Prevalence and impacts of ants colonizing active bird nests in Illinois

Arthropod communities are often dependent on the availability of ephemeral microhabitats. Many organisms, including birds, mammals and social insects, build nests which create unique and suitable microhabitats for a variety of other animals. These constructed environments can protect their residents from predation and from harmful abiotic conditions. In turn, the communities of arthropods that colonize animal made habitats may positively or negatively influence the nest owner. Bird nests, for example, are known to harbor arthropods communities that include both harmful (e.g. ectoparasitic fly larvae) and possibly beneficial species. I quantified the arthropod communities of nests of ten species of birds in Illinois along a land-use gradient. I found workers of eight species of ants in nests, and for three of these species (Tapinoma sessile, Temnothorax curvispinosus, and Crematogaster cerasi) there was evidence that at least part of their colonies inhabited the nest. Tapioma sessile was the most common species and maintained the largest colonies in nest material. I found that the percentage of forest cover surrounding bird nests best predicted the presence of T. sessile colonies, with colonies being negatively associated with forest cover. In addition to ants, members of 19 other arthropod orders were found living in the nests. There was little evidence that ant presence influenced the abundance or prevalence of other arthropods within nests with one exception. Brown Thrasher nests with T. sessile colonies had fewer fly larvae and pupae than nests without ants. Fledging success did not differ between nests with and without T. sessile colonies for any bird species. Polydomy and a high degree of nomadism are characteristics which likely predispose ant species like T. sessile to colonizing active bird nests. The association between these ant species and bird nests likely is a facultative commensalism benefiting ants that is widespread in North America, and warrants further investigation