Category: PhD

Or specifically: great tit personalities! While the idea that other animals have their own characters may seem a tad anthropomorphic, on a fundamental level it is not that strange a thought that certain individuals of a species will, for example, be more aggressive than others. If such behaviour is consistent across time and contexts, then that can be quite objectively described as a character trait.

Of course, the basic concept that animals can have their own personalities will be no surprise to many pet owners, but it raises many questions: what species have personalities? While a dog may be expected to, does a mouse? A lizard? A snail? What factors lead to an individual having a certain personality, and in turn, how does having that personality affect that individual’s behaviour?

For the last few decades, personalities have been studied across a range of animals, from mammals to birds and fish^[1–6]. Across these species, one common finding has for instance been that individuals vary along a “shy–bold” spectrum, a seemingly simple value that determines many aspects of their behaviour. Exposed to a changed or new environment, “bold” individuals would be more explorative or aggressive, while “shy” ones would be more careful and passive. In a natural context, the “bold” behaviour can be seen as a “high risk–high reward” strategy: these individuals might encounter things like new territories or food sources, but they are also more prone to encountering and falling prey to a predator. Both extremes have their own benefits, and the “best” strategy will depend on many environmental factors. Since environments are rarely stable, it then makes perfect sense that a population will contain different individuals: in some years or areas, one type may thrive, while in other contexts the other will.

Although my own research does not directly focus on these character traits, the birds I work with do possess them still (in fact, one species, the great tit Parus major, has been a very common study subject for research on personalities), which can sometimes be quite noticeable. Below, I will detail two specific cases involving some particularly noteworthy individuals.

The first time a bird’s personality really struck me was when visiting a certain nestbox for a routine check regarding nest progression. Peeking into the nestbox, I could see that the female great tit was inside, incubating her eggs. When simply approaching the box, or otherwise when carefully moving a bird aside if having to check for incubated eggs or hatched chicks underneath, some of these birds will attempt to escape from the box, while others will fan their feathers and make loud noises (at an impressive volume for such a small bird) to try and repel you. Not only did this particular bird stay completely silent initially, she continued to be surprisingly calm even when I gently grabbed her to investigate the colour-rings on her legs. Instead of trying to either get away or get me to go away, she instead appeared to pull downwards, seemingly only desiring to get back to incubating her eggs.

Weeks later, performing another routine check on some other nest, the bird in that box behaved in the exact same unusual manner; behaviour I hadn’t encountered in dozens of other nests besides the one mentioned before. When I looked at the identifying colour-rings on her legs, those looked somewhat familiar, and once I checked my earlier notes I could confirm: it was indeed the same bird as before, now attempting a second brood after the first one had fledged.

The next year, I had all but forgotten about those encounters, when I once more found myself holding a particularly dedicated mother-bird, an individual seemingly more interested in incubating her eggs than her own wellbeing. I wasn’t able to verify her identity until I got home and looked at last year’s data, but of course: it was the same bird again! Her behaviour was so unique it had essentially been possible to identify her on that basis alone.

The second case offered an interesting glimpse at the underlying mechanisms for these character traits. This involved an unringed male great tit, who we tried to catch in order to give him rings so that we could recognize him in the future. Using a simple trap that prevents escape from the nestbox after a parent bird enters it to feed the chicks, he was successfully caught, and all seemed fine. As soon as I took him from the box however, from the very first touch, he screamed, in a terrible manner. This was not the normal shouty hissing call used to repel potential predators I heard so many times, but something that appeared to indicate the bird was in some kind of pain.

As I turned the bird in my hand, I looked for any external injuries, but could find none. In addition, it didn’t appear to matter how I held the bird or where I touched it, the screaming persisted. The situation only slightly improved over the following minutes as I proceeded to ring and measure him as normal. Once done, I immediately released him, but stayed around to look for any unusual behaviour that might indicate some underlying issue after all. However, he simply flew to a nearby tree and adjusted his feathers a bit, all calmly and in complete silence. Whatever had been the issue, it appeared to have been restricted to being handled.

Last summer, on what I remember being a particularly sunny morning, I was again patiently waiting near a box full of hungry great tit chicks, hoping to catch the father bird there. On a prior visit, we had already seen that this particular bird had an aluminium ring around one leg, indicating he was ringed as a chick in our study area, but had not been ringed as a parent yet (at which point they receive one or more colour-rings).

As you will have guessed from the way this story has been going, when he was caught the little thing started screaming awfully, as soon as my hand as much as approached him. Having encountered this behaviour before and now knowing it may not mean anything, I continued to treat this fellow like any other bird, though once again checking (and failing to find) any injuries or issues.

While the previous case of a bird with a recognizable personality concerned the same individual, I knew that could not be the case here: after all, the previous bird shouting as if his life depended on it had expressly been given colour rings already; that had been the reason for handling him at the time. This bird only had the aluminium ring, though the unique code on that did allow me to trace back where he had been born the year before. Turns out, yes, he was indeed born in that nest where I had captured the first shouting bird last year. Whether learned or genetically encoded, the adage “like father, like son” can apparently also be true in birds..!

• ^[1] Gracceva G, Herde A, Groothuis TG, Koolhaas JM, Palme R, Eccard JA (2014) Turning Shy on a Winter’s Day: Effects of Season on Personality and Stress Response in Microtus arvalis. Ethology 120(8): 753–767.
• ^[2] Michelena P, Sibbald AM, Erhard HW, McLeod JE (2009) Effects of group size and personality on social foraging: the distribution of sheep across patches. Behavioral Ecology 20(1): 145–152.
• ^[3] Cole EF, Quinn JL (2014) Shy birds play it safe: personality in captivity predicts risk responsiveness during reproduction in the wild. Biology letters 10(5): 20140178.
• ^[4] Sasaki T, Mann RP, Warren KN, Herbert T, Wilson T, Biro D (2018) Personality and the collective: bold homing pigeons occupy higher leadership ranks in flocks. Philosophical Transactions of the Royal Society B: Biological Sciences 373(1746): 20170038.
• ^[5] Toms CN, Echevarria DJ, Jouandot DJ (2010) A methodological review of personality-related studies in fish: focus on the shy-bold axis of behavior. International Journal of Comparative Psychology 23(1).
• ^[6] Sneddon LU (2003) The bold and the shy: individual differences in rainbow trout. Journal of Fish Biology 62(4): 971–975.

Previously, I wrote about regularly visiting a nest of pied flycatchers in order to see how the little chicks were rapidly developing: from tiny pink monsters to complete birds in just two weeks time. Last summer, I decided to do the same for our two other box-breeding birds: blue tits (Cyanistes caeruleus) and their larger cousins, the great tits (Parus major). Having these picture series also happens to be very practical as reference material for determining the age of newly encountered birds: their development is so rapid, that clear differences can be seen in for example the emergence and growth of particular feathers from one day to the next.

These two species don’t develop quite as quickly as the pied flycatchers, leaving the nest a few days later (when about 18 days old). I only visited them until the age at which we measure and ring them (13 days for blue tits, 14 days for great tits) as additional disturbances in the later days might have led to them trying to fledge early. In these last few days in the nest, it’s especially their wing and tail feathers that would have grown some more — they can’t quite fly with what you see in the last pictures here, which is a good thing too: when handling them, the energetic little things are enough of a flight risk even without being capable of actually flying!

Aside from their size, the blue tits (above) and great tits (below) are largely similar for the first few days, with their distinctive colours only becoming apparent in the second week.

Although, more accurately, they don’t actually really have their distinctive colours yet. While their plumage as youngsters isn’t as dissimilar to that of adults as it is in flycatchers, it still is markedly different. Compared to an adult great tit, the plumage of the young one looks like it had a yellow filter put over it: the white areas are yellow, the glossy black parts a dull brown. For the blue tit, the characteristic colour from which it gets its name is still entirely absent, instead appearing some shade of green!

For adult birds, their colours are known to play an important role in attracting a partner^[1]: some of these pigments cannot actually be produced by the birds themselves, and instead originate from their food^[2], which means that only those birds who are capable of finding a lot of the right type of food can produce the right shade. Demonstrating an ability to efficiently find food makes you a pretty attractive suitor, as the female birds largely rely on their partners to provide them with food when they themselves are incubating eggs, and it will take the combined efforts of both parents to feed all those hungry mouths of their offspring. For the chicks, signalling to prospective partners is not something they have to worry about until next year, so the more subdued colours are more practical by offering some camouflage. Looking different from the adults may also prevent getting into trouble with other birds of the same species, as they won’t be regarded as potential competitors when entering these birds’ territories.

• ^[1] Norris KJ (1990) Female choice and the evolution of the conspicuous plumage coloration of monogamous male great tits. Behavioral Ecology and Sociobiology 6: 129–138.
• ^[2] Fitze PS, Kolliker M, Richner H (2003) Effects of common origin and common environment on nestling plumage coloration in the great tit (Parus major). Evolution 57(1): 144–150.

Let’s start the new year with a new series, detailing the ongoing research of my PhD. As is customary in scientific articles and a good idea in general, I will commence with some background information to explain why we do the things we do.

As nowadays can’t have escaped anybody’s attention, the world is undergoing a bit of change. Many of the warmest years, months and days on record have taken place in the last decade, and whichever of the predicted scenarios of climate warming one follows, such extreme weather events are only going to become more common. (I will not be tackling the existence of climate change and greenhouse gasses here, but perhaps at a later date, after establishing that evolution is real and the Earth is a sphere. Well, technically an oblate spheroid, but.. anyway.) In many different fields of study, people are now trying to predict how the world around us may be altered under the influence of these changing environmental conditions.

Some decades ago now, researchers started to monitor how the seasonal timing in various organisms was shifting with increasing temperatures, and found that from a biological point of view, spring was indeed occurring earlier. Deciphering clear annual patterns for some species could prove difficult, however, let alone understanding whole ecosystems: complex networks of many different species interacting in countless ways.

In 1998, Visser, van Noordwijk, Tinbergen and Lessells^[1] calculated whether the breeding times of a widely studied small bird, the great tit Parus major, might be shifting, and crucially whether this was in line with the peak abundance of the prime food for their young: caterpillars. Those caterpillars, in turn, would want to emerge earlier in the year too, so as not to miss the optimal timing for their own food: the new leaves of deciduous trees (locally predominantly oak, Quercus spp.). They found that while the availability of caterpillars was advancing, the birds were not shifting their egg laying dates accordingly. This suggested that over time, a problematic (for the birds at least) mismatch between food availability and food requirements might start to arise.

But, as often is the case in research, that was not the end of it. While they found no shift in the birds’ timings in the Netherlands, this was in contrast with a long-studied population in the United Kingdom, near Oxford^[2]. When a later study compared more populations across Europe of the same and a closely related species (the blue tit Cyanistes caeruleus), no consistent pattern could be found^[3]. Overall, breeding periods typically advanced more in southern than in northern populations, and at the time the temperatures in those northern areas had not been rising much yet. Neighbouring populations differed in their responses, however, indicating that other factors must be playing a role.

One potential factor was the timing window for the birds. The period of peak food requirements, when the chicks are rapidly growing (see this previous post), occurs quite a while after the parents commence their breeding behaviour. A nest has to be built (which takes a few days), after which they lay their eggs (around 10 days, as their clutches average about 10 eggs and they lay 1 egg/day), which have to be incubated (another 2 weeks), after which the chicks finally emerge. As those young require the most food when they are around 1.5–2 weeks old, well over a month has now elapsed since the start of the breeding period. Not only does this make the optimal timing difficult to predict based on the conditions at the start of the season, the increase in temperatures is not necessarily uniformly distributed over the year. Furthermore, weather in any individual year can of course simply vary a lot, which means any natural selection towards early breeding is not very straightforward.

And then we haven’t even considered many other factors that can play a role. In some populations, the birds frequently have second broods, and those birds may breed earlier in order to have the ability to raise another brood in the same year. The birds may further be constrained in time by a demanding overwintering period before the breeding season, or the timing of their moult in summer, although these latter factors are unlikely to significantly differ between neighbouring populations.

Then, of course, there are the prey themselves. The insect community in oak-dominated woodlands is unlikely to be the same as that found in birch forests or pine plantations. Different species have different overwintering strategies, and will emerge at different times in different forms (for example as tiny, freshly hatched caterpillars, or as adult moths). In studies on the birds’ food availability and requirements, it has been common to either assume that all local caterpillars would be on the birds’ menu, or that a particularly common prey species would be representative of all others in the diet. Such simplifications were basically required, as identifying hundreds of different prey species quickly and accurately is practically impossible. Or at least: it used to be. New DNA-based techniques have opened the doors to studies not thought to be possible before, and they have been particularly useful for otherwise tricky dietary studies. With these techniques, we will for the first time delve into the availability and consumption of all the different prey species in this food web. This should give us a much better understanding of how the populations of these birds may actually be affected by climate change, and with that, how the natural world could be altered in the decades to come. In the next blog post, I will go into more detail about these methods, how they work, and how we are using them.

• ^[1] Visser ME, Van Noordwijk AJ, Tinbergen JM, Lessells CM (1998) Warmer springs lead to mistimed reproduction in great tits (Parus major). Proceedings of the Royal Society of London B 265: 1867–1870.
• ^[2] McCleery RH, Perrins CM (1998) . . . temperature and egg-laying trends. Nature 391: 30–31.
• ^[3] Visser M, Adriaensen F, Van Balen JH, Blondel J, Dhondt AA, Van Dongen S, Du Feu C, Ivankina EV, Kerimov AB, De Laet J, Matthysen E, McCleery R, Orell M, Thomson DL (2003) Variable responses to large-scale climate change in European Parus populations. Proceedings of the Royal Society London B 270: 367–372.