The Tumbesian region of Ecuador and Peru contains high numbers of endemic and vulnerable species and is one of the most threatened bioregions in the world due to extensive human use. I examined the effects of small-scale livestock grazing and clearing of trees on mixed species bird flocks in arid scrub and tropical dry forest vegetation in Machalilla National Park by comparing flocks in more and less disturbed plots in each vegetation type. Habitat degradation is expected to change vegetation structure, which may lead to changes in food availability and predation pressure. Since the fitness benefits of flocking come from either a reduced risk of predation or enhanced feeding efficiencies or both, habitat degradation could lead to changes in the benefits of flocking or optimal flock sizes. I predicted that flocks in more disturbed vegetation would have fewer numbers of species and individuals, that species would show lower flocking propensities, and that feeding efficiencies would be lower in flocks in more disturbed vegetation.
Results/Conclusions
Mixed flocks in tropical dry forest had significantly greater numbers of total species participating in flocks, proportion of the total community participating in flocks, and average species richness per flock than arid scrub. Less disturbed tropical dry forest had significantly greater average species richness per flock than more disturbed tropical dry forest, but average species richness per flock did not significantly differ between more and less disturbed arid scrub. Flock species composition differed across the disturbance gradient in tropical dry forest and different between tropical dry forest and arid scrub. However, flock species composition did not differ across the disturbance gradient in arid scrub. An indicator species analysis gave high numbers of indicator species in flocks in less disturbed tropical dry forest, but very low numbers in more disturbed tropical dry forest and in arid scrub. Many species had highest flocking propensities in less disturbed tropical dry forest and greatest foraging efficiencies in less disturbed tropical dry forest and arid scrub compared to the more disturbed vegetation. I conclude that mixed species flocks are more affected by habitat degradation in tropical dry forest than in arid scrub, perhaps because flocks are smaller and overall species richness is lower in arid scrub. Conservation of the endemic avifauna of this highly threatened region depends on understanding and maintaining their unique interspecific associations.
