Biparental nest-attendance in Chilean Swallows (Tachycineta meyeni) breeding in Ushuaia, Argentina

Intermittent female incubation, where eggs are left unattended periodically while the female forages, is common among passerines. In extremely cold environments, unattended eggs may be at risk of freezing or exposed to suboptimal developmental temperatures. Our aim was to examine incubation behaviour of Chilean Swallows nesting in a cold environment (daily average temperatures similar to 10 degrees C) at the southern tip of Argentina, and the temperature regime of incubation. Females had bare, vascularised brood-patches, whereas males had false brood-patches, where feathering of the abdomen was somewhat sparse. Video-cameras were used at three nests, and at all three two adults were observed to attend eggs, day and night, confirming incubation by males. Temperature records of eggs showed that one adult (assumed to be the female) had high rates of attendance and maintained egg temperatures that approached 38-39 degrees C when incubating, and the other adult (assumed male) had lower and variable rates of attendance and maintained egg temperatures generally above 34 degrees C when incubating. The assumed female rarely left the eggs unless the assumed male was there to replace her. Researchers should be cautious when making conclusions using data from temperature loggers in situations where male attendance at nests during incubation is possible.