Life Lines

Nest Decorations

Bowerbirds are certainly my favorite examples of animals that build elaborate courtship and mating structures, such as the structure shown below being decorated with blue bottle caps by a male satin bowerbird (image credit: Alan Fear).

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Also check out this video clip from the BBC showing bowerbirds in the midst of decorating.

Home decorating may not be simply for mate attraction and courtship, however. A recent study of black kites (Milvus migrans) a species of raptor, shows evidence for increased offspring survival with highly decorated nests. In black kites, breeding is usually controlled by social dominance but threats to breeding may include individuals from the same species attempting to takeover territories, trespass, or steal food resources.

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Black kite nests are built by mating pairs of birds and are often decorated with man-made objects that are mostly white and plastic. Researchers studied the nests of these birds in Donana National Park to determine if the decorations might be used for more than simply mating or courtship. What they found was that nest decorating was correlated with age, such that young birds built sparsely decorated or undecorated nests. Nest decorations peaked around 10-12 years old and then declined again as the birds aged.

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The number of offspring raised and their survival were actually higher with greater nest decorations. Trespassers were also found to avoid nests with more decorations resulting in less interactions and time spent away from the nest fighting. The authors concluded that the amount of nest decorations is thought to relay information about social dominance and fighting ability, thereby reducing the risk of trespassers.

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Source:
Sergio et al., Raptor Nest Decorations Are a Reliable Threat Against Conspecifics. Science 331: 327-329, 2010.