Thermal ecosystem engineering by songbirds promotes a symbiotic relationship with ants

Abstract

Nesting birds can act as thermal ecosystem engineers by providing warm habitats that may attract arthropods to colonise the nest structure. This cohabitation of birds and nest-dwelling invertebrates may foster symbiotic relationships between them, but evidence is lacking. We investigated whether ants are attracted to bird nests by the heat generated by the hosts, and/or the nests’ structural insulation properties, to raise their broods (larvae and/or pupae) in advantageous thermal conditions. We found that the endothermic activity of birds within their nests created ‘heat islands’, with thermal conditions potentially promoting the survival and development of ant larvae in cool environments. We experimentally confirmed that the presence of heat within bird nests, and not the structure itself, attracted the ants to colonise the nests. As ants might benefit from exploiting warm bird nests, this may be a previously overlooked commensal, mutualistic or parasitic relationship which may be ecologically significant and globally widespread among various nesting birds and reproducing ants. Similar interspecific interactions may exist with other arthropods that reproduce in avian and mammalian nests. Further research is needed to reveal the nature of these relationships between such taxa, and to understand the role of warm-blooded animals as thermal ecosystem engineers.

Interspecific attraction between ground-nesting songbirds and ants: the role of nest-site selection

“…Cool weather further enhanced colonisation of Wood Warbler nests by ants, and also when higher rainfall coincided with low ambient temperatures. These findings supported the hypothesis that ants colonised bird nests to raise their own broods in a more advantageous microclimate than in the ants’ own nests elsewhere on the forest floor [ 20 ]. It is possible that longer exposure of nests containing older nestlings increased the potential colonisation of bird nests in the later nestling stage, relative to earlier nest stages.…”

“…Bird nests are often composed of insulative materials and are warmed from within by their owners when brooding eggs or chicks and maintaining their own body temperature [ 14 , 15 ]. As such, these warm nests may be a resource of a warm microclimate valuable for the survival, growth and development of arthropods, including ant larvae or pupae [ 16 – 20 ]. Moreover, as bird nests can be inhabited by numerous other invertebrates, and contain animal debris, these places can supply a rich resource of protein food that may be important for ant broods [ 18 , 21 ].…”

“…We hypothesised that ants would colonise bird nests to raise their own broods under more advantageous, warmer conditions than within their own nests elsewhere, as demonstrated previously [ 20 ]. As such, we assumed that ant broods would occur within bird nests most frequently in the late nestling period, when large chicks warm the nests most intensively, and when the temperature disparities between the birds’ and the ants’ own nests are greatest [ 20 ].…”

“…We hypothesised that ants would colonise bird nests to raise their own broods under more advantageous, warmer conditions than within their own nests elsewhere, as demonstrated previously [ 20 ]. As such, we assumed that ant broods would occur within bird nests most frequently in the late nestling period, when large chicks warm the nests most intensively, and when the temperature disparities between the birds’ and the ants’ own nests are greatest [ 20 ]. The colonisation of bird nests by ants would be also more frequent during cool and wet weather, when the microclimate of the ants’ own nests, which are reliant on ambient temperatures and solar radiation alone, would be less suitable for raising ant broods than in the warm nests of birds [ 16 , 17 , 29 , 30 ].…”

“…The colonisation of bird nests by ants would be also more frequent during cool and wet weather, when the microclimate of the ants’ own nests, which are reliant on ambient temperatures and solar radiation alone, would be less suitable for raising ant broods than in the warm nests of birds [ 16 , 17 , 29 , 30 ]. Conversely, we presumed that ant broods would become less common in the bird nests that were inspected long after they were vacated by the birds, when the thermal conditions had deteriorated [ 20 ].…”