Foraging at Army Ant Swarms by Fifty Bird Species in the Highlands of Costa Rica

Much has been written concerning the use of raiding swarms of army ants by Neotropical birds in lowland forests. Bird-ant foraging associations are much less common at higher (subtropical) elevations, and no highland species have been identified as professional or regular ant-followers. All known professional army ant followers in the Neotropics occur below 1000 meters elevation (Ridgely & Tudor 1994). Even attendance at army ant swarms by montane birds has seldom been reported, and previous authors have described observations made at a single, or at most six, highland army ant swarms (Hilty 1974, Gochfeld & Tudor 1978 and Dobbs & Martin 1998). In this note, I report observations of 50 species of birds attending 17 separate raiding army ant swarms in subtropical forests of the Cordillera de Tilaran at Monteverde, Costa Rica. During January, November and December 1996, and from January through May 1997, I made opportunistic observations of birds at army ant swarms in the vicinity of Monteverde. Elevations were estimated using topographical maps. Army ant swarms were encountered at sites ranging in elevation from 1100 to 1620 meters, on both the east and west slopes of the Cordillera de Tilaran. Thirteen swarms were found inside forest and four were found in edge (disturbed) habitats. When birds were located feeding at an army ant swarm, I recorded the species and number of individuals present. All swarms were watched for at least one hour to ensure that all species and individuals using the swarm were detected. Species were considered to be “ant following” if they remained with the swarm for the total period of observation and were observed capturing, or attempting to capture, prey flushed by the advancing ants. I recorded a total of 244 individuals at 17 separate army ant swarms. The total number of individuals detected at any one swarm ranged from 5 to 24 (average = 15.59, SD = 4.887). The number of species attending a single swarm ranged from 3 to14 (average = 8.76, SD = 3.134). Of the 50 species at these swarms, 20 have apparently not previously been reported following ants (Table 1). Only one species, Myrmeciza immaculata (present at