Featured paper: nest predation

Corey Tarwater. 2008. Predators at nests of the Western Slaty Antshrike (Thamnophilus atrinucha). The Wilson Journal of Ornithology, volume 120(3): pp 620-624.

Nest predation is a major worry for most birds. Chicks in the nest are essentially helpless, and parents usually can’t do much to stop predators, although they will try. Losing broods to predation is common. In this study, 79% of nests were lost to predation. If you’re a Western Slaty Antshrike, you can expect 4 out of every 5 broods of chicks you have to get eaten!

Yet actually seeing these—or any—predation events is extremely rare. Dr. Tarwater describes five nest predation events on Western Slaty Antshrikes (small, cryptic brownish birds) in Panama, one she observed in person and the rest of which she caught on video.

The descriptions of the events themselves are as sad to read as you might imagine, so I won’t subject you to that. Less grim to consider is the response of the parents in each case. Western Slaty Antshrikes, like many tropical birds, have a relatively low mortality rate as adults. This means that although losing a brood to predation is bad, they will probably survive long enough to try again; it is much worse for an adult to die than for it to lose a brood. (Contrast this with House Wrens in temperate New York, which often live only one year, and have only one or two shots at reproduction.)

So the adults are careful around predators. When a Keel-billed Toucan eats their nestlings, they stay away, alarm calling from a distance. For a White-faced Capuchin (a monkey), the incubating female flees the nest, and barely makes it out before the capuchin reaches in to grab the eggs. The capuchin would no doubt have eaten her too if it had caught her. However, when a Fasciated Antshrike comes to peck at the eggs, the male slaty antshrike attacks, striking the invader three times. The Fasciated Antshrike is no risk to the slaty, so it’s safe to attack (although it doesn’t work).

The parents are nowhere to be seen or heard during predation events by a Double-toothed Kite and a snake. It’s unclear whether they know that a predation event is occurring; but certainly a kite or a snake would be a risk to an adult, and it is a good idea to stay away.

Another striking aspect of this paper is the parents’ behavior once their broods are lost. Sometimes they call at the nest. Sometimes they hop in and out, as if searching. One female returns to her empty nest and sits on it for 61 minutes, as if the nestling is still there.

Documenting these five predation events is a great feat. Science nowadays is not particularly welcoming to papers like this one. These predation events are so rare to witness that they cannot be replicated or used to test a hypothesis—at least not without decades of field work—and direct hypothesis testing is the gold standard these days. Yet it’s exactly because they’re so rarely seen that it is essential that these events be documented. Predation exerts strong selective pressure on these birds and is a major part of their lives; we should know what it looks like.

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