Expert: Julia Booth - 3/24/2010

Question
please try to find the answer to the question as quickly as possible.
for five years now ive had a sparrow birdhouse and they have nested in it there have always been 4 eggs but every year the parents and i have watched them do this they take 2 of there fully fledged babys out of the nest and killed them the babys heads were off i am wondering why they kill there own young thanks

Answer
Infanticide among birds is not as rare as one would think.

I'm copying and pasting the text of what I believe to be the best article on this subject. It's a bit dull. For that I apologize.

Avian infanticide is recognized as an adaptive behavior which may increase the reproductive success of the perpetrator. Among birds, infanticide is often committed by individuals who replace other individuals on territories containing offspring of the previous resident. Replacement can occur after one member of a pair dies or deserts the territory, or through physical eviction of one member of a pair by a non-resident bird. Two hypotheses for the adaptive value of infanticide in this context have been proposed. First, by eliminating dependent offspring, the perpetrator may shorten the interval between replacement and the time that the resident is capable of starting a new breeding attempt. Second, destruction of offspring may provide the perpetrator access to a resource critical for attracting a mate, such as a nest site. Despite these potential benefits, replacements do not always commit infanticide. In some species, replacements have been observed to act indifferently towards resident offspring or provide them with some form of parental care. Rohwer (1986) outlined conditions under which infanticide, adoption or indifference would maximize the future reproductive success of a replacement male (i.e., by rapid acquisition of a mate). For short-lived species where the potential benefits of a response are not likely to be realized in subsequent breeding seasons, infanticide should be favored when only enough time remains in the breeding season to complete either the resident female's current breeding attempt or one new attempt. Infanticide will also be favored, regardless of the number of broods that can be raised that breeding season, when there is a high probability that the resident female will pair with the replacement male following infanticide, or if there is a high probability that the replacement male can attract a new mate to the territory if the resident female deserts. Indifference or adoption will only be favored over infanticide if the resident female represents the only mate a replacement male is likely to obtain that year, i.e., there must be a high probability that the resident female will desert the replacement male if he kills her offspring, and a low probability that the male will attract a new mate if the resident female deserts. In addition, enough time must remain in the breeding season to complete both the resident female's breeding attempt and at least one additional attempt.

And here is a link to a site which specifically discusses infanticide in House Sparrows, which might prove to be more informative for you.

http://beheco.oxfordjournals.org/cgi/content/full/15/2/219

Let me know if you need more information,

Julia