Seasonal changes in the survival of the black-capped chickadee

A population of Black-capped Chickadees (Parus atricapillus) living in a favourable environment was colour-banded, and its survival was followed, to find out what prevents continual increase in its numbers. A weekly census of the population was made throughout the two years of the study. All nests were found and the young were banded before they flew. Every two weeks throughout both winters checks were also made on an unhanded population in a control area one and a quarter miles from the main population. Nesting success was high in both years, with 5.0 young per pair being fledged in 1964 and 4.5 young per pair in 1965. Juvenile survival until family break-up was almost 100% in both years; juvenile survival until September seemed to be high. The survival rate of the adults was uneven: there were two periods when it was lower than it was during the rest of the year. The less sharply defined period occurred during the post-breeding moult of the adults. The more sharply defined period of two weeks or less was exactly correlated in both years with a change in behaviour from flocking to territorial behaviour. With the exception of one unmated female in each year, every bird which survived this critical period remained to breed or attempt to breed; hence this change in behaviour in the spring evidently removed surplus birds from the area, and thus prevented continual increase in the population. This behaviour may be the common factor that limits the breeding populations of other species with similar ecology.Science, Faculty ofZoology, Department ofGraduat