SOME BIRDS TAKE CARE OF SIBLINGS

by Whit Gibbons

March 22, 2009


Spring is the time to enjoy nature's productivity. As we approach the official first day of spring each year (around March 21), we can hear more birds singing, see pairs flitting here and there, and watch nest-building activities. Wrens started a nest in our garage this week, which means we have to leave the doors open till the young leave. It's worth it to be able to watch the birds and their fledglings.

But one aspect of nesting birds is less than enjoyable and might send chills flying through a typical bird-watcher: siblicide, the killing of a brother or sister. Siblicide in the nest has been documented for several bird species. Siblicidal birds are not cannibalistic. They do not eat their siblings; they just kill them.

Apparently, siblicide is the order of the day for some birds. For example, the hatchlings of black eagles of Africa and the Middle East seem to be a species on a death mission. Black eagles lay two eggs. Yet of two hundred nests examined by a research team, only one contained two young that survived to fledge. The deaths resulted from direct attack by the victim's sibling. In one nest, the larger eaglet pecked the smaller one 1,569 times before death occurred. In no instance did the parent birds become involved in fights between siblings; maybe they assumed that baby birds will be baby birds.

Black eagles are not the only birds that engage in such behavior. According to studies, siblicide is characteristic of many other birds, including pelicans, ospreys, and other eagles that lay two eggs. Some egrets are also notorious killers of their siblings in the nest. One scientific publication documented that a pair of young cattle egrets pecked a third one senseless. The helpless victim was driven out of the nest. The evicted baby finally fell to the ground and died.

In searching for similarities among bird species in which siblicide occurs, it comes as no surprise that birds that kill one another in the nest possess the weapons to do so. Large, pointed or hooked beaks aid in pecking one's nestmate to death. Also, such species are confined to an enclosed nest area, in contrast to many other birds, such as killdeer and bobwhite quail that lay their eggs on the ground. In these, the young scatter soon after they hatch. Perhaps they heard cautionary tales about what happens to little birds that hang around the nest too long.

Another aspect of siblicide relates to food resources. Siblicidal species are usually ones for which an adequate supply of baby bird food cannot always be assured. This is not the result of parental neglect; it is caused instead by a situation, such as drought, in which parents cannot find the constant, abundant supply of food necessary to keep all their young healthy and happy. This sets up a competitive relationship for the babies.

A trait identified as very important in siblicidal behavior is that some disparity in size exists between the nestmates. For example, in the black eagle, the mother hatches the two eggs three days apart, assuring that one will be larger than the other. Then, if food supplies are low in a particular year, the smaller nestling is killed by the other. If the first nestling is unhealthy in some way, the second one becomes dominant. Either way, the parents can count on at least one well-fed offspring.

In seasons when food is plentiful, more than one nestling may be fed successfully and siblicide may not occur. From the human perspective, siblicide as a means to ensure survival of the fittest may seem severe. But the phenomenon is widespread in the bird world and is clearly successful.

This spring, if you see a baby bird that's fallen from the nest, it may not represent an accident but a decisive act by a sibling. But don't pass judgment on such behavior. The actions of any animal species, even if they seem cruel to us, are generally those behaviors with the highest probability of passing the parents' genes on to the next generation. Nature's ways are exceedingly complex.


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