Partial Incubation and Hatching Asynchrony in the Red-Shouldered Hawk (buteo Lineatus)1

Like many raptors, Red-shouldered Hawks (Buteo lineatus) exhibit asynchronous hatching, which is thought to be a parental strategy for rearing the maximum number of offspring under conditions of unpredictable prey availability. Current knowledge of incubation patterns and the associated behavioral mechanisms by which as Itronous hatching occurs in raptors is limited, and few detailed quantitative studies are available for ally species of Accipiniformes. We investigated the effects of egg-laying intervals and parental behavioral incubation patterns during egg-laying on hatching asynchrony in the Red-shouldered I latck. We used 24-hr/d digital color/infrared video cameras at 15 Red-shouldered Hawk nests to record egglaying and hatching, and to quantify the proportion of time parents spent incubating, standing, or absent from the nest. The mean egg-laying interval was 2.9 +/- 0.1 d or 69.8 +/- 2.2 hr (range =65.8-74.2 hr), and the mean duration of incubation (from the onset of full incubation to hatch) for last-laid eggs was 33 d (32-34 d, n= 15). Red-shouldered Hawks exhibited partial incubation behavior, with the delay in the onset of full incubation varying by clutch size and among individuals; full incubation generally began with or just after the laying of the penultimate egg of the clutch. The mean interval between the hatching of the first and second egg was 0.56 +/- 0.49 d; between the second and third eggs, 1.24 +/- 0.71 d (P= 0.0215); and between the third and fourth eggs, 2.42 +/- 0.38 d (P < 0.0004). The total amount of time parents at each nest spent incubating during the egg-laying period explained most of the variation in the amount of time between the hatching of their first and last egg (the hatching span; r = 0.83, P < 0.0001). Hatching patterns were the result of both the egg-laying intervals and the pattern of partial incubation, which may represent an adaptive mechanism to reduce the hatching interval while maintaining the hatching asynchrony.