Eugenia Bianca Bonaparte and Kristina L. Cockle
Linked paper: Nest niche overlap among the endangered Vinaceous-breasted Parrot (Amazona vinacea) and sympatric cavity-using birds, mammals, and social insects in the subtropical Atlantic Forest, Argentina by E.B. Bonaparte and K.L. Cockle, The Condor: Ornithological Applications 119:1, February 2017.
Have you ever seen two animals fighting over a tree hole? Around the world, more than a thousand species of birds use tree holes or cavities for nesting, and they have to somehow share this critical resource with other birds, mammals, social insects, lizards and snakes. If two or more species of animals need cavities with similar characteristics, in the same place and at the same time of year, they overlap in nest niche. Nest niche overlap can lead to fierce competition for nest sites, lowering reproductive output and preventing the recovery of threatened species.
We got interested in how birds share the cavity resource because of our long-term work to conserve Vinaceous-breasted Parrots (Amazona vinacea), endangered cavity-nesters found only in the subtropical Atlantic Forest of South America. About 90% of the Atlantic Forest has already been converted to farmland, and nearly all the rest has been selectively logged, resulting in an extreme scarcity of nesting cavities. Vinaceous-breasted Parrots have declined throughout their range over the last century. In Argentina, where they were reported by early naturalists to “darken the sky” in “flocks of thousands,” less than 300 remain today. And this remnant population must share a dwindling supply of tree cavities with more than 70 other cavity-nesting species.
Our initial field work suggested that Vinaceous-breasted Parrots have low reproductive output. We wondered whether they might compete for nest sites with other species of birds, mammals, and social insects. On the other hand, theory and data from temperate forests suggested that cavity-nesting species reduce competition by partitioning the nest niche. For example, one species might use cavities high in the tree canopy, while another species uses cavities close to the ground. We reasoned that if such niche partitioning occurred in the Atlantic forest, we could identify specific cavity characteristics that we could then target for conservation of Vinaceous-breasted Parrots.
To find out the extent of nest niche overlap among Vinaceous-breasted Parrots and co-occuring cavity-nesters, we studied timing of breeding, characteristics of cavities, trees, and habitat, and interspecific reuse of the same individual cavities, by large (> 140 g) birds and mammals (parrots, owls, toucans, forest-falcons and opossums) and social insects (bees and wasps). Every spring for 10 years, we shadowed adult birds, snooped on their eggs with our tiny pole-mounted spy cameras, and tested the limits of our tree-climbing equipment to access cavities up to 24 m high in the Atlantic Forest canopy.
Contrasting with reports from temperate forests, our data showed very little evidence of nest niche partitioning among cavity-nesters in a diverse subtropical community. Except for White-eyed Parakeets (Psittacara leucophthalmus), which nested later in the season, all bird species nested at the same time of year. Furthermore, and unfortunately for conservation efforts, no combination of cavity, tree and habitat characteristics was used exclusively by Vinaceous-breasted Parrots. Indeed, our models were unable to distinguish among the cavities used by the various species, and 8 of the 10 species reused each other’s cavities. The high level of overlap in nest niche, combined with previous evidence that cavities can limit bird density in our study area, support the idea that competition among species could play an important role in structuring the community of cavity nesters in the Atlantic Forest. Such competition could potentially inhibit the recovery of threatened species like Vinaceous-breasted Parrot.
What can be done for Vinaceous-breasted Parrots? Although we had little success classifying cavities by species, some characteristics of cavities, trees and habitat were selected more by Vinaceous-breasted Parrots than by other taxa, and we recommend targeting conservation efforts toward cavities and trees with these characteristics: >10 m high, entrance diameter 7–40 cm, tree diameter (DBH)>55 cm. Additionally, we found 62% of Vinaceous-breasted Parrot nests on farms (vs.≤50% for any other taxon), which highlights the importance of working with local farmers to conserve cavities in human-altered habitats as well as protected areas. As a start, through Proyecto Selva de Pino Paraná we organize outreach activities in schools and on farms, an annual parrot census, and a small scale reforestation program, all of which involve local farmers in the conservation of Vinaceous-breasted Parrots and their nest trees. We’re now busy interviewing farmers to study how they manage remnant native trees and what they think about cavity-nesting birds on their land. Stay tuned for the results!