THE BREEDING BIOLOGY OF THE SWIFT APUS APUS

SUMMARY1. The Swift Apus apus was studied round Oxford 1946‐50, for the last three years in nesting boxes.2. The adult weight greatly varies with the weather, and is heavier in May than later in the summer.3. The clutch‐size in England is two, sometimes three, the proportion of clutches with three eggs differing markedly in different years. Average clutch‐size is higher in Switzerland than in either England or Fenno‐Scandia, and is lower late in the season.4. The average weight of the eggs in May differed in different years. The second egg of the clutch is usually heavier than the first.5. The apparent incubation period is several days longer if there is bad weather at the start.6. Nestling mortality is higher in large than small broods. It is also higher in bad than in good weather, the harmful influence of bad weather being much greater in large than in small broods. In three bad summers the broods of two produced more young per brood than did the broods of three, but in two good summers the broods of three produced a larger average per brood.7. The nestling period is remarkably variable (35–56 days), being longer in bad weather.8. The young increase to a peak weight well above that of the adult and then gradually get lighter. This general trend is interrupted by marked losses on days of bad weather when the parents bring little food, and the weight averages for different periods vary markedly in relation to the weather.9. The wing‐feathers grow more slowly in undernourished than well nourished nestlings. Retardation at one period does not prevent rapid growth later.10. The daily increase in nestling weight is higher with sunshine and warm temperatures and lower with rain and wind.11. At Oxford individual meals were of similar size in good and in poor weather, but the feeding frequency was higher in good than in poor weather.12. The feeding frequency is higher in broods of larger size, but it does not increase proportionately, so that individual nestlings receive fewer feeds in large than in small broods. The increase in feeding frequency with better weather is least in broods of one and greatest in broods of three.13. In Switzerland, where the weather is usually finer than in England, the average clutch‐size is higher than in England, nestling survival from broods of three is higher, the nestlings are heavier, and the meals are probably heavier, but other measurements are similar in the two countries.14. Swifts differ from small passerine birds and resemble Procellarii in having a smaller clutch‐size, longer incubation and nestling periods, a peak nestling weight well above, instead of similar to, that of the adult, a widely variable nestling period, a marked ability to withstand starvation and a capacity to retard feather development when undernourished. All these points can be related to the fact that Swifts and Petrels have safe nesting sites but an uncertain food supply, whereas small passerines are raised when food is dependable but predation is high.