Foraging and its consequences in the breeding season of the Blue Tit (Parus caeruleus)

Abstract

This dissertation deals with the direct and indirect effects of food availability in different phases of breeding in a small insectivorous bird, the Blue Tit (Parus caeruleus). Previous studies have emphasised the dual nature of food influencing reproductive decisions in birds. On the one hand, food constitutes energy and nutritional resource for the individual. This thesis has focused on the effects of food as a resource in two highly demanding phases: (1) the period of egg laying and (2) the period of brood rearing. On the other hand, food in the laying phase could also function as a cue predicting the best time for rearing the brood. This hypothesis was tested by means of a series of additional feeding experiments in which extra food was offered to the parents throughout the nestling period. Female Blue Tits experiencing additional food during the nestling period laid relatively later the next year than unfed females, controlled for between-year changes in the environment. As a result, those females mis-timed reproduction and raised the brood far from the caterpillar peak the next year. This suggests that food levels experienced during breeding are involved in fine-tuning the timing of breeding the next year. The additional feeding experiments provided the opportunity to investigate the provisioning rules of parents that experience different degrees of food availability. The parents that had access to extra food delivered similar amount of food as control, unfed parents, but with a different combination of feeding frequency and size of prey. They fed the chicks less frequently, but with larger prey items. This suggests that the change in the state of the parents (which spent less time self-feeding because of the food addition) and of the nestlings (because the parents delivered some extra food to them) produced a significant change in the parents' provisioning rules. The availability of more time caused the food-supplemented parents to get access to larger prey, presumably through an increase in selectivity. This is because larger prey could be obtained only by making longer foraging excursions