Brood reduction

Infanticide Brood reduction often occurs as infanticide, the killing of nestlings by members of the same species. Infanticide can be done by siblings, which is referred to as siblicide, or by the parents, which is called filial infanticide. In virtually all species, brood reduction only occurs via siblicide, however in some species such as the white stork, filial infanticide has been observed. Siblicide often occurs via harassment and intimidation, preventing the smaller chicks from having enough food. Brood reduction by means of siblicide can be facultative or obligate. Facultative brood reduction depends on the conditions of that particular year, and only occurs when there is a limit to the resources available for the nestlings. Obligate brood reduction always occurs and does not depend on food availability. Hatching asynchrony results in a dominance hierarchy between the nestlings, which often leads to the death of the youngest sibling. Parasitic nestlings can also cause brood reduction by out-competing the host species nestlings, or by simply killing them. Two species of cowbirds, the shiny cowbird, and the screaming cowbird, frequently parasitize the nests of the brown-and-yellow marshbird, resulting in an increased amount of brood reduction in this species. == Lack's hypothesis ==

The brood-reduction hypothesis was first proposed by David Lack in 1947 to explain the evolution of hatching asynchrony and dominance hierarchy in the broods of some bird species. Hatching asynchrony would have no cost during years when resources are plentiful, and the whole brood would survive. Criticisms of the hypothesis Lack's assumption that there is no cost associated with hatching asynchrony during years where food is plentiful can not be proven. Studies show that competition remains a factor even when there are enough resources and that the last nestling to hatch is still at a disadvantage. In orders to test Lack's hypothesis, brood reduction must be adaptive, which can be determined by comparing the success of asynchronous broods and synchronous broods. Birds that have naturally synchronous broods must have higher nestling mortality when there are insufficient resources. It has been suggested that this can be tested using artificially synchronous broods, however, this artificial manipulation may have an effect on resource use. Also, in order to test this, the brood reduction threshold must be crossed. In other words, the conditions must be poor enough for brood reduction to occur. It is also possible that brood reduction could occur even when resources are plentiful, in this case being a maladaptive trait. Another criticism of the hypothesis is that there is no general threshold for brood reduction; conditions are not static and are not always the same for all individuals. An alternate hypothesis that builds off of Lack's is the Offspring Quality Assurance Hypothesis. This hypothesis states that the benefit to hatching asynchrony is the hierarchy created from it. The hierarchy guarantees that at least some of the nestlings will survive, and whether or not some of the weaker nestlings die is irrelevant. == References ==