INCUBATION BEHAVIOR AND HATCHING PATTERNS IN THE AMERICAN KESTREL FALCO-SPARVERIUS

Asynchronous hatching within clutches is a pervasive phenomenon in altricial birds and the subject of considerable controversy. Intraspecific variation in the degree of hatching asynchrony is usually assumed to be the product of individual differences in parental incubation behaviour, yet few studies have reported on how variable such behaviour might be or the patterns of hatching within clutches. Wild American Kestrels in northern Saskatchewan started incubating their eggs prior to clutch completion, but the timing of the onset of incubation varied among pairs. There was considerable variation in the number of eggs to hatch per day within clutches. Many patterns of hatching could not be explained by conventional thinking about parental incubation behaviour, for the order in which eggs hatched was often not the order in which the eggs were laid. Small females were more likely to have such irregular hatching patterns and have hatching failures than large females, possibly because of problems in effectively covering all eggs during incubation.