Saturday, April 27, 2013

Avian Infanticide

On April 24th, the turkey vultures in Missouri laid their first egg. The egg was captured by video-maker Priscillash and celebrated by cam watchers. We were all perplexed when the egg disappeared. What happened to it? A quick review of footage revealed the culprit - a male turkey vulture. He spent roughly 20 minutes rolling, banging, scraping, rubbing, and biting the egg on and off camera view. The barn's owner was not able to find the egg or eggshells, so we assume the vulture consumed the egg after cracking it.

This behavior appears to be infanticide. Although we haven't seen it at the nests we watch, it is not an uncommon behavior in birds. Avian infanticide may involve killing eggs or hatched offspring. It includes:
  • Exploiting young as food
  • Sexually-selected or paternity-influenced infanticide
  • Resource competition
  • Social pathology, gender selection, reproductive fitness
We've seen falcons and eagles exploit dead young as food, although we haven't seen them kill young for food - the difference between infanticide and cannibalism. In the case of the bald eagles at Fort St. Vrain, and the falcon Belinda at the Allen S. King plant, the dead young were readily available and the live young needed feeding. Although it seems awful to many human watchers, exploiting dead young as a resource very probably saved live young, especially at Fort St. Vrain, where the eagles were struggling with cold weather and a heavy snow fall that reduced food availability.

The vulture destroys the egg
The turkey vulture may have been committing sexually selected or paternity-influenced infanticide. He - we think it was a male given the brow ridge - could have destroyed the egg because it was not his egg. There is some very lively discussion going on about whether or not this male was last year's father. If he was, he may have destroyed the egg because it wasn't his egg. In this scenario, he arrived at the barn to find his mate had already paired with another male and laid an egg. Since the egg wasn't his egg, he destroyed it. If it wasn't the same male, he may have driven off the original male. Once again, since the egg wasn't his egg, he destroyed it.

In this scenario, paternity was uncertain. The male bird was assuring that all of the offspring produced in the barn would be his. The female could most likely still be fertilized if the egg was destroyed, given that she hadn't laid all of her eggs yet. We haven't seen re-clutching in the presence of eggs, but we have when failed eggs have been removed from the nest. By eating the egg, the male effectively removed it.

Infanticide can also be compelled by resource competition. In this scenario, a parent does not have enough food or nest space for all of its young.  It may kill weaker young or refuse to feed or incubate them. Although Cain and Abel syndrome occurs between offspring instead of adult-to-offspring, it is an example of resource competition that can result in death. As cruel as it seems, the young best able to adapt to limited resources are the ones who will survive.

Finally, we have the last trio: social pathology, gender selection, and reproductive fitness. Social pathology occurs when infanticide occurs for no discernible reason.  Social pathology is a highly maladaptive behavior that doesn't benefit the individual or group. While I have tried to avoid using loaded terms, the closest human analogy would be murder. Eclectus parrots practice infanticide based on sex: that is, mothers kill male offspring. And some birds appear to practice brood reduction to maximize reproductive success, killing particular offspring while leaving others that are presumably more fit. Less mouths are easier to feed. We talk about the role that natural and sexual selection play in driving specialization. Could infanticide also be playing a role?

While this behavior is very difficult for us to understand, it is part of what birds do. The social and emotional lives of birds are complex, and it is my feeling - and I stress feeling here - that birds and humans have some states that are analogous to one another, and others that are not. We shouldn't judge the birds for living their lives - this is not a part of bird life we've seen very often, but it is still a part of bird life. We'll keep watching the barn.

Things that helped me write about this topic: