Conditions and Significance of Night Feeding in Shorebirds and Other Water Birds in a Tropical Lagoon

--On a small bay of the Chacopata lagoon complex in northeastern Venezuela, we analyzed the temporal feeding activity patterns of shorebirds and other water birds, and we determined the factors or conditions related to these patterns. During daytime and nighttime observations, we measured the abundance of each species as well as environmental factors (time, wind velocity, cloudiness, tide level, presence of moonlight and bioluminescence). A night vision module (light intensifier) was used during nighttime observations. Samples x species matrices were summarized by reciprocal averaging (RA) analysis, and the information was related to e'nvironmental factors. Feeding activity patterns were related most significantly to time of day, i.e. daytime and nighttime. Some species like "egrets," the Whimbrel (Numenius phaeopus), and the Black-bellied Plover (Pluvialis quatarola) fed principally during daylight, but most other species (including a majority of shorebird species) fed more regularly and in higher numbers at night. Lesser and Greater yellowlegs (Tringa fiavipes and T. melanoleuca, respectively) and Willets (Catoptrophorus semipalmatus) foraged with comparable frequency during the day and the night. Daytime and nighttime data were analyzed separately, and tide level best explained both the variations of nocturnal and diurnal abundance of foraging birds. Nocturnal feeding may be a natural habit in response to regularly limited feeding space and time mainly induced by tide. Received 4 January 1988, accepted 24 August 1988. FEEDING in shorebirds has been studied, for the most part, in daylight. For several shorebirds and other waders (see Bent 1926, Palmer 1962, Krebs 1974, Black and Collopy 1982, Burger 1984, Goss-Custard 1984, Wood 1986, Powell 1987), the relative importance of night feeding is largely unknown, except in night-herons (Nycticorax; see Watmough 1978) and Great Blue Herons (Ardea herodias; see Powell 1987). In addition, daytime and nighttime feedings have been observed almost exclusively in birds that winter in temperate latitudes. At these latitudes, night feeding has been related to high energy requirements under severe winter conditions. Thus, shorebirds presumably feed at night because they cannot obtain enough food during the shortened daylight periods (GossCustard 1969; Heppleston 1971; Goss-Custard et al. 1977; Pienkowski 1981, 1982; Puttick 1984). It is also believed that some shorebird species that forage at night (e.g. the Black-bellied Plover wintering in England) may take advantage of increased availability and activity of prey at night (Dugan 1981). 1 To whom reprint requests should be sent. 94 There are indications that night feeding also occurs in tropical latitudes (Engelmoer et al. 1984, Schneider 1985, McNeil and Robert in press, Robert and McNeil in press), where temperatures are higher and daylight longer than in temperate latitudes. We attempted to analyze the temporal feeding activity patterns of shorebirds and other water birds in a tropical habitat and to determine the factors or conditions related to these patterns. STUDY AREA AND METHODS The study was carried out at the Chacopata lagoon complex (10ø41'N, 63ø46'W) on the north side of the Araya Peninsula, State of Sucre, in northeastern Venezuela. The lagoon complex extends over 830 ha and comprises several areas where feeding birds congregate (Limoges 1987). Tidal amplitude is low (30 cm). Because of a very gentle slope, low tide expanse on the mudflats often ranges from 80-100 m. We scored a total of 156 observation periods (54 by day and 102 by night) in a small bay (2.0 ha) surrounded by mangroves from 12 October to 8 December 1985. Each observation period lasted the time necessary to count all birds. In addition to the abundance of each species, we noted wind velocity (according The Auk 106: 94-101. January 1989 January 1989] Night Feeding in Shorebirds TABLE 1. Relative abundance of species constituting each group in the Chacopata lagoon complex. 95