Red-winged Blackbird Nestlings Go Silent When Predators Are Near

Red-winged Blackbird nestlings alter their behavior to avoid detection when predators are nearby.

If you’re a predator that eats baby birds — say, an American Crow — eavesdropping on the begging calls of nestlings can be an easy way to find your next meal. But do baby birds change their begging behavior when predators are nearby to avoid being detected and eaten? Very few studies have investigated whether nestlings react to the sounds of predators, but new research published in The Auk: Ornithological Advances shows that when their parents are away, baby Red-winged Blackbirds beg less often and stop begging sooner if they hear recordings of predators’ calls.

Beloit College’s Ken Yasukawa and his students used recordings of both crows and Cooper’s Hawks to test the reactions of blackbird nestlings when one or both of their parents were nearby and when both parents were absent. When neither parent was around, nestlings who heard the calls of either predator adjusted their begging behavior to attract less attention. When a parent returned, however, nestlings begged noisily whether the researchers played predator recordings or not. Getting the food their parents had brought (and being noisier than their siblings in order to do so) was important enough that it outweighed the potential danger from predators.

Nest predation is so common that the researchers had trouble locating enough surviving nests to collect their data. “We spent a lot of time and effort searching for nests and then checking them daily, only to have more than half fail before the nestlings were old enough to be experimental subjects. Quite often we would go to a nest expecting to do a playback experiment, only to find that it no longer contained nestlings,” says Yasukawa. “We even had one case of snake predation during our playback. We know because we had a video recording of the snake removing the nestling!”

One surprise in the study’s results was that nestlings reacted to the call of Cooper’s Hawks as well as crows, even though hawks are not typically nest predators, preying on adult birds instead. The researchers suspect that adaptations to avoid Cooper’s Hawks predation have been so strongly favored by natural selection that they’re expressed at all ages.

Human activity can alter habitats in ways that make predators more common and make it easier to find nests, so deciphering the interactions between nestlings and predators can be crucial for planning conservation efforts. “Understanding this complex system is important, because human activity has been causing bird populations to decline,” says Yasukawa. “Even the very common Red-winged Blackbird has been affected by habitat conversion and destruction, as well as increases in rates of predation.”

Calling in the face of danger: do nestling Red-winged Blackbirds (Agelaius phoeniceus) suppress begging in response to predator playbacks? is available at https://academic.oup.com/auk/article-lookup/doi/10.1093/auk/ukz071.

About the journal: The Auk: Ornithological Advances is a peer-reviewed, international journal of ornithology published by the American Ornithological Society. The Auk commenced publication in 1884 and in 2009 was honored as one of the 100 most influential journals of biology and medicine over the past 100 years.

COMMENTS

  1. This possibility has been a concern for field ornithologists for many decades. For example, in 1979, Sarah Lenington showed that predation rates for Red-winged Blackbirds nesting in marshes increased from the first to the second year that nests were monitored, but she also showed far less of an effect for upland-nesting Red-winged Blackbirds (my birds nested in a prairie habitat). Frank Götmark published an extensive literature review in 1992 and found considerable evidence for such effects in many kinds of birds, but not for Passerines (perching birds), and a 2012 meta-analysis by Juan Ibáñez-Álamo, Olivia Sanllorente, and Manuel Soler showed no negative effects of researcher activity on nest predation. They even described cases in which such activity increased nest success. We attempted to reduce any negative effects by following different routes to nests, minimizing the time we spent checking each nest, wearing rubber boots and rain pants, and avoiding wearing anything with a strong odor, but we cannot say for certain that our activities had no effect.

  2. Thank you for the well stated reply. I was mainly wondering if this was a consideration in your analysis and I see that it was.
    As a second question you seem to imply that reaction to Cooper’s Hawk is an innate response. Is this your point of view on their behavior?

  3. Studies of Cooper’s Hawks have shown that they are important predators of birds, but although they are known to take nestlings, they are a minor item in the hawk’s diet at most. I have been observing Red-winged Blackbirds since 1973 and I have seen cases of Cooper’s Hawk predation on territorial adults. I also know that adult Red-winged Blackbirds react strongly to these hawks. If a Cooper’s Hawk is detected at a distance, adult Red-winged Blackbirds will give alarm calls that spread throughout the neighborhood, take flight, and circle above the hawk until it leave the area, but if the hawk is not seen until it is close by, adult Red-winged Blackbirds will move to cover, become silent, and, in the case of males, will conceal their red-and-yellow epaulets. In contrast, reactions to American Crows and Red-tailed Hawks include aggressive mobbing, probably because these predators represent no threat to adult Red-winged Blackbirds. These observations indicate that adult Red-winged Blackbirds recognize Cooper’s Hawks and respond appropriately given the risk these hawks represent. But because Cooper’s Hawks pose little treat to nestlings, it was surprising to us that Red-winged Blackbird nestlings responded so strongly to their calls. We suggested that this response is innate because it occurs so much earlier in development than the actual risk would predict. There are probably other explanations, however, and we would welcome any suggestions that our readers might offer.

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