Associations of waterbirds with harvesting and ploughing events in rice fields: increasing foraging opportunities or just gathering for the feast?

Abstract

Surges in food availability create localized intense foraging opportunities, often attracting multi-species consumer groups. Agricultural practices can trigger these surges, prompting bird associations. However, the strength and duration of associations and its drivers, remain unclear. This study examines waterbird association with harvesting and ploughing events in rice fields. The duration and magnitude of these associations were determined and three hypotheses addressed to explain them: (1) increased food availability, (2) enhanced foraging success and (3) reduced time allocated to vigilance. Waterbird counts and GPS tracking revealed strong associations with management events. Bird numbers spiked during events but declined within one to two days. Food availability increased significantly during events - crayfish and rice during harvesting, worms and soil penetrability during ploughing - supporting Hypothesis (1) However, this did not improve foraging performance (intake rate, foraging success), rejecting Hypothesis (2) Higher competition, interference or kleptoparasitism in these large mixed-species flocks may offset the benefits of increased food availability. Alternatively, prey intake may be constrained by a saturating functional response, with birds operating near maximum intake despite higher food availability. Hypothesis 3 was also unsupported, as birds did not reduce vigilance. Overall, birds gain no additional foraging advantage from attending actively managed fields relative to those managed over three days earlier. We hypothesize they are attracted by the expectation of improved foraging opportunities, with associations driven by gregariousness and local enhancement. Results highlight complex responses to food surges, suggesting waterbirds maintain stable foraging performance during and after agricultural management, indicating resilience to management timing shifts.