Unfamiliarity generates costly aggression in interspecific avian dominance hierarchies

Dominance hierarchies often form between species, especially at common feeding locations. Yet, relative to work focused on the factors that maintain stable dominance hierarchies within species, large-scale analyses of interspecific dominance hierarchies have been comparatively rare. Given that interspecific behavioral interference mediates access to resources, these dominance hierarchies likely play an important and understudied role in community assembly and behavioral evolution. To test alternative hypotheses about the formation and maintenance of interspecific dominance hierarchies, we employ an large, participatory science generated dataset of displacements observed at feeders in North America in the non-breeding season. Consistent with the hypothesis that agonistic interference can be an adaptive response to exploitative competition, we find that species with similar niches are more likely to engage in costly aggression over resources. Among interacting species, we find broad support for the hypothesis that familiarity (measured as fine-scale habitat overlap) predicts adherence to the structure of the dominance hierarchy and reduces aggression between species. Our findings suggest that the previously documented agonistic hierarchy in North American birds emerges from species-level adaptations and learned behaviors that result in the avoidance of costly aggression.

Status and distribution of an introduced population of European Goldfinches (Carduelis carduelis) in the western Great Lakes region of North America

Despite the importance of monitoring introduced species, reports of non-native birds are often initially dismissed by observers. This leads to serious information gaps about source localities, founding numbers, and growth and expansion of potential new populations. Here, we report on European Goldfinches in North America between 2001 and 2021, focusing on the western Great Lakes region. We compiled over 7000 records of European Goldfinches from multiple sources; over 3300 records were from the western Great Lakes. This species was initially reported widely in this region, but over time, birds were most consistently reported between Milwaukee, Wisconsin and Chicago, Illinois. They have been breeding in this area continuously since 2003 and the number of reported observations has increased in recent years. From our compiled records, we describe their distribution, breeding status, nesting phenology, and natural food sources. From import records, we provide evidence that the likely primary founding event of this population was release or escape from a cage bird importer in northern Illinois and provide information on possible origins. We briefly discuss possible ecological impacts. We highlight weaknesses in the way data on non-native species are currently collected and how it has impeded our ability to thoroughly reconstruct the recent history of this species in the western Great Lakes region. Formal study is needed on this population of European Goldfinches, including their potentially increasing population and range, ecology, and an evaluation of the potential effects on native ecosystems.

Motivations and Specialization of Birders Are Differently Related to Engagement in Citizen Science Projects of Different Complexity

Citizen Science (CS) projects are an important aspect of scientific data collection and biodiversity conservation. In ornithology, various CS projects exist, and even laypersons can contribute, but advanced birdwatchers also spend considerable time and effort in data collection. Here, different CS projects for birders were analyzed and compared with respect to recreation specialization and motivations for birdwatching. Established, psychometrically valid, and reliable scales were applied in this study. N = 2856 respondents from Austria, Germany, and Switzerland were grouped into no, low, and sustained engagement clusters. Sustained engagement was related to more complex programs, such as the breeding bird census and waterfowl counting. When comparing the engagement clusters, effect sizes were considerable, ranging from 0.098 (attraction) to 0.306 (skill/knowledge). Thus, birders of the three engagement clusters differed significantly in birding specialization, especially skill/knowledge, psychological commitment, social motivations, and the psychological construct centrality to lifestyle. No differences were found in enjoyment and achievement motivations. In conclusion, low-threshold projects are needed to attract participants, but keeping people within programs or moving them to a higher level of engagement might be easier when social dimensions are addressed.

Drivers of winter population cycles in the Varied Thrush Ixoreus naevius

The drivers of year-to-year difference in winter abundance patterns, particularly dramatic in the “eruptions” of many boreal seed-eating birds, are poorly understood. Varied Thrush (Ixoreus naevius Gmelin, 1789), endemic to the Pacific Northwest of North America, is a boreal species that exhibits pronounced, often biennially cyclic, changes in winter abundance within most of its normal wintering range. Although the drivers of this variability have not previously been explored, it has been suggested that differences in acorn abundance, a key winter food resource, might be important. Here we examine three hypotheses for the drivers of this pattern: the acorn crop within the bird’s normal winter range, weather within the bird’s winter range, and weather during the previous breeding season within the bird’s breeding range. Analyses supported the importance of breeding season conditions, particularly breeding season rainfall, with more birds wintering following wetter years. No support was found for the hypotheses that winter conditions, neither the acorn crop nor winter weather, correlate with winter abundance patterns. For this forest species, year-to-year differences in winter abundance patterns is apparently not driven by the “pull” of winter food supply or winter conditions, but by environmental factors during the prior breeding season that presumably affect reproductive success and subsequent population size.