BILL LENGTH VARIABILITY IN BIRDS OF THE TRES MARÍAS ISLANDS, MEXICO

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Abstract
Variability of bill length was compared between populations of species present on the Tres Marías Islands and appropriate nearby mainland. Four species were significantly less variable on the islands, one species was significantly more variable, and 11 species were not significantly different in the two regions. Explanations are offered in terms of available food supply: availability may be influenced by competitors. Data from other islands are consistent with this interpretation. However, no clear trend towards greater or less variability on islands has so far been revealed.