Old nest material in nest boxes of tree swallows: Effects on nest-site choice and nest building

Abstract

In natural cavities, old nest material accumulates with successive use, thereby reducing the size of the cavity, and allowing the numbers of certain haematophagous ectoparasites to increase. For this reason and because researchers studying birds breeding in nest boxes typically remove old nests from boxes, the results of such studies have been questioned. The accumulation of old nest material might affect nest-site selection and nest building by hole-nesting birds, so we tested this hypothesis by manipulating the presence and amount of old nest material in nest boxes of Tree Swallows (Tachycineta bicolor). Our experiment also allowed us to examine whether costs are incurred by females due to nest building in terms of their subsequent reproductive output. When a choice of boxes was available, swallows preferred empty and clean boxes, or those where the old material had been microwaved, over those with old, untouched material. Clean boxes and those with microwaved material had more space inside, so our experiments support two hypotheses: swallows avoid potentially high numbers of parasites in nests with old material; or they prefer large cavities. Empty boxes affected nest building. The mass and volume of nests built in clean boxes were greater than for nests built on old material. Females did not add more feathers to the nest lining in boxes with old material as compared with clean boxes. Correlation analyses suggested that females building large nests began egg laying earlier in both years. Otherwise, there were no associations between the sizes of nests built by females and subsequent reproductive output (e.g. clutch size) or nestling size (e.g. body-condition index). Our results show that the common habit of removing old nests from boxes can affect nest-site choice and nestbuilding behavior. Nest building does not influence reproductive output by Tree Swallows.