Food, space and time constraints on reproduction in the common treecreeper (Certhia familiaris L.)

Abstract

The effects of food supply and day length on the reproductive success of the common treecreeper were studied comparatively and experimentally, with particular attention to seasonal changes in these factors. Data are presented and analysed from a long term study of treecreepers breeding in special nest boxes mostly in Southern Finland (1973 - 1988) and Central Finland (1983 - 1988). The seasonal reproductive pattern was based on an initial increase in clutch size with time, eventually followed by a decrease. A similar seasonal trend was also found in the number of hatchlings and fledglings, but the recruits were observed to survive better the earlier they had hatched. The potential food supply was observed to increase during the spring, as did the clutch size. The diet of the nestlings was as variable as the food supply, with both of them including 72 % spiders by biomass. The treecreepers seemed to favour relatively large food items, however. The home range averaged 3.3 hectares in Central Finland. The minimum forest size accepted by a breeding treecreeper pair agreed well with this figure and increased markedly northwards. The treecreeper favoured large, uniform forest stands in southern Finland. The feeding rate and time budget of the adult birds during the breeding season were also studied, and their response to the seasonal change in day length was assessed. They were observed to change their behaviour in parallel with day length, foraging and feeding nestlings more the longer the day was. They did not seem to be able to increase their hourly feeding rate when the brood size was higher, but they could increase the feeding rate per day, which agrees well with the observations on seasonal variation in clutch size and brood size. Experiments were conducted by preventing feeding of the nestlings for three hours each day over a period of six days. These nestlings developed significantly more slowly than those in control nests. The time budget analysis proved that the adult birds used most of their time during the breeding season for foraging and feeding the nestlings. The environmental circumstances under which treecreepers live are relatively predictable (food supply, day length, mature spruce forest), and this may allow the population to develop in a closer response to the environmental conditions and resources than most foliage-gleaning passerine birds studied. The results support the idea that day length is also of some importance in determining the feeding capacity of treecreepers. Nevertheless the major factor govering reproduction seems to be the food supply.