Size-dependent costs of migration: Migrant bird species are subordinate to residents, but only at small body sizes

Long-term increases in wing length occur independently of changes in climate and climate-driven shifts in body size

Recent widespread reductions in body size across species have been linked to increasing temperatures; simultaneous increases in wing length relative to body size have been broadly observed but remain unexplained. Size and shape may change independently of one another, or these morphological shifts may be linked, with body size mediating or directly driving the degree to which shape changes. Using hierarchical Bayesian models and a morphological time series of 27 366 specimens from five North American migratory passerine bird species, we tested the roles that climate and body size have played in shifting wing length allometry over four decades. We found that colder temperatures and reduced precipitation during the first year of life were associated with increases in wing length relative to body size but did not explain long-term increases in wing length. We found no conclusive evidence that the slope of the relationship between body size and wing length changed among adult birds in response to any climatic variable or through time, suggesting that body size does not mediate shifts in relative wing length. Together, these findings suggest that long-term increases in wing length are not a compensatory adaptation mediated by size reductions, but rather are driven by non-climatic factors.

Color as an Interspecific Badge of Status: A Comparative Test

AbstractAnimals as diverse as cephalopods, insects, fish, and mammals signal social dominance to conspecifics to avoid costly fights. Even though between-species fights may be equally costly, the extent to which dominance signals are used between species is unknown. Here, we test the hypothesis that differences in color are associated with dominance between closely related species that aggressively interact over resources, examining between-species variation in colors that are used in within-species badges of status (black, white, and carotenoid coloration) in a comparative analysis of diverse species of birds. We found that dominant species have more black, on average, than subordinate species, particularly in regions important for aggressive signaling (face, throat, and bill). Furthermore, dominant species were more likely to have more black in comparisons in which the dominant species was similar in size or smaller than the subordinate, suggesting that black may be a more important signal when other signals of dominance (size) are missing. Carotenoid colors (i.e., red, pink, orange, and yellow) were not generally associated with dominance but may signal dominance in some taxonomic groups. White may have opposing functions: white was associated with dominance in species in which black was also associated with dominance but was associated with subordinance in species in which carotenoid-based dominance signals may be used. Overall, these results provide new evidence that colors may function broadly as signals of dominance among competing species. Such signals could help to mediate aggressive interactions among species, thereby reducing some costs of co-occurrence and facilitating coexistence in nature.

Modelling migration in birds: competition's role in maintaining individual variation

Animals exhibit extensive intraspecific variation in behaviour. Causes of such variation are less well understood. Here, we ask when competition leads to the maintenance of multiple behavioural strategies. We model variability using the timing of bird migration as an example. Birds often vary in when they return from non-breeding grounds to establish breeding territories. We assume that early-arriving birds (counting permanent residents as 'earliest') select the best territories. But arriving before the optimal (frequency-independent) breeding date incurs a fitness penalty. Using simulations, we find stable sets of return dates. When year-round residency is viable, the greatest between-individual variation occurs when a small proportion of permanent residents is favoured, and the rest of the population varies in their return times. However, when fitness losses due to year-round residency exceed the benefits of breeding in the worst territory, all individuals migrate, although their return dates often vary continuously. In that case, individual variation is inversely related to fitness risks and positively related to territory inequality. This result is applicable across many systems: when there is more to gain through competition, or when its risks are small, a diversity of individual strategies prevails. Additionally, stability can depend upon the distribution of resources.