COMPARATIVE DIURNAL AND NOCTURNAL FORAGING BEHAVIOR AND ENERGY-INTAKEOF PREMIGRATORY GRAY PLOVERS PLUVIALIS-SQUATAROLA AND WHIMBRELS NUMENIUS-PHAEOPUS IN SOUTH-AFRICA

Estimates of the energy budgets of nonbreeding waders (Charadrii) have frequently been hampered by ignorance of the birds' nocturnal activit ies. During the premigratory period (March), Grey Plovers Pluvialis sq uatarola and Whimbrels Numenius phaeopus at the Zwart-kops estuary, So uth Africa, foraged throughout the nocturnal as well as the diurnal lo w-tide period. Although both species continued to forage visually at n ight, they exhibited behavioural changes to compensate for the reduced visibility. Both species foraged more slowly at night, taking fewer s teps per minute, and Grey Plovers paused for longer to search for prey . Prey items were sighted from shorter distances at night than during the day. Energy intake rates, however, did not differ significantly by day and night for either species, and both achieved over 40% of their daily low-tide period intake at night. The energy intake rates and to tal daily intake of Grey Plovers, which fed exclusively on the study a rea, exceeded allometrically predicted values.