Seasonal Variation in Diurnal and Nocturnal Distributions of Nonbreeding Shorebirds at North Humboldt Bay, California

Recent studies of nocturnal foraging by shorebirds (Charadriiformes: Charadrii) suggest that many species feed at night. Much of this research has been qualitative and/or restricted to a small portion of the annual cycle (e.g., a few nights or one season) making it difficult to evaluate the extent to which nocturnal foraging varies seasonally. Consequently, we examined seasonal variation in abundance and distribution of diurnal and nocturnal foraging shorebirds from 10 Jan 1992-10 Jan 1993 at North Humboldt Bay, California. Shorebirds foraged primarily during the day. Overall, day/night frequency of occurrence (percent of censuses with birds) was 87%/48%. In fall, frequency of occurrence of shorebirds differed less between day and night (day/night: 82%/64%) than during spring (day/night: 79%/14%) or winter (day/night: 100%/42%). Moreover, nocturnal abundance of Marbled Godwit (Limosa fedoa), Willet (Catoptrophorus semipalmatus), dowitchers (Limnodromus scolopaceus and L. griseus), and Black-bellied Plover (Pluvialis squatarola) peaked in fall, whereas diurnal abundance of these species peaked in winter (Marbled Godwit, Black-bellied Plover, and dowitchers) or spring (Willet). Taxa varied in day/night patterns. For Scolopacids, diurnal abundance significantly exceeded nocturnal abundance. However, abundance of American Avocet (Recurvirostra americana), Black-bellied Plover, and Semipalmated Plover (Charadrius semipalmatus) did not differ significantly between day and night. Our results suggest that a researcher's choice of season or taxa may influence observed patterns of diurnal and nocturnal distributions of shorebirds considerably.