Migratory insectivorous songbirds of Missouri establish territories in late May/early June to provision adequate food resources for offspring in late June/early July (food-value theory). Songbirds may use environmental cues such as food density (direct-monitoring hypothesis) or other site characteristics (structural-cues hypothesis) to predict future food densities. This research will 1) compare the relative value of these two sets of cues to predict June food abundance, 2) compare the species-specific degree to which birds utilize cues in territory establishment, and 3) determine the species-specific success of birds at tracking June food abundance. To our knowledge, only one other study has compared the strength of food density vs. structural cues to predict territory space within a three-dimensional habitat volume; that study was limited to one bird species and one structural cue. We test these hypotheses using a complete foliage-gleaning community in a large geographic area (~3000 ha.) over three years. We considered foliage density (leaf weight/m²) and ecological landtype (slope/aspect) as potential structural cues. Vegetation and site characteristics were evaluated within 380 plots and food density was evaluated within 54 stands. Bird territory centers were established by spot-mapping over the entire area, and territory boundaries were estimated by the distribution of centers.

Results/Conclusions

Preliminary rmANCOVA results suggest that both direct-monitoring and structural-cues strategies effectively predict June insect densities. June insect density was weakly positively correlated with May insect density and foliage density. Foliage density was a better overall predictor of June insect density than May insect density, but its effectiveness varied by year. The power of the other measured structural cue, ecological landtype, to predict June insect density also varied by year. The observed trade-off between strength and reliability of these predictors could explain persistence of both direct-monitoring and structural-cue behaviors. Preliminary ordinal logistic regression analysis showed no evidence that the worm-eating warbler Helmitheros vermivorum utilized the structural cues of foliage density or ecological landtype, or directly monitored May insect abundance in any year to predict June insect density. There was ultimately no relationship between H. vermivorum territory location and June insect abundance. An analysis of the rest of the foliage-gleaning bird community may suggest whether H. vermivorum is incapable of tracking food resources or is being excluded from these high quality habitats by interspecific competition.