher pitch in urban noise Nature, 424 (6946), 267-267 DOI: 10.1038/424267a

Great tits hunt for Pipistrelle bats

ResearchBlogging.org
When under strong ecological pressure, or when a good opportunity arises, animals have often shown themselves to be surprisingly innovative in how they adapt to new pressures or take advantage of new resources. Many examples of this have been observed in the wild including the discovery of tool use by chimpanzees, problem solving in guppies and the development of a novel ‘body-slapping’ behaviour as a means of communication in grey seals. No behaviour has surprised me more however than the discovery that in Hungary a population of a small seed-eating song bird, the great tit (Parus major), has switched from its staple diet of seeds and insects and has learnt to search for, kill and eat hibernating bats (Pipistrellus pipistrellus).

At around five inches long great tits are small birds, but pipistrelle bats are even smaller at just an inch in size. During the winter these bats hibernate in cracks and crevices in dark caves or old buildings where they are safe and well hidden, but when they awaken they start making noises which draws the attention of nearby predators, including great tits.

A Great tit (Parus major). Not a typical carnivore.
A Great tit (Parus major). Not a typical carnivore.

The earliest suggestion that great tits might hunt for bats goes back to at least 1947 when a Swedish biologist named Olaf Ryberg observed dead bats in Sweden with “injury, caused e.g. by titmice (possibly also bigger birds)“. It was to be almost half a century before the subject was raised again when in 1996 a great tit was seen feeding on a dead bat in a cave in Poland. Three years later at the same site in Poland three more bats were found, one dead and two alive, with injuries which looked like they were caused by tit beaks. Despite these observations it was still not clear that in any of these cases great tits were actually hunting for bats actively and it remained a possibility that they were simply scavenging on bats which had already died. A chance observation of a great tit capturing a live pipistrelle in a cave in Hungary in 1996 provided the only evidence at this point that great tits ever actively preyed on live bats.

That first observation was made by Péter Estók from Germany’s Max Planck Institute for Ornithology and intrigued by what he had seen he and his research team returned to the cave in Hungary on three separate occasions from 2004 to 2009. Using experiments and old-fashioned observation  they aimed to discover whether feeding on bats by great tits was simply opportunistic, or whether great tits had learnt to deliberately and systematically hunt for and feed on pipistrelles.

A common pipistrelle bat (Pipistrellus pipistrellus). Image from http://www.naturephoto-cz.com/
A common pipistrelle bat (Pipistrellus pipistrellus). Image from http://www.naturephoto-cz.com/

The research team quickly found their answer. During the first winter of observations they witnessed great tits capture and consume live bats seventeen times in just ten days. Yet despite this it was still not known why this behaviour had developed in the first place.

One possibility was that great tits used bats as a last-ditch food source when their regular food was in short supply. To test this possibility the researchers left a mixture of sunflower seeds and bacon in feeders around the cave entrance to provide an easy and irresistible meal for any passing great tits. Sure enough, when plentiful food was provided they found that hunting for bats by great tits stopped almost completely with only one case observed over a ten-day period. This provided good evidence that feeding on bats was driven by an urgent need for food and did not represent a more general shift in diet.

Now just one question remained to be answered. How do great tits find the bats in the first place? It was thought that they might be able to home in on the bat’s calls so to test this possibility Estók recorded the bats and played their calls back to great tits from a speaker. Around 80% of the birds reacted strongly to the sounds often turning their heads towards the speaker and approaching to investigate. This was particularly interesting because in one study bat calls were shown to act as a deterrent to mammalian predators, possibly by signalling that the bats are awake and cannot be caught. For great tits however it seems that bat calls are far from a deterrent, possibly because they can easily outmanoeuvre a bat in flight.

Eight years passed between the first observation of a great tit preying on a live bat and the start of Estók’s study. Given that the typical lifespan of great tits is three years the birds observed in 2004 couldn’t possibly have been the same birds that were seen in 1996. This raises the fascinating possibility that the bat killing behaviour is passed from one generation to the next by some form of cultural transmission. Whether this is or is not the case is not yet known and so it seems there is still much to learn about the unassuming great tit.

                                                                            

References

For the study of great tits hunting bats

Estók P, Zsebok S, & Siemers BM (2010). Great tits search for, capture, kill and eat hibernating bats. Biology letters, 6 (1), 59-62 PMID: 19740892

Bat calls as a deterrent to mammalian predators

Martin, K., & Fenton, M. (1978). A possible defensive function for calls given by bats (Myotis lucifugus) arousing from torpor
Canadian Journal of Zoology, 56 (6), 1430-1432 DOI: 10.1139/z78-196

Innovative behaviour in other animals

Body slapping seals
Bishop, A., Lidstone-Scott, R., Pomeroy, P., & Twiss, S. (2013). Body slap: An innovative aggressive display by breeding male gray seals (Halichoerus grypusMarine Mammal Science DOI: 10.1111/mms.12059

Problem solving guppies
Laland KN, & Reader SM (1999). Foraging innovation in the guppy. Animal behaviour, 57 (2), 331-340 PMID: 10049472

Tool use in chimpanzees
Goodall, J. (1964). Tool-Using and Aimed Throwing in a Community of Free-Living Chimpanzees Nature, 201 (4926), 1264-1266 DOI: 10.1038/2011264a0

Nature on my doorstep

I know I’ve been away for a long time (has it really been four months?) but I have a good reason. I have just moved from Durham in the north of England all the way to Wales where I am working on a PhD project looking the effects of urban noise. It’s all very interesting and a lot of fun. Wales is also a beautiful place with lots of amazing countryside to explore and unique animals to see. I will talk more about my research soon but today I wanted to share a few pictures that I took while walking home from the beach this afternoon.

If you live in the UK you will know just how wet it has been recently, I don’t think I’ve seen the sun for two weeks and there is water everywhere. That’s why when I woke up this morning and saw the sun I planned to get out and make the most of it. There is a footpath that runs from just by my house and along the cliff tops by the beach. The views are fantastic and it is a great place to see seabirds wheeling and diving out in the bay, red kites flying overhead and I’m told that come summer it will also be possible to see dolphins swimming just offshore which I am inordinately excited about.

Thinking today would be a good opportunity for some wildlife photography I took my camera with me for a walk along the coast and managed to get what I think are some great shots of the town and a house sparrow (Passer domesticus*) perching in some bushes. I’ll be back with some proper posts soon but for now here are some pictures for you to enjoy. Ten points to whoever can guess the name of the town (without looking at my about page).

House sparrow (Passer domesticus) (7)

House sparrow (Passer domesticus) (3)

House sparrow (Passer domesticus) (4)

House sparrow (Passer domesticus) (5)

House sparrow (Passer domesticus) (6) _MG_0006

* Incidentally the house sparrow is the type species after which all song birds (Passerines) are named.

Gouldian finches’ head colour reflects their personality

Gouldian finches, Erythrura gouldiae, are an extraordinarily colourful species of passerine bird endemic to subtropical woodlands of northern Australia. Both sexes are brightly coloured with red, green, black, yellow, red and purple markings, it is for this reason that they are also sometimes known as rainbow finches.

Just look at those colours! Note the different yellow, red and black head colouration.

In the wild the birds exhibit two main head colour morphs, black and red. There is also a rare yellow colour morph as shown in the image above. Interestingly, studies of captive birds have shown that males with red heads are on average more aggressive than those with black heads and that females have a preference for red-headed over black-headed individuals. Red headed males were also found to have higher levels of testosterone and corticosterone than black headed males when faced with socially challenging situations.

What this suggests is that behavioural characteristics, such as aggression and other traits, may be correlated with particular head colour morphs meaning that head colour is indicative of different personality types. This idea has been tested in a new paper by Leah Williams and her colleagues.

In order to determine if head colour really does indicate personality traits in Gouldian finches Williams and her colleagues tested a number of predictions. First they looked at pairs of black-headed birds which were expected to show less aggression towards each other than pairs of red-headed birds, this makes sense since red-headed birds had previously been found to exhibit higher levels of aggression.

The second prediction was that red-headed birds should be bolder, more explorative and take more risks than black-headed birds. This hypothesis is based on previous studies of other species that have shown a correlation between aggression and these behavioural characteristics. However, there is another possibility, red-headed birds could take fewer risks for two reasons; first, they may be more conspicuous to predators due to their bright colouration and second, it may pay black headed birds to take more risks and be more explorative so they find food resources before the dominant red-headed birds do.

In order to test the first prediction paired birds of matching head colour were moved into an experimental cage without food. After one hour of food deprivation a feeder was placed into the corner of the cage where there was only enough room for one bird to feed at a time. aggressive interactions such as threat displays and displacements were then counted over a 30 minute period.

The results as shown in the figure below were striking. Red-headed birds were significantly and consistently more aggressive than black-headed birds.

Figure shows the mean (+SE) number of aggressive interactions by individuals in relation to their head colour.

To test the birds willingness to take risks they were deprived of food for one hour before their feeder was replaced. After the birds had calmly begun to feed a silhouette of an avian predator was moved up and down in front of the cage to scare the birds from the feeder. The time it took for them to return to the feeder was taken as a measure of their willingness to take risks, birds that returned quickly were considered to be greater risk takers than those that were more cautious.

This time the results were surprising. Red-headed birds were considerably more cautious than those with black heads at returning to the feeder after a “predator” had been introduced. As the figure below shows they took on average 4x longer to begin feeding again than the less aggressive black-headed birds.

Figure shows the mean (+SE) time taken for birds to return to their feeder after a “predator” was introduced.

Finally, the authors investigated the birds interest in novel objects or “object neophilia” which is defined in the paper as “exploration in which investigation is elicited by an object’s novelty“. To do this a bunch of threads was placed on a perch within the cage, the time taken for the birds to approach the threads within one body length and to touch them were recorded over a one hour period. In line with the results from the risk taking experiment it was found that the aggressive red-headed birds showed less interest in novel objects than did black-headed birds. The difference is not so striking as the previous experiments but was statistically significant nonetheless.

Figure shows the mean (+SE) time taken for birds to approach a novel object relative to their head colour.

These experiments were repeated after a two month interval and showed that different birds differed in their responses but the responses of individual birds were consistent over time. Head colour was found to predict the behavioural responses of the birds. Red-headed birds were more aggressive than black-headed birds but took fewer risks and were not explorative.

What is surprising about these results is that aggression does not correlate with risk taking behaviour, however, the authors do provide a convincing explanation, suggesting that…

…red coloration has been found to be conspicuous against natural backgrounds, and more conspicuous birds have been found to suffer higher predation rates. Thus, selection could favour more conspicuous red-headed birds taking fewer risks.

Interestingly boldness and risk taking behaviours were found to be strongly correlated, regardless of head colour they always occurred together forming a “behavioural syndrome”. This implies that there is selection in favour of specific combinations of traits and of head colour in relation to those traits. Selection favours aggression in red-headed birds and the boldness/risk taking behavioural syndrome in black-headed birds. This makes sense when you consider the high risk of predation faced by red-headed birds if they take too many risks and the need for black-headed birds to find food away from the dominant red heads which occupy the safest foraging locations.

Williams and her colleagues suggest that if red-headed birds are aggressive, and black-headed birds take more risks, this could lead to differences in foraging tactics. For example, black headed birds could increase their foraging opportunities by feeding at more risky sites away from interference by the dominant red-headed birds which feed in safer locations. The lower conspicuousness of their black heads means they are at less risk of predation at exposed sites that red-headed birds would be.

The results of this fascinating study strongly support the hypothesis that head colour does indeed signal personality in Gouldian finches. I would love to see some more research in this area. The authors themselves suggest that more research is needed to find out what roles head colours play in social situations. It would also be interesting to find out how widespread this phenomenon is, given that birds frequently use plumage colouration as signals it seems likely to me that colour may indicate personality in other avian species.

                                                               

Williams L.J., King A.J. & Mettke-Hofmann C. (2012). Colourful characters: head colour reflects personality in a social bird, the Gouldian finch, Erythrura gouldiae, Animal Behaviour, 84 (1) 159-165. DOI: