Population fluctuations and survival of Great Tits Par us major dependent on food supplied by man in winter

• Published in: Ibis
• Main Author: ORELL, MARKKU
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1989
• Subjects: Animal Science and Zoology; Ecology, Evolution, Behavior and Systematics; Taskila; Northern Finland
• Online Access: http://dx.doi.org/10.1111/j.1474-919x.1989.tb02750.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1474-919X.1989.tb02750.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1474-919X.1989.tb02750.x

The 16‐year data of a Great Tit population, breeding and overwintering in a nest box area at Oulu, northern Finland, were studied to reveal the factors governing the annual fluctuations in density and local survival. The breeding density at Taskila was high after a cold spring (March‐April) and a warm August the year before was also followed by a large breeding population. A similar relationship resulted for the local survival of fledglings, and there was a tendency for immigrant yearlings to settle to breed in large numbers when the spring was cold. In the north, Great Tits cannot survive the winter in the forests but gather around feeding tables near human settlements. A cold spring evidently suppresses the birds' dispersal before the breeding season. For a population which is time‐limited, it is adaptive to remain in order to breed nearby instead of searching for territories out in the forests when conditions become favourable late in the spring. Thus, the negative correlation between spring temperature and the size of the following breeding population may be the result of a spatial distribution of juveniles achieved by behavioural responses to unfavourable conditions. The causality behind the high survival of resident juveniles after a warm August may be attributable to food availability during the time when the birds undergo their postjuvenile moult, which is an energy‐demanding process. The survival rate of breeding adults was highly dependent on the intensity of nightly predation upon individuals roosting in nest boxes. Stoats Mustela erminea , the prime predators, preyed on tits when the populations of their main prey species, small rodents, were low. When the effect of predation was eliminated, the density‐dependent survival of adult birds disappeared. During low predation years the adult survival rate was negatively related to the number of fledglings produced per pair; this suggests that investment in descendants is a substantial survival cost for reproducing individuals.