Does fresh vegetation protect avian nests from ectoparasites? An experiment with tree swallows

Abstract

Some species of birds incorporate fresh green vegetation into their nests. The adaptive significance of this behaviour, however, is unclear. The "nest protection hypothesis" suggests that volatile chemicals contained in green plants reduce populations of nest-dwelling ectoparasites. I tested this hypothesis with tree swallows, Tachycineta bicolor (Vieillot, 1808), a species that has large infestations of ectoparasites in its nests. I experimentally added green leaves of yarrow (Achillea millefolium L.) to nests and compared these nests to control nests. My results showed that nests with yarrow had significantly higher levels of infestation by fleas, and yarrow had no effect on the number of blow flies detected. There was no effect of yarrow on condition of parents, size of offspring, or the probability of nests fledging at least one young. I did, however, find that second-year (SY) females with yarrow added to their nests hatched significantly more eggs than control SY females. In addition, pairs with yarrow added to their nest were significantly less likely to suffer hatching failure and (or) brood reduction. These beneficial effects of yarrow cannot be explained by reductions in numbers of fleas or blow flies, and I suggest that these results may be due to reductions in numbers of other types of parasites not quantified in my study, such as flying insects or the diseases that they may vector. I discuss my findings for swallows in the context of results from previous studies of other species that habitually use green plant material such as yarrow in their nests.