They Grow Up So Fast

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!

On the left a 14-day old great tit chick, and an adult bird on the right.

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.

Interlude: Wait.. what?!

Research can involve a lot of reading — especially when you find yourself in a new field of study that has a long history, which means there is a substantial pile of previous research to catch up with. While my current project uses methods that have only been developed over the past few years, some of the study species have been observed for decades, and various fundamental aspects of their behaviour and lifestyle are exactly the things people were describing in the early studies.

Over the past two years, I have read (parts of) hundreds of scientific articles about all manner of vaguely related subjects. For some of these, I have looked back at particular sections so often that it’s perhaps only a matter of time before I can quote them from memory! Others I have only quickly glanced over, and sometimes I find myself reading something, only realizing when I’m already halfway through that I had in fact read it before…

Especially when browsing a lot of text for something specific, most of what you encounter is not particularly memorable. It all just blurs into one, all irrelevant for one reason or another: wrong species, different context, outdated method, and so on, and so on, and.. wait, what? Did I seriously just read that?!

Case in point:

Royama, T (1970) Governing the Hunting Behaviour and Selection of Food by the Great Tit (Parus major L.). Journal of Animal Ecology, Vol. 39, No. 3, pp. 619-668.

Yes.

Moreover, these larvae were not particularly distasteful to the human tongue

In the middle of a perfectly unassuming text about some small birds and the various insects they have been observed eating, Mr. T. Royama included a little throwaway line about, apparently, having tried some of these caterpillars for himself too. Nowhere in the 50-page text could I find any further reference to the culinary experiments that led to this conclusion. Although the article has 561 citations at the time of writing according to Google Scholar, I can find no further references to this particular peculiar line anywhere on the web.

Earlier this year, I have tried to reach out to Mr. Royama’s last known place of work and listed email-address, but with no luck. Given that the above is already half a century old and some of T. Royama’s articles predate these, there is a chance this intriguing opportunistic insectivore is perhaps unfortunately no longer alive, although he at least appears to have authored an article as recently as 2017. I have now tried to contact one of his last known collaborators, in the hope they might have any means of contacting him. Within seconds, I received a reply: “I will be away from my office for an extended period. For important matters, I can be reached at […]”. I guess this doesn’t *quite* qualify as an “important matter”, so I will simply wait and see.

Maybe whatever happened in the woods near Oxford during the 1960s will simply remain a mystery forevermore…

A brood of Parus major chicks — the more usual consumers of caterpillars.

Current research, part 1: Introduction

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 particulary 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.