Ornithologists are well aware that closely related species of birds often coexist in the same habitat. For example, five Parus species occur together in English broad-leaved woodlands: the blue tit (P. caeruleus), the great tit (P. major), the marsh tit (P. palustris), the willow tit (P. montanus) and the coal tit (P. ater). All have short beaks and hunt for food chiefly on leaves and twigs, but at times on the ground; all eat insects throughout the year, and also seeds in winter; and all nest in holes, normally in trees. However, the closer we look at the details of the ecology of such coexisting species, the more likely we will find ecological differences - for example, in precisely where within the trees they feed, in the size of their insect prey and the hardness of the seeds they take. Despite their similarities, we may be tempted to conclude that the tit species compete but coexist by eating slightly different resources in slightly different ways. However, a scientifically rigorous approach to determine the current role of competition requires the removal of one or more of the competing species and monitoring the responses of those that remain. Martin and Martin (2001) did just this in a study of two very similar species: the orange-crowned warbler (Vermivora celata) and virginia's warbler (V. virginiae) whose breeding territories overlap in central Arizona. On plots where one of the two species had been removed, the remaining orange-crowned or virginia's warblers fledged between 78 and 129% more young per nest, respectively. The improved performance was due to improved access to preferred nest sites and consequent decreased losses of nestlings to predators. In the case of virginia's warblers, but not orange-crowned warblers, feeding rate also increased in plots from which the other species was removed (Figure 8.4).
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