Army Ant Raid Attendance and Bivouac-checking Behavior by Neotropical Montane Forest Birds

—We quantified resident and migrant bird attendance at army ant swarm raids (n 5 48) in a neotropical montane forest. All observations were during seasons when Nearctic migrant birds are present. Bird species differed in army ant raid-attending behavior. Resident bird species attended 2 to 54% of raids, while migrants attended at lower maximum frequencies (2 to 21% of raids attended per species). Some resident and migrant bird species attended raids more frequently than expected based on capture rates in mist-net studies and point-count density surveys. Army ant raid attendance may be a regular element of foraging behavior for some resident species, and important in the wintering ecology of some Nearctic migrant species. The bird species that attended raids most frequently were predicted to show behavioral specializations for exploiting army ant swarms. Eight resident bird species (but no migrants) performed a specialized behavior, bivouac checking, by which birds sample army ant activity. Resident bird species’ frequencies of raid attendance were positively associated with frequency of checking bivouacs (r 5 0.68). We hypothesize the absence of obligate army ant-following birds in montane forests has favored performance of specialized behaviors for exploiting army ant raids by some resident birds. Received 3 October 2009. Accepted 9 March 2010. Neotropical army ants (Formicidae: Ecitoninae) are top predators, and a diverse array of animal species associate with army ant colonies (Franks 1982, Franks and Bossert 1983, Brady 2003, Koh et al. 2004). Birds attend army ant foraging-raids to feed on arthropods and small vertebrates that flee from the advancing ants. Birds primarily attend the swarm raids of Eciton burchellii and Labidus praedator (Willis and Oniki 1978, Wrege et al. 2005). Bird flocks at army ant raids often include multiple species, and their composition is largely distinct from sympatric mixed foraging flocks of insectivores (Willis 1972, Willis and Oniki 1978, Otis et al. 1986, Willson 2004, Peters et al. 2008). Some bird species are obligate army ant raid attendants that obtain most or all of their food at army ant swarms (Willis and Oniki 1978, Swartz 2001, Willson 2004, Brumfield et al. 2007). Other bird species attend raids opportunistically (Swartz 2001, Chaves-Campos 2003). Opportunistic army ant raid-attending bird species vary in their reliance on army ants (Willis 1972, Willis and Oniki 1978). Obligate army ant-following birds are agonistic toward other birds at raids in lowland forests. This interference competition reduces the value of ant raids as a food source to other birds (Willis 1966, Willis and Oniki 1978, Brumfield et al. 2007). Obligate army ant-following birds are poorly represented or absent from montane forests (Willis and Oniki 1978, Brumfield et al. 2007). There are no obligate army ant-following birds at our study site near Monteverde, Costa Rica (Kumar and O’Donnell 2007). Birds from a diverse array of families attend army ant raids in the Monteverde area, including some resident and Nearctic migrant species (henceforth migrants; Vallely 2001, Kumar and O’Donnell 2007). We hypothesized that some montane bird species would exhibit behavioral specializations for exploiting army ant raids in the absence of local competition from obligate army ant-following birds. We asked whether some montane bird species attend raids more often than expected as a first test of this hypothesis. The frequency of raid attendance varies among Monteverde area birds, but attendance frequency alone does not account for possible effects of local abundance (Vallely 2001, Kumar and O’Donnell 2007, Peters et al. 2008). We extended our previous analyses estimating the effects of species’ relative abundance on army ant raid attendance in this study. Bird abundance estimates were derived from previously published mist-net captures and point-count densities (Young et al. 1998, Jankowski et al. 2009). We predicted the most frequent raid attendant birds would be more likely to exhibit specialized behaviors for exploiting army ant swarms (Willis 1972, Willis and Oniki 1978, Swartz 2001, Chaves-Campos 2003). We ascertained whether 1 Animal Behavior Program, Department of Psychology, Box 351525, University of Washington, Seattle, WA 98195, USA. 2 University of Cambridge, Department of Experimental Psychology, Cambridge CB2 3EB, United Kingdom. 3 Corresponding author; e-mail: [email protected] The Wilson Journal of Ornithology 122(3):503–512, 2010