Anti-Predator Behavior in Birds: Reactions to Raptor Calls
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Anti-predator behavior in Birds: Reactions to Raptor Calls Robin Elahi Department of Biology, Northeastern University _____________________________________________________________________________________ ABSTRACT Birds display a number of defense mechanisms against their predators, including camouflage, mobbing, fleeing, and aggregational behavior. I studied the responses of 32 bird species to songbird and raptor calls to see if birds displayed anti-predator behavior in response to raptor calls in comparison with songbird calls. One focus of my study was avian defensive strategies, namely flocking and concealment, so observations took into consideration different conditions: whether the bird was solitary or in a group, and whether it was exposed or hidden in vegetation. A previous study showed that crows could distinguish between raptor calls of their predators and non-predators, so I examined if birds specifically recognized resident raptors, or if they had a general response to resident and non-resident raptor calls. I also attempted to correlate bird size with reaction, specifically to see if smaller birds flew away more often than larger birds due to a greater perceived threat. For each bird or group of birds. I played a songbird call, followed by a raptor call, and timed the raptor call to see how long the bird stayed till it flew away. Birds flew away 27% of the time in response to raptor calls, and only 3% of the time to songbird calls (X² = 16.11, df = 1, n = 26). Birds did not distinguish among resident and non-resident raptor calls, and had nearly equal flight frequencies (X² = 0.48, df =3, n=40). Trends showed that solitary and exposed birds tended to fly more often than birds in groups and hidden birds. Bird size was not a reliable indicator of reactions, although a trend demonstrate that average bird size was larger for flight reactions, than for no reactions or look reactions (One-way ANOVA, p = 0.1314). These results suggest that birds use the anti- predator strategies of flocking and concealment, however, further research should be done on raptor call recognition on a single genus or species of birds to determine if raptor calls recognition is specific on a lower taxonomic level as previous studies have suggested. RESUMEN Los pájaros exhiben una variedad de mecanismos defensivos contra sus predadores, incluyendo camuflaje, ataque, huir o hacer grupos. Estudie las reacciones de 32 especies de pájaros al sonar cantos de aves canoras y aves de rapiña, para ver si los pájaros exhibieron un comportamiento anti-predador. El enfoque de mi estudio fueron las estrategias de las aves, específicamente en grupos y escondidos, por eso en las observaciones se tomaron en consideración si los pájaros estuvieron en grupo o solitarios o si estuvieron escondidos en la vegetación o no. Un estudio anterior demostró que los cuervos distinguieron entre las llamadas de rapaces, por eso examine si los pájaros específicamente distinguieron entre llamadas de aves de rapiña residente y no-residente, o si tuvieron una respuesta general a todas las llamadas de ave de rapiña. También trate de tomar en cuenta el tamaño del pájaro con la reacción, específicamente si los pájaros pequeños volaron más que los pájaros grandes. Para cada pájaro o grupo de pájaros, soné una llamada de ave canora e inmediatamente después una llamada de ave de rapiña y conté el tiempo que le tomo al pájaro volar. Los pájaros volaron en 27% del tiempo en respuesta a las llamadas de rapiña y solamente 3% del tiempo a las llamadas de aves canoras (X² = 16.11, df = 1, n = 26). Los pájaros no distinguieron entre las aves de rapiñas residentes y no-residentes y tuvieron casi la misma frecuencia de vuelo (X² = 0.48, df = 3, n = 40. Las tendencias mostraron que pájaros solitarios y expuestos volaron más que las aves en grupo o escondidas. El tamaño del pájaro no fue un indicador para las reacciones pero una tendencia mostro que el tamaño fue un factor para cuando volaron, que para cuando no lo hicieron o para cuando solo miraron (One-way ANOVA, p= 0.1314). Estos resultados sugieren que los pájaros usan estrategias anti-predador, pero se debe estudiar más el reconocimiento de una especie de pájaro de cómo distingue las llamadas de aves de rapiña para determinar si el reconocimiento es específico en especies como mostraron los estudios anteriores. INTRODUCTION Predators confer Strong selective pressure on their prey, resulting in varied defense mechanisms. Animals have developed two basic types of strategies to prevent predation – anti-detection and anti-capture (Alcock 1984). Anti-detection strategies rely on remaining invisible to the predator, through cryptic coloration and background matching. This is seen in many different taxa, such as lizards, insects and birds. Anti-capture strategies are behaviors displayed after the prey has been spotted by their predator, such as flash colors, misdirecting an attack, chemical repellants, Batesian mimicry, fleeing and fighting back (Alcock 1984). Birds avoid predation by raptors using both anti-detection and anti-capture strategies. Many birds, such as Amazonia parrots, blend in with their environment and are very difficult to see, which serves as an anti-detection strategy. Once a raptor has spotted its prey, anti-capture strategies include diving into thick bushes or water, and simply fleeing and dodging. These actions are often accompanied by alarm calls (Perrins 1979). Birds have also evolved anti-capture behaviors such as mobbing and aggregational behavior. For example, European kestrels mobbed by foraging birds flew significantly farther from foraging areas, and thus supports the assumption that mobbing decreases predatory efficiency, mainly by driving the predator from the vicinity (Pettifor 1990). Aggregational behavior has been shown to be an effective defense against predators in numerous studies on leks, foraging flocks, and escape tactics (Buchanan et al. 1988, Caldwell 1986, Glodman 1980, Trail 1998). Flocks provide increased vigilance because of the greater number of eyes to spot predators. As a result, individuals can increase the time they spend foraging since they spend less time watching for danger. Lima (1998) has shown that solitary birds that are not overtly vigilant (i.e. birds that are feeding) are more vulnerable to predator attack. Additionally, the selfish-herd hypothesis states that an individual’s chances of being eaten decrease when they join a group, simply because of probability (Terborgh 1989). The dilution effect explains that groups will likely satiate their predators, and again, the individual’s chances of survival increase due to probability, since only a few prey will be taken (Alcock 1984). Since aggregational behavior and concealment are key defense mechanisms for birds, I tried to measure the effects of these variables on the reaction of birds in response to songbird and raptor calls. The reactions to songbird calls provide a control to compare the reactions of raptor calls. Based on avian anti-predator strategies, solitary birds should fly away more often and more readily than birds in groups. Likewise, exposed birds should fly away more often than hidden birds. I am also interested in whether birds respond specifically to the calls of resident raptors, or whether they have a general raptor call response. Western American Crows (Corvus brachyrhynchos hesperis) not only distinguish between raptor and non-raptor calls, they can also distinguish the calls of Red-Shouldered Hawks (Buteo lineatus), a major predator, from the Madagascar Harrier Hawk (Gymnogenys radiatus) that has a spectrally similar call (Hauser and Caffrey 1994). Therefore, it appears birds will distinguish between songbirds and raptors, and that at least some birds will recognize the calls of raptors that are common in the area, and as a result, fly more often and more readily from those calls. The Great Blawk-Hawk (Buteogallus anthracinus) and the Collared Forest-Falcon (Micrastur semitorquatus) were chosen as representative of the San Luis area, while the Semiplumbeous Hawk (Leucopternis seminplumbea) and the Slaty-Backed Forest-Falcon (Micrastur mirandollei) frequent other areas, mainly the Caribbean slope (Mauricio Ramirez, Manuel Leitón, pers. comm.; Fogden 1993, Stiles and Skutch 1989). As a last inquiry, I wished to examine the relationship between bird size and reaction. Only larger birds were found to mob kestrels because it seemed unlikely that the kestrels posed a great threat to them (Pettifor 1990). I hypothesize that smaller birds will fly more often than larger birds, because of the greater perceived threat. MATERIALS AND METHODS Calls were played to 87 birds of 32 different species in San Luis Arriba and Invu, Costa Rica, located on the Pacific slope of the Tilarán mountain range at 1100m elevation between October 25 and November 14, 2000. The Holdridge life zone is premontane wet forest (Holdridge 1967, Haber et al. 2000). Data were collected from 5:30 – 8:30 AM and 3:00 – 5:00 PM, along roads and trails in pasture, secondary growth forest and gardens. I walked trails, stopping to make observations at certain points for durations of five to twenty minutes depending on bird activity. Once a bird was perched and identified using binoculars, I played a call sequence once, at medium to high volume depending on distance from the observed bird (i.e. volume increased with increased distance) with a portable cassette player, pointed straight up in the air. The distance ranged from two meters to 25m. There were four different call sequences, each consisting of a songbird call, immediately followed by a raptor call, as follows (time in seconds is noted in parentheses): Sequence 1: Tropical Pewee (39s), Great Blawk-Hawk (39s) Sequence 2: Buff-Throated Wood-Creeper (44s), Semiplumbeous Hawk (35s) Sequence 3: Striped-Breasted Wren (45s), Collared Forest-Falcon (54s) Sequence 4: Yellow-Faced Grassquit (29s), Slaty-Backed Forest-Falcon (33s) The specificity of bird responses to native raptors was tested by sequences one and three, and to non-native raptors by two and four, as explained above.