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Multiple invasive cavity-nesting bird species can be present in a nest web, the network linking birds using cavities. We investigated the nest preferences and breeding phenologies of the cavity-nesting guild in the region surrounding Miami, Florida, USA, where invasive starlings, mynas, and parrots potentially usurp cavities from native woodpeckers and secondary cavity-nesters. We asked if the timing of reproduction determines which invasive species will usurp cavities from native birds with similar nest preferences. Nest usurpations between European Starlings (Sturnus vulgaris) and the woodpecker species present in Miami is well documented, but we predicted that a recently arrived sturnid species and introduced psittacids would also usurp nests. European Starlings had the largest breeding population of any species in our nest web, breeding during the peak of nesting season, and usurped the largest number of active nest cavities. We found that a small population of Common Mynas (Acridotheres tristis) usurped nests, sharing the peak-season nesting period with starlings and native woodpeckers. Parrots bred later than we expected, avoiding nest-site overlap with similarly large native birds that use cavities with similar characteristics. Parrots did not usurp any active nest cavities from native birds. Our results demonstrate how to use analysis of cavity characteristics and reproductive timing to evaluate threats to a cavity nest web posed by multiple invasive species. Common Myna currently usurp few nests; if they increase greatly in population, they could pose a problem for native cavity-nesters.

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Background: Exotic parrots have established breeding populations in southeast Florida, including several species that nest in tree cavities. We aimed to determine the species identity, nest site requirements, relative nest abundance, geographic distribution, and interactions of parrots with native cavity-nesting bird species. Methods:We searched Miami-Dade County, Florida, and nearby areas for natural cavities and holes excavated by woodpeckers, recording attributes of potential nest trees. We inspected all cavities with an elevated video inspection system to determine occupancy by parrots or other birds. We mapped nearly 4000 citizen science observations of parrots in our study area corresponding to our study period, and used these to construct range maps, comparing them to our nesting observations. Results: Not all parrots reported or observed in our study area were actively breeding. Some parrots were observed at tree cavities, which previous studies have suggested is evidence of reproduction, but our inspections with an elevated video inspection system suggest they never initiated nesting attempts. Several parrot species did successfully nest in tree cavities, Red-masked Parakeets (Psittacara erythrogenys) and Orange-winged Parrots (Amazona amazonica) being the most common (n = 7 and 6 nests, respectively). These two parrots had similar nesting requirements, but Orange-winged Parrots use nests with larger entrance holes, which they often enlarge. Geographic analysis of nests combined with citizen science data indicate that parrots are limited to developed areas. The most common parrots were less abundant cavity nesters than the native birds which persist in Miami's urban areas, and far less abundant than the invasive European Starling (Sturnus vulgaris). Conclusions:Exotic parrots breeding elsewhere in the world have harmed native cavity-nesting birds through interference competition, but competitive interference in southeast Florida is minimized by the urban affinities of parrots in this region. The relative abundance and geographic distribution suggest that these parrots are unlikely to invade adjacent wilderness areas.

Abstract:Canopy gaps create a temporary spatial heterogeneity, often allowing pioneer species to establish and grow in mature forests. In this study, we asked whether the above model holds for tropical dry forests in the Florida Keys. Six hundred and forty-eight canopy gaps in an extensive Key Largo forest were identified with a LiDAR digital canopy model. The structure and composition of juvenile trees were examined in 45 selected gaps in three stands of known age, and weighted averaging calibration and regression were applied to the data to determine the successional age optimum for each tree species, and the inferred age for each gap based on its sapling composition. Less than 1% of the forest area was recorded as canopy gaps in the LiDAR model. The inferred stand ages were about 70 y greater in canopy gaps in young forest than in the surrounding, unimpacted forest. This suggested that gap formation advanced succession rather than reversing or resetting it. The apparent lack of recruitment by early-successional species may be due to the small size of canopy gaps in this forest, and the minimal contrast between gap and understorey environments; light and water conditions in the small gaps may favour late-successional rather than pioneer species. Establishment of pioneer species may not take place without intense, large-scale disturbances such as fires and hurricanes that remove the entire canopy and consume or erode soils.