Christiaan Both

The ultimate reason why birds should advance their phenology in response to climate change is to match the shifting phenology of underlying levels of the food chain. In a seasonal environment, the timing of food abundance is one of the crucial factors to which birds should adapt their timing of reproduction. They can do this by shifting egg‐laying date (LD), and also by changing other life‐history characters that affect the period between laying of the eggs and hatching of the chicks. In a long‐term study of the migratory Pied Flycatcher, we show that the peak of abundance of nestling food (caterpillars) has advanced during the last two decades, and that the birds advanced their LD. LD strongly correlates with the timing of the caterpillar peak, but in years with an early food peak the birds laid their eggs late relative to this food peak. In such years, the birds advance their hatching date by incubating earlier in the clutch and reducing the interval between laying the last egg to hatching of the first egg, thereby partly compensating for their relative late LD. Paradoxically, they also laid larger clutches in the years with an early food peak, and thereby took more time to lay (i.e. one egg per day). Clutch size therefore declined more strongly with LD in years with an early food peak. This stronger response is adaptive because the fitness of an egg declined more strongly with date in early than in late years. Clearly, avian life‐history traits are correlated and Pied Flycatchers apparently optimize over the whole complex of the traits including LD, clutch size and the onset of incubation. Climate change will lead to changing selection pressures on this complex of traits and presumably the way they are correlated.

Advances in the phenology of organisms are often attributed to climate change, but alternatively, may reflect a publication bias towards advances and may be caused by environmental factors unrelated to climate change. Both factors are investigated using the breeding dates of 25 long–term studied populations of Ficedula flycatchers across Europe. Trends in spring temperature varied markedly between study sites, and across populations the advancement of laying date was stronger in areas where the spring temperatures increased more, giving support to the theory that climate change causally affects breeding date advancement.

Summary: 1.Descriptive studies have shown that the annual mean fecundity and survival in bird populations decline as density increases. Experimental studies in which breeding density has been manipulated show that density causally affects reproduction in some but not other species.2.In a 3‐year study on great titsParus majorwe manipulated density by removing about one‐third of territorial great tit pairs from half the study area each year. The removal resulted in an increase in territory size of the remaining birds in the experimental sub‐area.3.Fecundity and survival did not differ between the experimental low‐density and the control sub‐areas. The experiment thus did not show a causal relationship between breeding density and fecundity and survival at the level of the population.4.The data were analysed further with respect to individual territory size, the level at which competition is assumed to operate. Pairs in the experimental sub‐area enlarged their territories. We argue that variation in territory enlargement is not caused by individual quality.5.Both pre‐manipulation territory size and territory enlargement positively affected the probability that a territorial pair nested, the growth rate of their chicks, the number of fledglings that recruited into the breeding population and the survival of the territorial adults. No effect of territory enlargement was observed on clutch size, nor on the probability that pairs started a second brood.6.The positive effect of the experimental territory enlargement argues for a causal effect of territory size on these reproductive parameters, independent of individual quality.7.An effect of the density reduction on fecundity may not be detectable at the population level, either due to the large variation in territory sizes within the experimental treatments or to the negative effect of the experimentper se, which counter balances the positive effect of territory enlargement.

The presence of density dependence of clutch size is tested in 57 long‐term population studies of 10 passerine bird species. In about half of the studies of tit species Parus spp. density dependence of clutch size was found, while none was found in studies of two flycatcher species Ficedula spp. One hypothesis explaining this difference is that migrants are less able to predict the final competitor density, because new pairs are still settling when the first females start laying eggs. Such unpredictability is only a problem for early laying females. If this explanation is true, the commonly observed negative correlation between clutch size and laying date should be stronger in high‐density years. I tested this prediction in three populations of Pied Flycatcher Ficedula hypoleuca , and compared the results with three populations of Great Tit Parus major . In none of the six populations was there a significant correlation between the strength of the seasonal decline in clutch size and population density. Thus the lack of density dependence of clutch size in Pied Flycatchers was not consistent with the idea that this is caused by the unpredictability of final density at the time of egg‐laying of the earliest females in the population. Furthermore, density does not have any adverse effect on reproductive output of Pied Flycatchers, and therefore they do not adjust clutch size to density.