BIRDS TAKE CARE OF SIBLINGS
March 22, 2009
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.
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.
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.
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.
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.
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.
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.
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
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