The use of foot web coloration for the estimation of prebreeder numbers in Wilson's storm-petrels, Oceanites oceanicus

The aim of this study was to develop a method to estimate the proportion of prebreeders in Wilson's storm-petrels (Oceanites oceanicus) in mist-net catches. A comparison of morphological parameters between birds captured on the nest and by mist-net revealed significant differences in two seasonally changeable parameters, body mass and brood patch size, as well as in a parameter independent of the time of the season, the presence of black spots in the yellow areas of the webbed feet. The difference in the occurrence of black markings between birds caught by mist-net and at the nest was highly statistically significant and was used to estimate the proportion of prebreeders, which was 86.6% in a sample of 336 mist-netted birds, compared with 85.3% for the same sample using cloacal measurements. The latter method is only applicable during the time window of the days after egg laying, while the absence of black pigment patches serves as an indicator for the classification of prebreeders throughout the season. This provides the basis for further studies on the behaviour of prebreeding petrels. Accepted: 3 June 2000