Food availability, mistiming, and climatic change

Most bird species do not breed at just any moment in the year: breeding is mostly confi ned to a restricted period and is rather synchronized within populations. Seemingly, each species breeds at the period when benefi ts in fi tness terms are maximal, and one of the major ultimate factors is food availability. For example, brent geese Branta bernicla migrate thousands of kilometres to the High Arctic to breed there in mid-summer, profi ting from the high quality of the growing vegetation. Eleonora’s falcons Falco eleonorae breed exclusively in the fall on Mediterranean islands, to match the nestling phase with the mass migration of passerines to their African wintering grounds. Honey buzzards Pernis apivorus have their chicks in the nest in mid-summer, when bee nests reach their maximal size. Great tits Parus major breed in the early spring in temperate forests, where they profi t from the bonanza formed by herbivorous caterpillars that forage on young oak leaves. It is not just temperate and arctic species that exhibit well-defi ned breeding seasons, many tropical birds also breed at certain periods in the year, for example when food is most abundant due to seasonality in rainfall ( Hau et al. , 2008 ). The restricted temporal abundance of food supplies within years shapes avian breeding cycles to a large extent, and understanding the effects of climate change on avian calendars therefore requires good knowledge of how the timing and abundance of food supplies changes along with climatic change. Whereas birds are normally well adapted to breed on average at the time with most abundant food, the exact timing of these food peaks could vary considerably between years. This betweenyear variation is to a large extent determined by local weather conditions. For instance, in warm springs the caterpillar peak in Wytham Wood (Oxfordshire, UK) could occur as early as midMay, whereas in exceptionally cold springs it is a month later ( Charmantier et al. , 2008 ). The Wytham great tits track this between-year variation in their food supply very well: in the warmest springs they lay about 25 days earlier than in the coldest springs ( Perrins, 1965 ; Charmantier et al. , 2008 ). Later we will see that not all species are able to track annual variation in the timing of their food supply so well. For a bird it is not an easy task to match the timing of its greatest food requirements with the peak in food availability. Nestlings need most food in the second half of the nestling phase. To reach this stage, parents should already have built the nest, and laid and incubated the eggs. After the start of egg-laying, birds have little scope for speeding up the hatching date of their eggs, except from reducing clutch size. Thus, parents predict when the food peak will occur at the time of egg-laying. For a female great tit that lays nine eggs, incubates 13 days and whose chicks reach maximal food needs at about 10 days after hatching, this means that she should start laying 32 days before the expected food peak. This necessitates considerable predictive power. If temperatures are higher than normal after the female started, caterpillars develop faster, pupation dates advance, and, hence, the caterpillar peak is earlier, her chicks will hatch too late to CHAPTER 11