AGE-DETERMINATION OF FREE-LIVING MALE BLACK-THROATED BLUE WARBLERS DURING THE BREEDING-SEASON

Male Black-throated Blue Warblers (Dendroica caerulescens) can be aged as yearlings (SY) or older (ASY) during the breeding season using bin oculars. Significant differences in wing color of SY and ASY males, as revealed be spectrophotometric analysis of museum specimens, are appa rent in free-living birds. Error rates in age determination in five he ld trials averaged 3.1% for ASY males and 0% for SY males (though samp le sizes were small for yearlings). Age data from multiple investigato rs should not be combined for analysis unless individual error rates c an be estimated. The statistical effects of error are especially criti cal when age ratios are calculated for small samples (n < 25).