Fitting the Bill in an Urban World: Hummingbird Beak Shape Responds to Anthropogenic Factors

Measuring organismal responses to environmental perturbation and determining whether species are capable of adaptation in a changing world is a perpetual challenge for biologists, particularly when anthropogenic processes are altering ecosystems at an alarming rate. Classic textbook examples of phenotypic change as a response to varying environmental conditions include beak size and shape fluctuations in Darwin's finches due to food scarcity from extended dry seasons (Grant and Grant 1989) and increased frequency of dark morphs in the Peppered Moth (and other insects) due to air pollution (McCulloch and Waters 2023). Most studies reveal that human-induced environmental modification has negative implications for species survival (English et al. 2021), and phenotypic responses generally buffer anthropogenically driven selective pressures. For instance, urban birds sing at night to avoid noise pollution from traffic (Lowry et al. 2013), and New Zealand stoneflies exhibit wing vestigialization as a result of deforestation (McCulloch and Waters 2023). When species thrive in the face of anthropogenic change, they tend to be ecological generalists with high levels of phenotypic plasticity enabling them to exploit human resources (e.g., urban foxes, racoons), and many reported adaptive or plastic responses to urbanization involve changes in behavior or life history traits as opposed to morphology, which is often more challenging to detect (Lowry et al. 2013). One such species that has undoubtedly benefitted from increased urbanization is Anna's Hummingbird (Battey 2019; English et al. 2021). Once endemic to the Pacific slopes of California (USA) and the Baja California Peninsula (Mexico), Anna's Hummingbird underwent an extraordinary range expansion north and eastwards during the twentieth century, with its current breeding range extending to British Columbia and its non-breeding range as far north as Alaska (Battey 2019; Clark and Russell 2020). Outside its native range, Anna's Hummingbird is closely associated with human-modified environments, and it is rarely found nesting far from urban areas (Clark and Russell 2020). Population increases and range expansion in Anna's Hummingbirds tracked progressive landscape alteration along the north American Pacific coastline between 1940 and the present day, with a likely cause being ecological release from its typical food source via a combination of introduced winter-flowering eucalyptus trees, tropical garden plants, and supplemental nectar feeders (Battey 2019). Anna's Hummingbird does not, however, fit the bill of a typical ecological generalist that we would expect to colonize urban environments; rather, hummingbirds are specialists that rely on nectar, with most species adapted to exploit only a few specialized plant species for food (McGuire et al. 2014; Rico-Guevara et al. 2019). Anna's Hummingbird is no exception; its main nectar source in its native range is just a handful of plant species endemic to the chaparral habitat of the Upper Sonoran life zone (Clark and Russell 2020). Nevertheless, birds have a remarkable capacity to diversify bill shape to adapt to new ecological roles (Grant and Grant 1989; Navalón et al. 2021), with hummingbirds exhibiting significant disparity in bill lengths and shapes related to feeding and defence across their species diversity (Rico-Guevara et al. 2019). But to what extent have alternative food sources influenced the distribution and success of Anna's Hummingbird, and is there any measurable phenotypic change associated with colonising urban environments in this trophic specialist? Alexandre et al.'s (2025) recent contribution in Global Change Biology attempts to address these questions in an ambitious study tracking beak shape change in museum specimens of Anna's Hummingbird that were collected throughout the 160-year time interval of this species' population and range expansion. Utilizing historical data from newspaper archives, together with citizen science records, human population density and climatic variables, the authors synthesized a multivariate framework of population expansion and morphological change through time for Anna's Hummingbird throughout California. Alexandre et al. (2025) estimated supplemental feeder and eucalyptus availability across California between 1860 and 2020 using mentions in newspaper articles as a proxy for their local availability in space and time. They estimated changing urbanization from human population records, as well as modelling Anna's Hummingbird abundance with Christmas Bird Count data, a long-running community science program. The authors found positive associations between urbanization, feeder and eucalyptus availability and increased Anna's Hummingbird abundance, particularly at higher latitudes. However, eucalyptus availability was not as strong a predictor for abundance as feeder availability and urbanization. In addition, Alexandre et al. (2025) collated data from 400 Anna's Hummingbird specimens from the Museum of Vertebrate Zoology at the University of California, Berkely to obtain morphometric measurements of bill shape and length. These specimens spanned the breadth of latitudes in California and were collected between the years 1861 and 2020. The results reveal that Anna's Hummingbird bill morphology responds to feeder abundance, but not to eucalyptus abundance. In a mere few decades, the shape of Anna's Hummingbird bills has changed to become longer and exhibit a larger bill volume as a response to supplementary feeding that coincides with major range expansion. Flowers offer diminishing nectar rewards compared to feeders because nectar in flowers may run out over time, whereas feeders offer an infinite food source if kept filled (Rico-Guevara et al. 2019). It follows, therefore, that feeders have become the preferred food source of these urban hummingbirds. Higher bill volume and length equate to more nectar collection during a given time at feeders (Rico-Guevara et al. 2019), hence modifying the bill to suit feeder shape in an urban world may be highly adaptive. Another major result from Alexandre et al. (2025) is differential bill shape between male and female Anna's Hummingbirds. The authors report that not only does bill shape change in general, but that in males, the bill becomes even more pointed than in females. This may be a direct result of increased male–male conflict at feeder sites, as males battle to secure resources and territories. This phenomenon has been documented for many hummingbird species, Anna's Hummingbird included (Clark and Russell 2020), whereby males fiercely defend a territory when food is abundant. Sexual selection results in male bills tending to be sharp, pointy, or hooked for greater success in agonistic encounters (Rico-Guevara et al. 2019). Conversely, the study reports that Anna's Hummingbird bills are shorter in length as a response to colder temperatures at higher latitudes, even when feeders are present. In fact, the results suggest that hummingbirds at higher latitudes were only able to colonise these cooler climes by relying on feeders during the winter months. Alexandre et al. (2025) investigated the thermoregulatory function of Anna's Hummingbird bills using infrared imaging and confirm that the bill acts as a thermoregulatory organ. Although they did not test its function in low temperatures, they did show that longer bills with larger surface areas are used to cool down in high temperatures. Their findings that colder, higher latitudes are coupled with short-billed Anna's Hummingbirds may indicate that this morphology may help conserve heat during winter. Future research investigating Anna's Hummingbirds at even higher latitudes, or other species, may shed further light on the role of bill dimensions in thermoregulation and its consequences for colonising new areas with different climates, as well as whether these responses are adaptive or due to phenotypic plasticity. The results from Alexandre et al. (2025) suggest that supplemental feeders have freed the bill of Anna's Hummingbird from the constraints imposed by flower shape, so that it can simultaneously adapt its shape for optimal feeder use, mitigating latitude-dependent climatic effects and as a response to sexual selection. Their study highlights the bill as a multifunctional organ that is subjected to numerous selective pressures, and provides evidence as to what may happen after a species undergoes ecological release from its historical niche (Battey 2019; English et al. 2021). Although the authors did not test whether the morphological changes observed here are plastic or adaptive responses, it does provide an exciting avenue for future enquiry into whether Anna's Hummingbirds are undergoing the adaptive processes that are typical when a species colonises new environments and occupies novel niches. Hummingbirds historically radiated and diversified largely due to Andean uplift in South America, generating astonishing levels of endemism and resulting in more than 300 species (McGuire et al. 2014). It seems plausible that a hummingbird species could rapidly colonise and adapt in this manner given its evolutionary backdrop (Navalón et al. 2021), particularly as Anna's Hummingbird is within the rapidly diversifying bee hummingbird clade (McGuire et al. 2014; Clark and Russell 2020), even if hummingbirds are not the typical ecological generalist found in urban environments. But this begs the question, why did Anna's Hummingbird fare so well from urbanization when other, closely related north American hummingbird species have sharply declined in recent decades for the same reason (English et al. 2021)? This remains to be explored, but it seems that Anna's Hummingbird has a high propensity for rapid phenotypic change when subjected to anthropogenic pressures and may make an ideal model system to explore plasticity and adaptation in the face of urbanization. Alexandre et al.'s (2025) work paves the way for exciting new research into these fundamental evolutionary questions, and bears the hallmarks of becoming another of those textbook examples of anthropogenically-driven phenotypic change. Elizabeth M. Steell: conceptualization, investigation, writing – original draft, writing – review and editing. Many thanks go to Guillermo Navalón and Juan Benito for insightful discussion and feedback on a draft of the commentary. I would also like to thank the editors at Global Change Biology who invited me to this opportunity. This work was supported by the Sarah Woodhead Research Fellowship in Earth Sciences from Girton College, Cambridge. The author declares no conflict of interest. The author declares no conflicts of interest. This article is a Commentary regarding Alexandre et al, https://doi.org/10.1111/gcb.70237. The author has nothing to report.