During 1995, the relationship between the nesting chronology of the Least Tern (Sterna antillarum) and the near-shore abundance of fish was examined at La Purinera, Sonora, the site of one of the largest known Least Tern colonies in northwest Mexico. Fish abundance was measured adjacent to the shoreline during early mornings using a beach seine net, and the sampling periods corresponded to different phases of the Least Tern’s annual cycle. During this year, the nesting season (April to July) coincided with peak prey fish abundance, and peak hatching of tern eggs (early July) coincided with the greatest abundance of fish of adequate size for chicks. Departure of the terns from the area coincided with a marked reduction in suitable food in September. Collections of fish dropped and left uneaten by Least Terns in the colony contained five species, which were also present in the fish samples obtained with the beach seine net. The observations on foraging adult Least Terns and the collections of fish dropped in the colony suggested that the terns were preying mainly upon silversides (Leuresthes sardina, Colpichthys regis) and anchovies (Anchoa spp.). Terns foraged mainly on the bay side of the barrier beach where fish were more abundant. The results support the idea that seabird breeding and food abundance or availability coincide at the time of maximum need, when parents feed rapidly growing chicks.
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Vol. 28 • No. 2