Communal roosting

Communal roosting is an animal behavior where a group of individuals, typically of the same species, congregate in an area for a few hours based on an external signal and will return to the same site with the reappearance of the signal. Environmental signals are often responsible for this grouping, including nightfall, high tide, or rainfall. The distinction between communal roosting and cooperative breeding is the absence of chicks in communal roosts. While communal roosting is generally observed in birds, the behavior has also been seen in bats, primates, and insects. The size of these roosts can measure in the thousands to millions of individuals, especially among avian species.

There are many benefits associated with communal roosting including: increased foraging ability, decreased thermoregulatory demands, decreased predation, and increased species-specific interactions. While there are many proposed evolutionary concepts for how communal roosting evolved, no specific hypothesis is currently supported by the scientific community as a whole.

Proposed by Peter Ward and Amotz Zahavi in 1973, the Information Center Hypothesis (ICH) states that bird assemblages such as communal roosts act as information hubs for distributing knowledge about food source location. When food patch knowledge is unevenly distributed amongst certain flock members, the other “clueless” flock members can follow and join these knowledgeable members to find good feeding locations. To quote Ward and Zahavi on the evolutionary reasons as to how communal roosts came about, "...communal roosts, breeding colonies and certain other bird assemblages have been evolved primarily for the efficient exploitation of unevenly-distributed food sources by serving as ' information-centres.' " Currently, there is only speculation as to how the information is conveyed. It has been suggested that the successful members first convey their knowledge through displays and the unsuccessful members then follow, or that the unsuccessful members circle in the air or slowly fly out and proceed to join the successful members when they take off. Ward and Zahavi approached the explanation for the ICH in several different ways, but each explanation is related to the ability to distribute knowledge of resources. In this case, roosting can be divided into several components: advertisement, synchronized breeding, seasonality, mood, and predation.

Ward and Zahavi cited a variety of species that demonstrate behaviors supporting the ICH but no species exhibited all of the behaviors. Red-winged blackbirds (Agelaius phoeniceus) exhibit synchronized breeding patterns, as well as have displays to attract birds to join the same nesting site. An observation Ward and Zahavi had done of red-billed queleas and the cattle egrets showed altered behavior after individually failing to find food in the morning and later in the afternoon. However, after resting in a secondary roost, these birds that failed joined other birds and flew off in a completely different direction. White wagtails and cattle egrets are two species proposed to demonstrate advertisement, with their coloring and pattern making them very noticeable and the two species often choosing open places for assemblages. There is also a large variety of finch species that have large, long-term roosting sites as well as large food searching areas.

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