Effects of sampling design on age ratios of migrants captured at stopover sites

Age classes of migrant songbirds often differ in migration timing. This dif ference creates the potential for age-ratios recorded at stopover sites to vary with the amount and distribution of sampling effort used. To test for these biases, we sub-sampled migrant capture data from the Middle Rio Grand e Valley of New Mexico. We created data sets that reflected the age ratios of migrants that would have been captured with stratified and concentrated designs at four levels of mist-netting effort. Analysis of these data indic ate that age-ratios of Neotropical migrants varied significantly with sampl ing design, but not with sampling effort. More after-hatch-year Neotropical migrants were captured with stratified than with concentrated sampling des igns. Age-ratio of temperate migrants did not vary with either amount of sa mpling effort or sampling design. Sampling design is an important considera tion in the interpretation of age ratios among stop-over sires, and standar dization of sampling among sites could improve our understanding of differe ntial migration of age classes.