Feeding ecology of Red Knots Calidris canutus roselaari at Golfo de Santa Clara, Sonora, Mexico

The Upper Gulf of California has recently been discovered as a summering area and as an important staging area during spring migration for the Pacific Red Knot Calidris canutus roselaari. In this paper we describe the feeding ecology of Red Knots using the beaches near Golfo de Santa Clara, Sonora, Mexico. During March–September 2010, we determined the abundance of knots, their abdominal profiles, agonistic relationships and the composition of benthic prey. The area includes two habitat types: muddy beach (in the north) and sandy beach (in the south). The highest numbers of knots were found on the sandy beach during March–April 2010, with a maximum count of 842 on 24 March. Bivalves were the most abundant potential prey on both beaches. However, in spring, high densities of eggs of Gulf grunion Leuresthes sardina occurred on the sandy beach, attracting shorebirds. Before the spawning of Gulf grunion eggs, no aggressive encounters between birds were observed; however, after the eggs appeared intra- and interspecific agonistic behavior became common (up 0.12 attacks/minute). In March we found mostly lean birds with low abdominal profiles, in April–May, birds became more robust (presumably mostly adults on migration to their breeding areas), and in summer and autumn birds had intermediate profiles (presumably oversummering juveniles, or juveniles and adults that had just arrived in the area). Our results suggest that Gulf grunion eggs are the most important food source during spring migration, of comparable ecological importance to horseshoe crab Limulus polyphemus eggs in Delaware Bay, USA. The distribution of eggs led to aggressive behavior, nevertheless the body mass of the birds increased to prepare for northward migration.