Growth of the Swainson's Hawk

Abstract

individuals of .many bird species, and for North American species of Buteo he could discuss only the Red-tailed Hawk (B. jamaicensis). Ricklefs' (1968, 1973) emphasis on the importance of growth rates in avian reproductive biology illustrates the need for continued fieldwork on bird growth and prompts me to report the growth of a brood of Swainson's Hawks (B. swainsoni). No published data describe the growth of wild-raised Swainson's Hawks, although Olendorff (1971) studied captive juveniles and tabulated all references of growth studies of raptors then available. In 1970 I studied the nesting of a pair of Swainson's Hawks 6.4 km east of Arnett, Ellis County, Oklahoma. The nest was a mass of sticks typical for this species, about six dm in diameter and four m up in a 6-m osage orange tree (Maclura pomifera) on the south side of a shelterbelt woodlot. A colony (six nesting attempts) of Mississippi Kites (Ictinia misisippiensis) used the same group of trees. When found, the hawk nest held two nestlings and one egg that did not hatch and eventually disappeared. The ages of the two nestlings were estimated at three and five days on the basis of my experience and comparison with Olendorff's (1971: 234, 243) data for weight and tarsal length. Weight and several body dimensions of the nestlings were measured at irregular intervals (table 1). Tarsal length was measured as the distance along the posterior surface of the tarsus from the base of the first digit to the posterior side of the distal head of the tibiotarsus at the heel; the tarsometatarsus was held perpendicular to the tibiotarsus. I measured the culmen as the chord from the anterior dorsal edge of the cere to the tip. The third-toe claw measurement was the chord from the tip of the claw to the proximal-most ventral surface of the claw at the toe pad. The greatest weights of the juveniles were estimates because the tares for a triple-beam balance were not available, and a spring scale graduated in English units was used to weigh the nestlings. Feather length was the distance from the point where the shaft exited the skin to the tip of the rachis minus natal down. The seventh primary remex and the longer of the two central rectrices were used. The younger nestling was recently dead in the nest when it would have been 32 days old. Its condition did not indicate malnutrition; rather, it died of injuries suggesting either intentional or unintentional