Chilean Swallows (tachycineta Meyeni) Adjust the Number of Feathers Added to the Nest with Time of Breeding

—Tachycineta swallows nest in secondary cavities and build nests made of a mat of dry grasses with a nest cup lined with feathers. The insulative quality of feathers may prevent hypothermia of the chicks and increase chick growth, but also may raise the risk of nestling hyperthermia if ambient temperature is high. The number of feathers added to the nest should vary throughout the breeding season according to ambient temperature. We describe nest structure and timing of nest building of Chilean Swallows (Tachycineta meyeni) nesting in Tierra del Fuego, Argentina. We analyzed the association between number of feathers in the nest and (1) daily ambient temperature during the period swallows add feathers to the nest, and (2) hatching success of eggs and survival and growth of the chicks. There was a negative association between number of feathers added to the nest and average daily ambient temperature during the nesting cycle. Hatching success was not associated with number of feathers at start of laying or at the end of incubation. There was no association between number of feathers and chick survival or between number of feathers and average weight of the chicks when they were 12 and 15 days of age. Chilean Swallows make temporal adjustments to the number of feathers added to the nest. We suggest these adjustments may help maintain reproductive success throughout the breeding season. Received 28 January 2009. Accepted 3 May 2009. An important characteristic of nests, which generally has an effect on reproductive success, is amount of insulating material placed within them. Many bird species use feathers, hair or other materials with good insulating properties as nest lining material. Differences in type and amount of insulation may affect timing of breeding (O’Connor 1978), incubation rhythms (White and Kinney 1974, Møller 1991), incubation energetics (Skowron and Kern 1980), incubation time (Lombardo et al. 1995), nestling growth (Møller 1991, Winkler 1993, Lombardo et al. 1995), brood thermoenergetics (Mertens 1977), and risk of nestling hyperthermia (Lombardo 1994). Tachycineta swallows nest in secondary cavities and build nests of a mat of dry grasses with a nest cup lined with feathers (Dyrcz 1984, Brown et al. 1992, Winkler 1993, Allen 1996, Bulit and Massoni 2004, Townsend et al. 2008). The effect of nest feathers on reproductive success has been experimentally examined in Tree Swallows (T. bicolor), where removal of feathers reduces chick growth, possibly as a result of chicks’ hypothermia before they become fully homoeothermic (Winkler 1993, Lombardo et al. 1995). Similarly, Dawson et al. (2005) experimentally demonstrated the importance of ambient temperature during this period. These authors showed that raising the nest temperature by 5u C when nestlings of Tree Swallows are 4 days of age enhances nestling survival, and results in heavier nestlings with longer primary feathers at 16 days of age compared to nestlings in control nests. The benefit of adding feathers to the nest may vary temporally throughout the breeding season. Well-insulated nests of Tree Swallows may be advantageous early in the season, when ambient temperatures are low and eggs and chicks are vulnerable to hypothermia, but may become disadvantageous late in the season, when higher ambient temperatures increase risk of nestling hyperthermia (Lombardo 1994). Within Tachycineta swallows, the Chilean Swallow (T. meyeni) has the southernmost breeding range. This species experiences low ambient temperatures during the temperate breeding season and the effect of nest feathers on reproductive success should be important. We describe the nest structure and timing of nest building of Chilean Swallows nesting in nest boxes in Tierra del Fuego, Argentina, the southern limit of the species’ range. To date, the reproductive biology of this species has not been studied. Our objectives were to: (1) analyze the association between number of feathers added to the nest and ambient temperature throughout the breeding season, and (2) analyze if the number of feathers The Wilson Journal of Ornithology wils-121-04-15.3d 21/9/09 17:26:51 783 Cust # 09-018 1 Centro Austral de Investigaciones Cientı́ficas, CADICCONICET, Bernardo Houssay 200, V9410BFD Ushuaia, Tierra del Fuego, Argentina. 2 Departamento de Ecologı́a, Genética y Evolución, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabellón II, Ciudad Universitaria, C1428EGA, Buenos Aires, Argentina. 3 Corresponding author; e-mail: [email protected] The Wilson Journal of Ornithology 121(4):783–788, 2009