DAY-TIME AND NIGHT-TIME PREY AVAILABILITY FOR WATERBIRDS IN A TROPICAL LAGOON

One of the main hypotheses formulated to explain why marsh birds, wild fowl, and shorebirds forage at night postulates that the birds prefer to feed at night because the feeding opportunities are most profitable then. To investigate this hypothesis, we compare day- and night-time availability and describe the diel abundance rhythm of swimming and be nthic organisms inhabiting the shallow waters of a tropical lagoon com plex in northeastern Venezuela. Three sampling techniques were used by day and by night: net sampling for swimming organisms, core sampling for those in the sediments (10 cm deep), and sight counts for organism s on the surface of substrata. Overall, in the case of swimming organi sms, fishes, isopods, amphipods, shrimps (Penaeus spp.), and corixids were 3-30 times more abundant at night than during daytime. In general , infaunal organisms were about equally abundant during the day and th e night, or slightly more abundant during the day. At the surface, iso pods, amphipods, and polychaetes were greater than 10 times more abund ant at night than during the day. However, gastropods were slightly mo re numerous during daytime on the substrata, while pelecypods were abo ut equally numerous during day- and night-time. Fiddler crabs (Uca sp. ) were generally more available during daytime. Considering all organi sms together, the prey for marsh birds, wildfowl, and shorebirds were significantly more abundant at night. In conclusion, tactile-foraging species (e.g., spoonbills, skimmers, ibises, dabbling ducks, and sever al species of shorebirds) should profit by foraging only at night and resting during the day, and foraging during daylight only to top up a nighttime deficit. On the other hand, despite limited nighttime visual capacity, some sight-feeding species (e.g., herons, plovers) may take advantage of increased prey availability at night, at least on moonli t nights.