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Egg tossing (behavior)

= Egg tossing (behaviour) = From Wikipedia, the free encyclopedia

Egg tossing or egg destruction is a behaviour observed in some species of birds where one individual removes an egg from the communal nest. This is related to infanticide, where parents kill their own or other's offspring. Egg tossing is observed in avian species, most commonly females, who are involved with cooperative breeding or brood parasitism. Among colonial non-co-nesting birds, egg-tossing is observed to be performed by an individual of the same species, and in the case of brood parasites this behaviour is done by either the same or different species. The behaviour of egg tossing offers its advantages and disadvantages to both the actor and recipient.

Behaviour
Tossing of eggs is non-accidental; the individual rolls the egg to the edge of the nest by repeatedly flicking it with its beak. In brood parasitic birds, such as the common cuckoo, the chick will push host eggs out using its back. During co-nesting, before a bird starts laying its own eggs it will toss out other eggs laid previously by another female. As a result, the last egg-layers may contribute more eggs to the common nest and this will increase the chances that newly laid eggs bearing the genetic material of that female will have a better chance of survival. In some species, egg-tossing is a strategy of clutch coordination; eggs are tossed until all birds in the common nest are ready to proceed with brooding. This helps to prevent early egg-layers from dominating reproduction.

Species[edit]
Some examples of communal breeders that demonstrate the egg tossing behaviour are: ostriches, grooved-bill anis, acorn woodpeckers, the gray-breasted jay, the guira cuckoo, the smoothed-bill anis and the common cuckoo.

Advantages[edit]
Performing the egg tossing behavior increases the number of offspring per individual compared to those in single pairs. Many species have learned to adapt to this behaviour to increase the chances of offspring survival.

The smooth-billed ani is one species that participates in communal breeding, where there are multiple females in a group. This has shown that the number of eggs produced per individual is greater in comparison to single female groups. Reasoning behind this is due to the higher competition between females, to have their own eggs successfully hatch, and from the large amount of egg loss. When there are more females in a group the majority of egg loss is due to egg tossing.

The acorn woodpecker showed that when in a group of 7-8 individuals, the success rate of reproduction increased, but would decrease if more members joined the group. When there were two females in the clutch, the success rate would decrease compared to a single female clutch due to conflicts such as egg tossing.

In the guira cuckoo, up to 7 females share a nest and perform egg tossing behaviour. Eggs that are laid in the early period of production are more likely to be tossed out of the nest by another female. When the group size increases, the behaviours that attempt to disrupt egg hatching or laying by others increases.

Disadvantage[edit]
Laying eggs late prevents the chicks from being tossed out of the nest, but it can have a negative impact on the offspring survival. Late egg laying causes later hatching which increases the probability of death, since these late chicks will be smaller than their nest mates, putting them at risk.

Adaptation[edit]
In the acorn woodpecker it has been observed that due to egg destruction behaviour, it causes egg laying to become synchronized between females. This synchronization of egg laying allows for all females to have the same opportunity to have a similar number of eggs in the nest. The larger the communal breeding group is, the longer it takes for the synchronization to occur.

Ostriches are usually found in a group of two to seven and there is only one major hen which will incubate the nest with the single male. The female ostriches will lay their eggs at the same time, leading to having too many eggs in the nest. The major hen is able to detect which eggs belong to her and will push the other eggs on the perimeter of the nest, which is not looked after. This adaption of abandoning these eggs protects the well kept eggs from predators.

The grooved bill anis and guira cuckoo are species that will stop tossing eggs once they have started to produce eggs in the nest. This behaviour prevents them from unknowingly tossing one of their own eggs from the nest.

Egg tossing by brood parasites[edit]
There are several species that will increase their offspring's chance of survival through a means that is slightly different than egg tossing, which is brood parasitism. These species will lay their eggs in nests of different species, allowing the offspring to survive without their direct contributions. Some bird species that exhibit this behaviour are the Black-headed duck, the common cuckoo and cowbirds. There are two types of brood parasitism; one which the females lay their eggs in the nest of the same species and one where the eggs are laid in the nest of a different species.

The common cuckoo is a species of cuckoo that exhibits brood parasitism in the nest of a different species. They accomplish this by watching the nest of a potential host and once the host leaves the nest the female cuckoo will remove one of the host's eggs and will replace it with one of their own. The female cuckoo will have no part in taking care of her offspring, instead she will leave the host's nest and look for another nest which she can lay more eggs. The common cuckoo will often stay in the nest and take advantage of the feeding of the host mother, even after the cuckoo is much larger and evidently not the host's offspring.

A common species nest that the cuckoo will choose to place its eggs in, is the Reed Warbler. The common cuckoo distinguishes the warblers nest and will choose what specific nest to brood in depending on the foliage and distance from the nest.

The common cuckoo demonstrates the egg tossing behaviour when they are just hatchlings. Once the cuckoo eggs are placed into the host nest and they hatch, they will evict the other species eggs out of the nest by pushing them out with their backs. This behaviour is very beneficial for the cuckoos survival as they are able to grow and feed without any competition from other members of the nest.

The cowbird is another parasitic species that lays their eggs in a different species' nest; the eastern phoebe. Although the cowbirds eggs differ in size and colour, the eastern phoebe will still choose to provide parental care unless there is a partial clutch reduction, or PCR.

There are different methods that brood parasites use to trick the host into raising their child, however some host's have developed counter adaptations to these. The adaptation between the host and brood parasites is an example of co-evolution.

References