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 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. 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. 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.
-  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.
-  McCleery RH, Perrins CM (1998) . . . temperature and egg-laying trends. Nature 391: 30–31.
-  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.