THE INFLUENCE OF FOOD ABUNDANCE CHANGE ON SEARCH AND ATTACK BEHAVIOR OF INSECTIVOROUS BIRD IN THE MONTE DESERT, ARGENTINA

Bird foraging is a combination of two behaviors, the search movements to locate preys and the attack maneuvers to catch them. However, the first behavior is poorly studies. Hear we determine the searching movements and attack maneuvers of three species of insectivorous birds in the central Monte desert, Argentina and evaluate if these two components of the foraging behavior change as a function of interannual variation in the availability of food resources. During three years we studied in the Ñacuñán Reserve (Mendoza, Argentina) the search movements and attack maneuvers of three insectivorous bird species in the foliage of Neltuma flexuosa trees and simultaneously evaluated the abundance and biomass of their prey. We found that arthropod abundance increased in the last year, although biomass was low, due to the greater number of small arthropods (less than 0.2 mg) in that year. During the first and second year the main capture maneuver of Grey-crowned Tyrannulet ( Serpophaga griseicapilla ) was sally-hover and it moved among the foliage using flight movements, but in the third year a greater use of gleaning maneuvers was observed and its main foraging movements were jumping on the branches.

Greater Wagtail-tyrant ( Stigmatura budytoides ) used both sally-hovering and gleaning maneuvers in the first and second years, however it used glean more frequently and performed more jumping movements in the third year Ringed Warbling-Finch ( Microspingus torquata ) showed very stereotyped behavior throughout the study period, using only gleaning and jumping as foraging strategies. The increase of small prey in the last year could have determined the changes in attack maneuvers and searching movements of Grey-crowned Tyrannulet and Greater Wagtail-tyrant. The gleaning maneuver might be more suitable for capturing small prey, probably because birds detect them better at close range, and moving with short movements such as jumping might allow them to explore branches and foliage in greater detail. For Ringed Warbling-Finch, given the characteristics of its feeding behavior, the above-mentioned changes did not influence the way it search and attack prey. The interannual changes observed in searching and capture behavior provide valuable information about ecological flexibility of insectivorous birds that may be advantageous when they confront variable climates and food fluctuations such as in desert environments.