Hybridization involving introduced species is becoming more common as humans modify landscapes and ecosystems. When two closely related species are introduced to the same area, their niche dynamics will dictate the level of sympatry and potential for hybridization. Amazona parrots offer a rare case where multiple closely related species have established as breeders in Southern California. Red-crowned Parrots (A. viridigenalis) and Lilac-crowned Parrots (A. finschi) are particularly interesting because they are sister species (Russello and Amato, 2004) with allopatric native ranges in Mexico. In Southern California, where they established in the 1980s and have since grown in numbers, they appear to occupy the same urban habitat. We sought to test whether introduced Red-crowned and Lilac-crowned parrots have shifted their niches compared to their native ranges, and if so, whether sympatry has led to hybridization. Using broad-scale environmental data collected from weather stations and orbiting satellites, we found that Red-crowned and Lilac-crowned parrots have partially divergent environmental niches in their native ranges, but now occupy the same environmental niche in their introduced range in Southern California. This new niche is largely different from what they experience in their respective home ranges, supporting a niche-shift model of species introduction. Due to this niche shift, the two species now come into contact across Southern California, leading to hybridization. Genomic markers support the existence of some recent hybrids as well as advanced backcrosses resulting from older hybridization events closer to the time of first introduction. Photographs from community scientists included as part of the Free-flying Los Angeles Parrot Project (FLAPP) also document hybrids, but underestimate their frequency compared to genetic data. Despite evidence for ongoing hybridization, the bimodal distribution of ancestry among the introduced population points to the existence of reproductive isolating mechanisms keeping the two species distinct. Further study is needed to understand if reproductive isolating mechanisms result from behavioral factors, such as conspecific flocking preferences, suggested by the community science data, or from genomic incompatibilities built up from a long history of isolation in Mexico. The integrity of these genetic lineages in Southern California carries important conservation implications, as both species are listed as endangered in their native ranges due to trapping for the pet trade and habitat loss.

The Tricolored Blackbird (Agelaius tricolor) is endangered (Birdlife International 2018) and declining rapidly (Cook and Toft 2005, Meese 2017), designated as threatened by the state of California (Calif. Dept. Fish and Wildlife 2019), and nearly endemic to the California ecoregion (Kelsey 2008). It breeds somewhat nomadically (Hamilton 1998), which behavior has hampered its conservation. For its declining population to be managed, it is essential that the resources on which it depends for food and habitat be understood (Skorupa et al. 1980). Here we describe a previously unknown behavior of the Tricolored Blackbird, foraging in the inflorescences of a common plant. On 20 April 2019, while on a class field trip, we were driving through the Kelso Valley in Kern County, California, at 1270 m elevation. The habitat was characterized by desert with fairly dense cover of Western Joshua Trees (Yucca brevifolia brevifolia), bordering agricultural fields along the bottom of the valley. We noticed a small flock of blackbirds flying between Joshua Trees and stopped to observe them. We identified them as Tricolored Blackbirds, and saw them probing into the trees' inflorescences (Figure 1). This first flock comprised 11 individuals, with seven or eight females and three or four males. We then noted two other similarly sized flocks engaging in the same behavior. As we observed these birds probing into the inflorescences, they appeared to be obtaining and eating items from among the flowers and developing fruit. They would forage in one tree, probing into and prying open fruits and flowers, then move as a flock to a nearby tree. Tricolored Blackbirds are opportunistic foragers that take advantage of superabundant resources of diverse nature. They generally eat arthropods, especially insects of the orders Orthoptera and Odonata, but also eat grains and mollusks when available (Crase and DeHaven 1977, Meese 2013, Beedy et al. 2018). They generally forage in open habitats with low vegetation such as grasslands, wetlands, pastures, fields, dairies, and scrub. The only previous description of Tricolored Blackbirds foraging in trees involved the Coast Live Oak, Quercus agrifolia (Wilson et al. 2016). Many other species of insectivorous birds occur in the southern Sierra Nevada, but we only observed Tricolored Blackbirds foraging in these flowers. The family Icteridae is characterized by an extended postarticular process to the mandible, the site of insertion of the depressor mandibulae muscle (Zusi 1959) and a structure that allows the birds to open the bill with force. This "gaping" behavior allows the Icteridae to obtain food unavailable to other birds, by prying objects open. The ability to pry open inflorescences might mean that the Tricolored Blackbirds have access to the arthropods living in these flowers that other insectivorous birds of other families do not have. If this is true, and a common behavior during periods of extensive Joshua Tree flowering, the inflorescences of Joshua Trees might be important for the foraging of...