Benjamin T. Walters, Tin Nok Natalie Cheng, Justin Doyle, Chistopher G. Guglielmo, Michael Clinchy and Liana Y. Zanette
Escaping from a predator is a matter of ongoing life or immediate death. While escape ability is clearly imperative for survival, the variables that impair or improve it are not well understood. Escape ability is frequently assumed to be dependent on body mass, whereby getting fat can compromise escape while slimming down enhances it. Consequently, prey living with a lot of predators around may be expected to strategically alter their body mass to enhance escape. This “mass-dependent predation risk hypothesis” makes good theoretical sense, especially for birds that fly to escape from a predator attack, requiring them to lift their entire body mass off the ground. If light birds do have more lift than fat birds do, then light birds should be superior escape artists.
We tricked birds living in semi-natural conditions into thinking that predators were around using sounds and taxidermic models to test whether scared prey alter both body mass and escape ability, and whether body mass has any effect on flying ability. We found that scared birds actually gained mass by packing on fat, which they lost again when exposed to non-predator cues. Although scared birds were heavier, this did not interfere with their ability to fly, and indeed, scared birds enhanced their escape prospects by taking off at steeper angles, relying on evasion rather than brute force. This experiment, and then a second one we conducted in the lab, both confirmed that no amount of mass loss makes any bird a better flier, and only individuals that became abnormally fat flew worse.
Our experiments reveal that predators cause prey to alter escape behaviour and actually gain weight but that the magnitude of weight gain is strategically orchestrated to ensure that flying ability is not compromised. Whenever a predator is perceived nearby, fatter prey can hide in safety and forgo looking for dinner to avoid becoming dinner. While fat gain is thus beneficial to frightened prey, the amount of fat gained is tempered so as not to tamper with flight, presumably because flight is just too important for predator escape.
Image caption: One of the large outdoor aviaries in which the experiment was conducted. The speaker boxes through which predator or non-predator sounds were broadcast are evident on the struts near the ceiling on the right-hand side. The apparatuses (2 in total) we used to measure the angle and speed of escape at take-off are evident on the ground. The study species (brown-headed cowbird) is evident in the far corner on the left.
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