The importance of using the proper methodology and spatial scale in the study of habitat selection by birds

Nest-site preferences of birds have traditionally been inferred from correlations between reproductive success and various habitat attributes. Because there are several problems associated with this method, we examined preferences by comparing characteristics of actual nest sites with those of sites available to females at the time of their settlement. We examined these preferences on two spatial scales using the marsh-nesting Red-winged Blackbird (Agelaius phoeniceus). On the broad spatial scale, univariate analyses established that female settlement was nonrandom with respect to four of five habitat characteristics. However, a multivariate analysis revealed that females settled nonrandomly only with respect to water depth (they preferred areas located in deep water). The other characteristics, including cattail density, were not important. On the narrow spatial scale, females preferred sites located in dense cattail clumps that were surrounded by unusually sparse cattail vegetation. The preference of female redwings for deep-water areas was not detected in an earlier study conducted in the same marsh using the traditional method. In addition, if we were to have examined female preferences on the broad scare only, we would have failed to recognize the importance of cattail density in nest-site selection. The results of this study demonstrate the importance of examining nest-site selection using an appropriate method and multiple spatial scales.