Competition and cooperation in three species of sympatric corvids

Background/Question/Methods

Competition is one of the driving forces in shaping ecosystems, and is typically divided into two types; exploitation and interference. Both types commonly occur between closely related species where they are sympatric. The most abundant species of corvids in Jackson Hole are Common ravens (Corvus corax), American crows (C. brachyrhynchos) and Black-billed magpies (Pica hudsonia). All three are opportunistic omnivores that are well-adapted to anthropogenic environments, though they vary in size and behavior.  Ravens and magpies are native to the valley, whereas crows are moving northward and their competitive influence on the ecosystem is unknown. There is considerable overlap in nesting preferences between the three species. Crows and magpies are known to breed cooperatively or colonially based on ecological factors such as territory quality or threat of nest predation. Social nesting behavior may provide a competitive advantage over ravens, which nest individually. We studied how exploitation and interference competition are manifested between these three species in the family Corvidae in Jackson Hole, focusing on the influence of social nesting behavior. Demographic information for all species was collected, as well as focal observations of nesting individuals competing for access to a proffered food source near their nest during the breeding season.

Results/Conclusions

We found that the raven population initially increased rapidly in the area, stabilizing near carrying capacity two decades ago and remaining at high densities with little growth since that time. Ravens nest singly and raven territories are found across an urbanization gradient in the valley of Jackson Hole. In contrast, crows are continuing to increase in density in urbanized areas of the valley and some are nesting colonially, in some cases occupying previous raven territories in these areas. Nesting magpies are present at high densities, and frequently appear in close proximity to crow nests.  Thus, magpies and crows do not exhibit interspecific exploitation competition for nest territories, though they will compete for access to food on the territory via interference competition. Further, crows and magpies will cooperatively mob ravens that approach proffered food on their territory, though this is rare as ravens exhibit high levels of neophobia (are unlikely to approach proffered food) and do not nest in close proximity to magpies or crows. This study demonstrates the importance of competition and cooperation to shaping distributions of closely-related species in a resource-rich environment.