Nestling growth and fledging success in manipulated American kestrel broods

To study factors regulating clutch size in American Kestrels (Falco sparverius), brood manipulation experiments were performed on captive and wild birds in southwestern Quebec during 1986 and 1987. The largest normally occurring brood size was 5 young. Manipulations enlarged or decreased broods to 7 or 2 young, respectively. Significantly more young fledged from wild control and enlarged broods in 1987 than from comparable groups in 1986. The average number of young fledging from enlarged wild broods in 1987 was slightly higher than for control broods, but fledging weight was significantly depressed in enlarged broods. Growth rates and tarsal and antebrachial length at fledging were not affected by brood size, but development of primary feathers was slower in enlarged wild broods. Parental ability to adequately feed all young appears to be the major factor limiting brood size in American Kestrels.