Food, foraging, and timing of breeding of the Black Swift in California

-The nestling diet and breeding seasonality of the Black Swift (Cypseloides niger) were studied in southem California 1990-1992. The peak (40%) of egg laying was in mid-June and the peak of fledging (60%) was midto late August (n = 87 nests). Winged ants comprised 91% (n = 1179 prey items, 10 boluses) of nestling diet. Three main prey size classes were found: 6, 8, and 13 mm. Food bolus mass increased and number of trips per day to feed the nestlings decreased with nestling age. The parents made short and long foraging trips during early morning hours and long trips from early to late afternoon. Short trips were observed only during the first half of the nestling period. During the last half of the nestling period, parent swifts made a single foraging trip per day that lasted about 12 hrs. Perhaps the short foraging bouts are for feeding the young, whereas the long foraging bouts are not only for feeding the young but also for parental energy storage. The single foraging bout, during the midand late nestling period, might also serve to store fat for migration by the adults. Received 13 Feb. 1998; accepted 24 Oct. 1998. The Black Swift (Cypseloides niger) is a member of the subfamily Cypseloidinae which consists of 12-13 species, most of which are tropical or subtropical in their breeding distribution. The Black Swift is found locally in the West Indies, Middle America, and north through much of western North America to southeastern Alaska (Bent 1940; AOU 1957, 1983). For a species with such a wide latitudinal distribution, quantitative data on diet and timing of breeding are rare. Most of what is known about the Black Swift is limited to breeding and distributional records (e.g., Vrooman 1901, 1905; Michael 1927; Dixon 1935; Knorr 1961; Foerster 1987; Foerster and Collins 1990; Stiles and Negret 1994). This reflects the difficulty of studying this species because of its aerial life style and its usually inaccessible nest sites. Here I present new information on diet and timing of breeding of this species. The Black Swift is a sumnner breeding visitor to western North America, and like many migratory species there, has a restricted breeding season. Furthermore, it has a proportionately large egg, a single-egg clutch, is single brooded, and has a long incubation and nestling period (Manrn 1997). These factors should constrain variation in the timing of breeding of the Black Swift. Swifts catch airborne insects and ballooning spiders (Lack and Owen 1955, Whitacre 1991). During the reproductive season, breeding cypseloidine swifts accumulate insects and arachnids in the back of the throat continuing into the esophagus and bind them with saliva to produce a sticky assortment of insects. This insect conglomerate or food bolus is produced exclusively to feed the nestling(s) and have never been reported outside the breeding season. Alive or dead these insects are complete, making them ideal for identifying and quantifying diets. Hespenheide (1975) pointed out that one difficulty in analyzing bird diets is knowing which is more important: the number or the size of food items. Some diet studies of New World swifts have addressed both parameters [e.g., Whitacre (1991)]; however, most have emphasized only one or they had small sample sizes (e.g., Collins and Landy 1968, Hespenheide 1975, Foerster 1987, Bull and Beckwith 1993, Marnn and Stiles 1993). Other authors have considered only the number of prey items (e.g., Beebe 1949; Rathbun 1925; Rowley and Orr, 1962, 1965; Manrn and Stiles 1992). One problem in quantifying swift diets is the source of prey samples, either stomach contents or food boluses. Stomach contents of adults are prey items that the adult bird has fed upon, whereas food boluses are prey fed to nestlings. Thus, examination of stomach contents versus food boluses might produce different results. Because my focus ' Dept. of Biological Sciences and Museum of Natural Science, 119 Foster Hall, Louisiana State Univ., Baton Rouge, LA 70803; E-mail: Zomari@LSU.edu 2 Present address: Western Foundation of Vertebrate Zoology, 439 Calle San Pablo, Carmarillo, CA 93012; E-mail: MMA@wfvz.org