Yearling-biased female mortality in tree swallows

Tree Swallows (Tachycineta bicolor) are among the first species of Hirundinidae to arrive on breeding grounds in the spring (Tyler, in Bent 1942) possibly enabling them to begin nest site selection before most of their interspecific nest-site competitors (Hersey 1933; Kuerzi 1941; Tyler, in Bent 1942; Chapman 1955; Erskine 1964; Zeleny 1976). However, this early arrival has potential costs because Tree Swallows must often encounter cold, unseasonable weather that lowers aerial insect abundance (Taylor 1963) and places birds under food stress. Although Tree Swallows can feed on plant foods (e.g., bayberry fruit, Myrica pennsylvanicus, Eliott 1939; Tyler, in Bent 1942; pers. observ.) during periods of low insect abundance, mortality after a period of adverse weather is not uncommon (Dence 1946, Whitmore et al. 1977, Weatherhead et al. 1985). In this note I report significant yearling-biased female mortality in Tree Swallows early in the breeding season. From 1980 to 1983, I studied Tree Swallow social behavior at a breeding area on the salt marshes of the J. F. Kennedy Memorial Wildlife Refuge located at Tobay Beach on the south shore of Long Island, New York (see Schaeffer 1972 for a description of the site). By 1983, 72 nest boxes were available for breeding; 40 with entrance holes 3.8 cm (1.5 in) in diameter and 32 with 3.2 cm (1.25 in) holes. Boxes were the same in all other dimensions including the distance from the floor to the entrance hole (10.8 cm). During April and May of each year, I found dead Tree Swallows in boxes often before nest building had begun at the study site. All dead birds (n = 20) were found alone (but see Weatherhead et al. 1985). Their tail feathers were covered with white, opaque excrement (see Christy 1940, Weber 1940) indicating that they may have been either (i) alive and unable to exit the box for a prolonged period, or (ii) on the bottom of a communal roost, crushed, and defecated on (see Weatherhead et al. 1985). Four of the dead birds were known to be males according to banding records (n = 2) or the presence of a cloacal protuberance (n = 2). Based on banding records, one male was yearling and the other an After Second Year (ASY) individual. Of the other 16, 14 were in brown plumage with little iridescence, indicating that they were yearling females (Cohen 1980, Hussell 1983). The two remaining birds were in full iridescent blue-green plumage and were found to be ASY females by necropsy. Mortality of known sex birds (n = 20) was significantly biased in favor of females (x2 = 7.20, df = 1, P 0.50). Seventy-five percent of all dead birds were known to be yearlings (females, n = 14; males, n = 1). There are many factors that could contribute to yearling mortality including inexperience in (i) using plant food as a supplemental food during periods of low insect abundance, (ii) finding and capturing insects (DeSteven 1978, R. R. Cohen, pers. comm.), and (iii) finding safe refuge during periods of adverse weather (R. R. Cohen, pers. comm.). Whitmore et al. (1977) found that Tree Swallows dying during unseasonably cold weather in Utah were physiologically less able to withstand the metabolic stress of cold weather than those that survived based on measurements of dry weight, % dry lipid weight, and nonfat dry weight. Greater susceptibility to unseasonable weather may help explain why yearling females (Kuerzi 1941; Tyler, in Bent 1942; Chapman 1955; Cohen 1978; Leffelaar and Robertson 1984), and yearling males (R. R. Cohen, pers. comm.) usually return to breeding areas up to one month later than older birds.