Whether birds share nests is not a simple yes or no answer, as the behavior depends entirely on the species and the context of the sharing. While many birds build and defend a nest exclusively for their own eggs and young, numerous species use complex social strategies. These strategies involve sharing the nesting structure, the duties of raising young, or the space used for sleeping. Understanding this behavior requires separating the three distinct ways birds engage in “nest sharing.”
Shared Nests for Laying Eggs (Communal Nesting)
Communal nesting is the most literal form of sharing, where multiple females deposit eggs into a single, shared nest structure. This strategy is relatively rare among birds but is notably practiced by species like the Greater Ani and the Ostrich. The primary motivation is often increased defense against predators, as a larger group of parents can more effectively guard the clutch.
The Greater Ani of Central and South America is a classic example, with two to five socially monogamous pairs laying eggs in one large cup nest. All group members share the duties of incubation and feeding the mixed clutch of nestlings. This group effort significantly increases the chance that the nest will successfully fledge young, as solitary pairs are rarely successful due to high predation rates.
Communal nesting involves costs, particularly fierce reproductive competition among the females. Prior to synchronized incubation, females often compete by ejecting the eggs of other group members from the nest. For the Greater Ani, the earliest-laying females are disproportionately affected by this egg rejection, which limits the group size.
Ostriches also exhibit communal nesting, where multiple unrelated females lay their eggs in a single nest scraped into the ground by the male. While the nest may hold up to 60 eggs from various females, only the dominant female and the male remain to incubate the joint clutch. The dominant hen ensures her own genetic success by recognizing her eggs and arranging them safely in the center of the nest, pushing the eggs of minor hens to the less protected periphery.
Shared Duties in Family Groups (Cooperative Breeding)
Cooperative breeding is a different form of sharing, where a breeding pair is assisted by non-parent birds, often called “helpers-at-the-nest.” Only one female, the primary breeder, typically lays eggs in the nest, distinguishing it from communal nesting. Helpers are usually older offspring from previous seasons who have delayed their own reproduction.
The helpers contribute substantially to the reproductive effort by feeding nestlings, defending the territory, and maintaining the nest structure. This assistance significantly increases the reproductive success of the breeding pair, allowing them to raise more young than they could alone. Furthermore, the reduction in parental care workload can also increase the lifespan and future survival rate of the breeders.
The evolutionary advantage for helpers is explained by inclusive fitness, as they assist close relatives with whom they share genes. By ensuring the survival of their siblings, the helper indirectly passes on their own genetic material. This behavior is also a practical strategy when suitable territories or mates are scarce, providing helpers with parental experience and a potential future inheritance of the territory.
Sharing Shelter for Safety and Warmth (Social Roosting)
The final type of sharing involves using a structure for rest, warmth, or safety, separate from reproductive activity. Social roosting is the aggregation of multiple individuals, often in large numbers, to rest together during periods of inactivity. Unlike the shared effort of breeding, roosting is a temporary, non-reproductive use of space.
One of the primary reasons for this mass aggregation is thermoregulation, especially during cold winter nights. Birds like Long-tailed Tits and Common Wrens huddle together in groups, which drastically reduces the individual heat loss and energy required to survive the night. Individuals occupying the inner positions of the huddle benefit the most from this shared body heat.
Communal roosts also function as a defense mechanism, where a large group provides greater protection against predators through heightened collective vigilance. These large gatherings can also act as information centers, allowing less successful individuals to locate others who know the location of abundant food sources, increasing the foraging efficiency of the group. Species such as European Starlings, Crows, and Red-billed Queleas are known for forming roosts that can number in the millions.