Nest box orientation affects internal temperature and nest site selection by Tree Swallows

ABSTRACT Orientation of nests can influence nest microclimate, particularly temperature. However, few investigators have examined orientation preference and microclimate simultaneously. We examined the possible correlation between entrance orientation of artificial nest boxes used by Tree Swallows (Tachycineta bicolor) and the internal temperature of boxes. Tree Swallows showed a preference for east- and south-facing boxes, but only during the first half of the breeding season (before 1 June). During the second half of the breeding season (after 1 June), Swallows selected boxes based on availability. We found that east- and south-facing boxes were warmer than north- and west-facing boxes, but only during the first half of the breeding season when those boxes were preferred. Entrance orientation and box temperature were only correlated during the morning (06:00–12:00); the temperature of all boxes was similar during the afternoon. Our results suggest that Tree Swallows show a preference for nest boxes with a certain entrance orientation only when orientation influences microclimate, suggesting that warmer nest temperatures may provide fitness benefits.