![]() These colonies consist of multiple nesting chambers and are used as roosts when birds are not breeding. ![]() We use extensive year-round data to investigate the thermal benefits of massive colonial structures built by sociable weavers in the arid savannahs of the Kalahari, South Africa. Nest structures that help buffer against extreme temperatures may play a crucial role in managing the costs of thermoregulation, especially those nests that are used and maintained year-round. In hot, arid environments, maximum daily temperatures can exceed a species’ upper critical temperature threshold and during the non-breeding season temperatures may also drop below freezing. Nests can serve a particularly important role in protecting or buffering birds from weather. The primary role of nests as structural support for eggs, nestlings, and incubating parents is well established, but our understanding of their secondary roles and their adaptive features is still limited. Despite the positive effects for native species, monk parakeets also facilitate nesting opportunities to other non-native species and may also transmit diseases to tenants, highlighting the complexity of biotic interactions in biological invasions. Multispecies communal nests triggered interspecific aggression between the monk parakeet host and its tenants, but also a cooperative defense against predators. Rural and invaded areas showed the highest abundance and richness of tenant species. A total of 2690 nests from 42 tenant species, mostly cavity-nesting birds, were recorded in 26% of 2595 monk parakeet nests. We compared the role of the monk parakeet as a nest-site facilitator in different rural and urban areas, both invaded and native, across three continents and eight breeding seasons. However, the ecological effects of these interspecific interactions are barely known. The monk parakeet (Myiopsitta monachus) is a worldwide invader and the only parrot that builds its own communal nests, which can be used by other species. Invasive ecosystem engineers can disrupt recipient environments however, they may also facilitate access to novel resources for native species. While most of the knowledge on invasive species focuses on their impacts, little is known about their potential positive effects on other species. Finally, we identify potential pathways that may have led to the evolution of nesting associations, discuss some possible implications for the populations involved and argue that nesting associations provide many novel research opportunities. In two cases the nesting association is obligatory for one of the species but there was also evidence for significant local adaptation to nesting near protective species, sometimes across a broad geographical range. Many studies show that nesting associations occur by active choice and not because the associate species choose similar habitat, but only one study does so experimentally. This is probably because few researchers set out to test this hypothesis specifically, but we tentatively conclude that the majority of nesting associations are commensal in nature, and that only a few are parasitic or mutualistic. In contrast, there is little evidence to suggest that the protective associates pay costs or gain benefits. Two studies demonstrate that protected species manipulate the trade-off between the costs and benefits of nesting near an aggressive associate, while only a handful of studies have found specific adaptations to help avoid costs. Protected associates sometimes also pay costs when they or their young are killed by their aggressive associate, or when they are forced to abandon their nest. Protected associates primarily benefit from a reduced predation rate when nesting near protective species although a variety of other, often unusual benefits have been described, including early warning of predators, lower parasitism, lower brood-parasitism, nest stability and higher mating success. Most ‘protected’ associates are found in the Anseriformes, the Charadriiformes and the Passeriformes, while most ‘protective’ associates come from the Charadriiformes and the Falconiformes. Here we review 62 studies that have looked at protective nesting associations in which at least one species is a bird. ![]() Exploiting the nest defence behaviour of another relatively aggressive species is one of the most unusual amongst a great diversity of strategies used by animals to evade predation.
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