Wide Disparity in Age of Nestlings in a Say's Phoebe Nest from New Mexico

Abstract

Say's phoebes (Sayornis saya) are common throughout open country of western North America (Schukman and Wolf, 1998), building nests of dry grasses, forbs, and animal hair on ledges in natural (cliffs) and man-made (buildings, bridges) sites (Ohlendorf, 1976; Harrison, 1979; Baicich and Harrison, 1997). Abundance of this species probably has increased since European settlement because man-made nest sites now occur in many areas where no natural nest sites exist (Ohlendorf, 1976; Schukman, 1993). Say's phoebes often attempt to raise two broods, beginning in April (Harrison, 1979; Baicich and Harrison, 1997; Schukman and Wolf, 1998). In Kansas incubation averaged 15 days and fledging occurred at 16-17 days (Schukman and Wolf, 1998). At 1430 h on 12 July 1996 I entered a small building at Stone Lake, Rio Arriba Co., New Mexico that is regularly used by nesting Say's phoebes. A nest was found that contained two large young, estimated to be within a few days of fledging. Body feather growth appeared complete, and flight feathers were near full length but still partially sheathed. Between these two large young was a naked nestling (likely <24 h old) and an unhatched egg. Neither the egg nor the small nestling were those of a brown-headed cowbird (Molothrus ater; Sogge et al., 1997); the egg's size and coloration were typical of a Say's phoebe (Harrison, 1979; Baicich and Harrison, 1997). I banded both large nestlings. I returned to the nest on 15 July at 1450 h. One large nestling had fledged and the other was out of the nest but on the same ledge. It was nearly capable of sustained flight but was easily recaptured. A nestling appearing no larger than the smaller one seen on 12 July was in the nest; it had light white down on its back, but no other feather growth and was assumed to have hatched from the egg observed three days before. I photographed the large and small nestlings (on file with New Mexico Game and Fish Department, Santa Fe) and returned them to their respective locations. On a final visit on 20 July at 0900 h, I found only the remaining large fledgling. Breeding biology of the Say's phoebe has not been as intensively studied as that of its congener, the eastern phoebe (S. phoebe). Assuming that Say's phoebe nestlings grow at the ame rate as eastern phoebes (Weeks, 1994), the two larger young found on 12 July were at least 10 and probably 12-14 days old. One had fledged 3 days later, though the other remained in the building another 8 days. The two smaller young were not seen together, but the one seen 15 July had no primary dots (23 days in eastern phoebe; Weeks, 1994) or visible size increase, suggesting the two eggs hatched asynchronously but that neither young survived more than a day or two. Once egg-laying is initiated, eastern phoebes generally lay eggs daily until the clutch is complete (Murphy, 1994); a similar laying pattern may occur in Say's phoebe. Therefore it is unlikely that a female Say's phoebe would have a gap of 10-12 days between laying dates for her second and third eggs. Further, incubation of second clutches in eastern phoebes is usually initiated with the penultimate egg (Weeks, 1994). Although all eggs in many eastern phoebe first clutches hatch within a 24-h period, hatching in many second clutches occurs over a 2-3 day period (Murphy, 1994; Weeks, 1994). Hatching asynchrony of less than 2 days, with the greatest asynchrony in second clutches, also is typical of the Say's phoebe (J. M. Schukman, pers. comm.) but asynchrony of more than 10 days is unusual. Two explanations of this mixed-age clutch seem feasible. Eastern and black (S. nigricans) phoebes occasionally are polygynous; in the former species five marked males concurrently fed young at two nests (Weeks, 1994) and in the latter species one verified and one suspected case of polygyny were documented (Wolf, 1997). Though common use of a nest by two female phoebes has not been docu-