Some Observations of Sunbathing in Swallows

--Tree Swallows (Tachycineta bicolor), Violet-green Swallows (T. thalassina) and Barn Swallows (ttirundo rustica) were observed sunbathing under conditions that produced apparent hyperthermia as evidenced by gaping, posture and timing of exposure. Sunbathing was observed only on days having air temperatures above 30 C, medium to high humidities and low wind velocities. Duration of sunning behavior was short, never exceeding 2 min. The substrate of sunning sites under these conditions always exceeded 47 C. It appeared that swallows were sunbathing in a fashion that led to short-term heat stress. This implies that the function of this form of sunbathing in these swallows is for a purpose other than thermoregulation. ALGUNAS OBSERVACIONES SOBRE BAi•OS DE SOL EN GOLONDRINAS Sinopsis.--Se observaron tomando barios de sol a individuos de Tachycineta bicolor, T. thalassina y Hirundo rustica, en condiciones que aparentemente produjeron hipertermia, basado en conducta de jadeo, posturn de Ins aves y perlodo de exposici6n. Esta conducta tan s01o se observ6 en dias en que la temperatura del aire estaba sobre los 30 C, la humedad relativa entre mediana y alta y la velocidad del viento era bain. La duraci6n de los barios de sol result6 cotta, nunca excediendo los 2 min. La temperaturn de los sustratos en donde se tomaron los bahos, siempre excedi6 los 47 C. Aparentemente los barios de sol dieron lugar a cortos perlodos de estr•s causados por el calor. Esto implica que la funci6n de estos barios de sol debe set otra diferente a la termoregulaci6n. Sunbathing, also called sunning or basking, has been documented for more than 170 species of birds in 48 families (Kennedy 1969) and has been described extensively (Simmons 1986, Terres 1980). It appears that many birds deliberately expose themselves to direct sunlight to accumulate heat for thermoregulation, but alternative functions have been suggested (Kennedy 1969, Mueller 1972, Simmons 1986, Terres 1980). There is little information in the literature regarding sunning by swallows (but see Barlow et al. 1963, Johnston and Hardy 1962, Prytherch 1981). The present paper describes observations of sunbathing in three species of swallows and presents evidence that such activity is not always related to conservation of energy. Throughout July 1990, we observed sunbathing of Tree Swallows (Tachycineta bicolor), Violet-green Swallows (T. thalassina) and Barn Swallows (Hirundo rustica) at Flathead Lake Biological Station, 30 km north of Polson, Montana (47ø41.4'N, 114ø09.3'W). All three species of swallows breed at the station and are roughly equal in abundance there. Swallows were seen basking in mixed groups one or more times each week, but it is not known if the same individuals were always involved. We were able to make observations of basking on 15 occasions over about 8 h on six different days. Most birds sunned at the same location, a roof on one of the buildings at the station. During each observation period we