Communal roosting
[4][6] 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.
It states that bird assemblages such as communal roosts act as information hubs for distributing knowledge about food source location.
This hypothesis proposes that different individuals join and participate in communal roosts for different reasons that are based primarily on their social status.
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.
[2] Proposed by Heinz Richner and Philipp Heeb in 1996, the recruitment center hypothesis (RCH) explains the evolution of communal roosting as a result of group foraging.
The RCH also explains behaviors seen at communal roosts such as: the passing of information, aerial displays, and the presence or lack of calls by leaders.
Group foraging decreases predation and increases relative feeding time at the cost of sharing a food source.
The decrease in predation is due to the dilution factor and an early warning system created by having multiple animals alert.
In the presence of patchy resources, Richner and Heeb propose the simplest manner would be to form a communal roost and recruit participants there.
A study by Guy Beauchamp explained that black-billed magpies (Pica hudsonia) often formed the largest roosts during the winter.
Communal roosting in this case would improve their reactivity by sharing body heat, allowing them to detect and respond to predators much more quickly.
[4] A large roost with many members can visually detect predators easier, allowing individuals to respond and alert others quicker to threats.
Similar to the selfish-herd theory, communal roosts have demonstrated a hierarchy of sorts where older members and better foragers nest in the interior of the group, decreasing their exposure to predators.
[17] Small scale communal roosting during the winter months has also been observed in Green Woodhoopoes (Phoeniculus purpureus).
[20] Eurasian crag martins (Ptyonoprogne rupestris) also form large nocturnal communal roosts during the winter months.
As with the tree swallows, research has shown that Eurasian crag martins also exhibit a high degree of fidelity to the roost, with individuals returning to the same caves within and between years.
It has been shown that the snowy egret determines the general location of the roost because the other three species rely on it for its abilities to find food sources.
In these roosts there is often a hierarchical system, where the more dominant species (in this case the snowy egret) will typically occupy the more desirable higher perches.
[1] Communal roosting behavior has also been observed in the neotropical zebra longwing butterfly (Heliconius charitonius) in the La Cinchona region of Costa Rica.
These species of tiger beetle have been observed to form communal roosts comprising anywhere from two to nine individuals at night and disbanding during the day.
