No abstract

Plumage and song are important signals used by birds to attract mates and repel rivals. Divergence in sexual signals can lead to reproductive isolation among incipient species, but the relative importance of each modality may vary among taxa. Tyrannid flycatchers exhibit evolutionarily conservative plumage coloration but distinct song structure among subfamilies and species. Thus, tyrannids are an interesting group in which to study the relative role of plumage and song in contributing to population divergence. In this study, we assessed character divergence among four willow flycatcher Empidonax traillii subspecies by measuring spectral reflectance of plumage modeled in tetrahedral colorspace from museum specimens collected on putative breeding grounds. We also quantified differences in song structure based on publicly available and field-recorded songs across the species range. Using unsupervised and unbiased clustering algorithms that assigned group membership independent of a priori taxonomic designations, we found that currently recognized subspecies did not consistently sort in accordance with subspecies designation based on plumage color. However, song analyses grouped birds into two clusters; one that included 89% of all putative E. t. extimus, and another that included 100% of specimens designated as E. t. adastus, E. t. brewsteri, E. t. traillii and a small percentage of E. t. extimus (11%). Our results are consistent with previous hypotheses of conservative plumage evolution in tyrannids and species differentiation based on song, and support the subspecific status of E. t. extimus.

Climate change and competition from invasive species remain two important challenges in restoration. We examined the hypothesis that non‐native tamarisk (Tamarix spp.) reestablishment after aboveground removal is affected by genetics‐based architecture of native Fremont cottonwood (Populus fremontii) used in restoration. As cottonwood architecture (height, canopy width, number of stems, and trunk diameter) is, in part, determined by genetics, we predicted that trees from different provenances would exhibit different architecture, and mean annual maximum temperature transfer distance from the provenances would interact with the architecture to affect tamarisk. In a common garden in Chevelon, AZ, U.S.A. (elevation 1,496 m), with cottonwoods from provenances spanning its elevation distribution, we measured the performance of both cottonwoods and tamarisk. Several key findings emerged. On average, cottonwoods from higher elevations were (1) two times taller and wider, covered approximately 3.5 times more basal area, and were less shrubby in appearance, by exhibiting four times fewer number of stems than cottonwoods from lower elevations; (2) had 50% fewer tamarisk growing underneath, which were two times shorter and covered 6.5 times less basal area than tamarisk growing underneath cottonwoods of smaller stature; and (3) the number of cottonwood stems did not affect tamarisk growth, possibly because the negative relationship between cottonwood stems and basal area. In combination, these findings argue that cottonwood architecture is affected by local conditions that interact with genetics‐based architecture. These interactions can negatively affect the growth of reinvading tamarisk and enhance restoration success. Our study emphasizes the importance of incorporating genetic and environmental interactions of plants used in restoration.

The elaborate ornamental plumage displayed by birds has largely been attributed to sexual selection, whereby the greater success of ornamented males in attaining mates drives a rapid elaboration of those ornaments. Indeed, plumage elaboration tends to be greatest in species with a high variance in reproductive success such as polygynous mating systems. Even among socially monogamous species, many males are extremely colourful. In their now-classic study, Møller and Birkhead (1994) suggested that increased variance in reproductive success afforded by extra-pair paternity should intensify sexual selection pressure and thus an elaboration of male plumage and sexual dichromatism, but the relatively few measures of extra-pair paternity at the time prevented a rigorous test of this hypothesis. In the nearly three decades since that paper’s publication, hundreds of studies have been published on rates of extra-pair paternity and more objective measures of plumage colouration have been developed, allowing for a large-scale comparative test of Møller and Birkhead’s (1994) hypothesis. Using an analysis of 186 socially monogamous passerine species with estimates of extra-pair paternity, our phylogenetically controlled analysis confirms Møller and Birkhead’s (1994) early work, demonstrating that rates of extra-pair paternity are positively associated with male, but not female, colouration and with the extent of sexual dichromatism. Plumage evolution is complex and multifaceted, driven by phylogenetic, ecological, and social factors, but our analysis confirms a key role of extra-pair mate choice in driving the evolution of ornamental traits.

Birds exhibit a vast array of colours and ornaments and while much work has focused on understanding the function and evolution of carotenoid‐based colours (red, orange, yellow), structural colouration (blue, green, purple, iridescent) can also play a key role in sexual signaling. Several studies have examined how factors such age may influence structural colour, however few studies have looked at how structural colour may be influenced by environmental conditions such as variation in weather conditions experienced during moult. In this study, we examined variation in structural colour expression in relation to age as well as rainfall and temperature during post‐breeding moult for a population of mountain bluebirds Sialia currucoides in western Canada over nine breeding seasons. Overall, we found structural colouration was explained by sex, age and weather patterns during moult. At a population level, tail and rump feathers from males were more colourful (higher brightness and chroma, hue values shifted more towards UV) than females, and adults were more colourful than juvenals. Male and female rump feathers generally became less colourful with age. Noise‐receptor colour models revealed colour differences were discernible among sexes, suggesting bluebird colouration is an important sexual signal. Tail and rump plumage variation was associated with weather during moulting periods, though the effects were sex‐ and age‐dependent. Female plumage was generally more colourful following wetter and warmer early summers, while males were more colourful following warmer late summers, and plumage of older birds was more resilient to colour variation due to weather patterns. We suggest that more rainfall may increase insect abundance and thus improve food intake and overall condition of mountain bluebirds. This is one of the first studies to examine how both age and weather conditions concurrently influence the expression of structural colours in birds.