page_title: Communal roosting
section_title: The two strategies hypothesis
hierarchical_section_title: Communal roosting / Evolution / The two strategies hypothesis
caption_attribution_description: English: A stylized example of a communal roost in birds with dominant individuals occupying the higher and safer positions.
context_page_description: 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 conspecific 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.
context_section_description: The two strategies hypothesis was put forth by Patrick Weatherhead in 1983 as an alternative to the then popular information center hypothesis. This hypothesis proposes that different individuals join and participate in communal roosts for different reasons that are based primarily on their social status. Unlike the ICH, not all individuals will join a roost in order to increase their foraging capabilities. This hypothesis explains that while roosts initially evolved due to information sharing among older and more experienced foragers, this evolution was aided by the benefits that more experienced foragers gained due to the fact that as better foragers they acquired a status of high rank within the roost. As dominant individuals, they are able to obtain the safest roosts, typically those highest in the tree or closest to the center of the roost. In these roosts, the less dominant and unsuccessful foragers act as a physical predation buffer for the dominant individuals. This is similar to the selfish herd theory, which states that individuals within herds will utilize conspecifics as physical barriers from predation. The younger and less dominant individuals will still join the roost because they gain some safety from predation through the dilution effect, as well as the ability to learn from the more experienced foragers that are already in the roost.
  Support for the two strategies hypothesis has been shown in studies of roosting rooks (Corvus frugilegus). A 1977 study of roosting rooks by Ian Swingland showed that an inherent hierarchy exists within rook communal roosts. In this hierarchy, the most dominant individuals have been shown to routinely occupy the roosts highest in the tree, and while they pay a cost (increased energy use to keep warm) they are safer from terrestrial predators. Despite this enforced hierarchy, lower ranking rooks remained with the roost, indicating that they still received some benefit from their participation in the roost. When weather conditions worsened, the more dominant rooks forced the younger and less dominant out of their roosts. Swingland proposed that the risk of predation at lower roosts was outweighed by the gains in reduced thermal demands. Similar support for the two strategies hypothesis has also been found in red-winged blackbird roosts. In this species the more dominant males will regularly inhabit roosts in thicker brush, where they are better hidden from predators than the less dominant individuals, that are forced to roost at the edge of the brush.
  The TSH makes several assumptions that must be met in order for the theory to work. The first major assumption is that within communal roosts there are certain roosts that possess safer or more beneficial qualities than other roosts. The second assumption is that the more dominant individuals will be capable of securing these roosts, and finally dominance rank must be a reliable indicator of foraging ability.
caption: A stylized example of a communal roost under the two strategies hypothesis, with the more dominant individuals occupying the higher and safer roosts.
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