Effects of Red Imported Fire Ants on Songbird Nest Survival

Authors: A. J. Campomizzi

Invasive species are often implicated in population declines of native species through competition and predation. Red imported fire ant (Solenopsis invicta) predation of songbird nestlings and eggs has been documented. I conducted a replicated manipulative experiment to determine the magnitude of the decrease in nest survival caused by S. invicta in addition to other predators. I conducted mensurative experiments to quantify the frequency of S. invicta foraging near active songbird nests and factors that influence the susceptibility of songbird nests to S. invicta predation. I hypothesized that predation by S. invicta reduced nest survival by 10%, potentially biologically significant, and that songbird nests would be more susceptible to S. invicta predation that were located: (1) closer to the ground, (2) closer to an edge, (3) closer to disturbed soils, and (4) initiated later in the breeding season. I monitored 235 songbird nests including 45 black-capped vireo (Vireo atricapilla), 67 white-eyed vireo (V. griseus), and 123 northern cardinal (Cardinalis cardinalis) nests on 9 patches of 36–103 ha each on private land in east-central Texas, USA in 2006–2007. I found preventing S. invicta from preying upon songbird nests increased nest survival 20% for white eyed vireos and 1% for black-capped vireos. I detected S. invicta near songbird nest on 60% of food lures on the ground and 7% of food lures 1 m high in vegetation (n = 122). Vireo nests <2 m high and <4 m from an edge were more susceptible to S. invicta predation indicating potential threshold conditions, below which songbird nests may be more susceptible. If my results are applicable to other areas then songbird populations of some species nesting below 2 m may have substantially lower nest survival in areas occupied by S. invicta. I suggest the negative impacts of S. invicta on songbird nest survival may be reduced by applying integrated pest management methods and increasing woody vegetation cover in breeding areas of songbird species susceptible to S. invicta nest predation.

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