Over-summering shorebirds in Guerrero Negro, Baja California Sur, Mexico and the particular case of the Marbled Godwit

Over-summering is particularly frequent in shorebirds and different hypotheses have been proposed to explain this behavior, which could be an important part of species’ life history. The Guerrero Negro wetland complex in Mexico is important as both a wintering and over-summering site for multiple species. This paper describes an ‘over-summering index’ (i.e., the percentage of the wintering population that remains during the boreal summer) of eight species of shorebirds in Guerrero Negro during 13 migratory seasons (2006–2019): Black-bellied Plover Pluvialis squatarola, Long-billed Curlew Numenius americanus, Red Knot Calidris canutus, Dunlin C. alpina, Western Sandpiper C. mauri, Short-billed Dowitcher Limnodromus griseus and Willet Tringa semipalmata, with particular emphasis on Marbled Godwit Limosa fedoa, for which we also collected morphometric data. The over-summering index varied among shorebird species, with Long-billed Curlew highest (50.4%) and Dunlin (3.4%) and Western Sandpiper (9.7%) lowest. The over-summering index tended to be higher for larger species. Marbled Godwit had a mean over-summering index of 29.4% with a maximum summer abundance of 27,678, representing 16% of its total population. Monthly trapping of Marbled Godwits showed that the percentage of adults and the body mass decreased towards the middle of the summer in May–June. Our data show that a greater percentage of juveniles than adults of both sexes remain on their wintering grounds and we suggest that this is associated with reproductive immaturity. However, some adults remain, possibly when pre-migratory physiological conditions are not met. We consider that the hypotheses proposed are not mutually exclusive and different factors could be responsible for the oversummering for the shorebirds.