Effect of Growth and Hatching Asynchrony on the Fledging Age of Black and Red Kites

Abstract

-We studied the effect of growth rate, final size, hatching sequence, hatching asynchrony, and fledging asynchrony on the fledging age of Black and Red kites (Milvus migrans and M. milvus). Feather growth explained 43% and 38% of the variance in fledging age of Black and Red kites, respectively. Fledging age increased with hatching sequence and increased fledging asynchrony in relation to initial hatching asynchrony of both species, especially in broods of three chicks. Once the effect of growth was removed, no differences in fledging age related to hatching sequence were found in the Red Kite, but there was still a delay in fledging of third-hatched Black Kite chicks. Synchronous fledging of firstand second-hatched Black Kite chicks seemed to occur when both chicks had grown at a similar rate, and was caused by a delay in the fledging of the first-hatched chick. First-hatched Black Kite chicks did not delay fledging if second-hatched chicks experienced reduced growth. Differences between the two species in the third-hatched-chick fledging delay could be due to differences in nest provisioning by adults during the postfledging dependence period. Red Kites reduced provisioning to the nest as soon as the first chick fledged, while Black Kites maintained nest provisioning longer. No evidence was found supporting the idea that parents may reduce feedings to hasten first flight of their offspring. Received 20 May 1991, accepted 10 February 1992. THE LENGTH of the nestling period in birds is thought to be correlated with growth rate (Ricklefs 1968, Skutch 1976, Zach 1982b, Poole 1989). Thus, it has been used in comparative studies of growth (e.g. Bortolotti 1986a) because growth rates are more difficult to obtain. However, species with similar rates of development may have nestling periods of different length, suggesting that selective factors affecting growth rate are not the only ones acting on fledging age (Maher 1964, Skutch 1976, Zach 1982a, Freed 1988). Some studies have explored the effect of growth rates on the length of the nestling period and the extent to which growth and fledging age are related (Zach 1982a, b, Bortolotti 1984, 1986b). Hatching asynchrony within a clutch is a widespread trait in birds that may result in the development of feeding hierarchies promoting differences in growth rates (Bryant 1978, O'Connor 1984) that can influence fledging age (Clark and Wilson 1981). There also may be a parent-offspring conflict over fledging age (Trivers 1974, 1985), the chicks trying to extend the nestling period and the parents trying to force the first flight of their chicks by a reduction in feeding rates, or by means of special behavioral mechanisms (Rowan 1955, Brown and Amadon 1968, Walker 1972, Skutch 1976, O'Connor 1984, Poole 1989). A prolonged nestling period would give extra parental care (food and defense against predators) to the chicks, increasing their survival expectancy, but also could increase the rearing costs beyond the optimum for parents. Some studies on fledging of species in the Passeriformes have shown a trend for synchronizing the date of first flight among siblings, but it is not clear if fledging synchrony is obtained by a delay in first flight of the oldest chicks (Freed 1988), by an earlier fledging of the youngest (Gibb 1950, Skutch 1976, Zach 1982a), or by both phenomena (Lemel 1989). Raptors follow the general trend of interspecific increase of fledging age with body size (Newton 1979, Bortolotti 1986a). Also, the smaller males fledge at a younger age than the females in size-dimorphic species (Scharf and Balfour 1971, Sherrod 1983, Wyllie 1985, Bortolotti 1986b, Poole and Bromley 1988, Delannoy and Cruz 1988), and this has been related to differences in growth rates and final size between sexes (Newton 1979, Bortolotti 1984). Hatching sequence in the asynchronous hatching Black Kite (Milvus migrans) has been said to