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By Marta Maziarz
As an ornithologist, I have focused on the reproduction of birds but often overlooked the fact that bird nests can also be home to many invertebrates that find shelter, food or a suitable microclimate within them. When we discovered ant workers and their larvae inside nests of the wood warbler Phylloscopus sibilatrix, curiosity drove us to study this phenomenon.
An initial literature review revealed just a handful of published records of ant broods found inside bird nests, including blue tits Cyanistes caeruleus breeding in nest-boxes in Corsica (Lambrechts et al. 2008), and great tits Parus major and marsh tits Poecile palustris occupying tree cavities in primeval stands of the Białowieża Forest, Poland (Mitrus et al. 2015). Blem and Blem (1994) reported ant colonies on the side of nests in nest-boxes used by prothonotary warblers Protonotaria citre but gave no further details. This surprising scarcity of observations of ants in songbird nests suggested that this phenomenon may be exceptional and occur only among cavity-nesting species.
Our discovery of ant workers and their larvae in wood warbler nests, which are domed structures composed of dry grass, moss, and leaves and situated on the forest floor, challenged this view. We made the original finding during long-term studies of wood warbler ecology in 2004-2015 in Białowieża Forest (Eastern Poland), which prompted us to document this phenomenon systematically during 2016-2017. In 2017, we also contacted researchers in Switzerland and the UK to ask them to inspect nests for the presence of ants and their broods. We wanted to find the frequency of ants colonising wood warbler nests, and whether ants are present in wood warbler nests elsewhere in the species’ breeding range.
During our systematic observations in 2016-2017, we found adult ants in 43% of warbler nests, and one-third of nests also contained ant larvae or pupae. These ant broods were situated within the sidewalls of the nests, at or just above ground level. The most frequent species were Myrmica ruginodisor M. rubra, and occasionally Lasius niger, L. platythoraxor L. brunneus. These numbers, compared to 30% of nests containing adult ants and 20% containing broods during the earlier (2004-2015) period, indicated a long-term association between the ants and the birds. The findings from Białowieża Forest contrasted with those from Switzerland and the UK, where we only found single cases of adult ants and their broods. The different frequencies of ant presence between regions could be due to varying densities of bird or ant nests between woodlands transformed by humans to a different degree, but further studies would be necessary to confirm this.
These first records of adult ants and their broods in wood warbler nests showed that occupation of bird nests by ants can be a locally common phenomenon, which may have been overlooked previously in this and other songbirds. Systematic examination of nests belonging to different bird species would be valuable in understanding this further.
Furthermore, the occurrence of ant broods in the walls of wood warbler nests showed that ants colonised these structures following their construction by birds. Why they do this remains unclear; are the ants attracted to the nests by their structure, the presence of other invertebrates as a source of protein, or by heat generated by the birds? More work is underway to answer these questions, but it seems that these potential ant-bird interactions could be much more widespread than has been suspected.
Blem CR, Blem LB (1994) Composition and microclimate of Prothonotary warbler nests. Auk 111:197–200.
Lambrechts MM, Schatz B, Bourgault P (2008) Interactions between ants and breeding Paridae in two distinct Corsican oak habitats. Folia Zool 57:264–268.
Mitrus S, Hebda G, Wesołowski T (2015) Cohabitation of tree holes by ants and breeding birds in a temperate deciduous forest. Scand J For Res 31:135–139.