Despite our growing understanding of these general patterns, we have yet to uncover the reason why they exist--or, in other words, the mechanism(s) linking the urban environment with variations in behavior and biodiversity of birds (or any other taxa, for that matter). One of the most likely candidates is the endocrine system, which plays a vital "role in mediating an organism's physiological and behavioral responses to novel environments." This possibility was the focus of a recent review paper, published in the academic journal Hormones and Behavior, by Frances Bonier of Queen's University. In particular, the paper focuses on two key endocrine axes that are likely to mediate avian responses to urban environments: the hypothalamic-pituitary-adrenal (HPA) axis, which is responsible for regulating processes related to metabolism, and the hypothalamic-pituitary-gonadal (HPG) axis, which is responsible for reproductive processes. Hormones associated with these axes, including glucocorticoids, estradiol, and testosterone, can be used as indicators of physiological responses to the stresses of city life.
(A curve-billed thrasher, Toxostoma curvirostre. Urban thrashers have lower baseline glucocorticoid levels than desert-dwelling thrashers, but only during the non-breeding and molting stages of their life histories; during the breeding season, glucocorticoid levels are similar in urban and non-urban birds.)
Bonier identifies three major questions that researchers have sought to answer by measuring these and other hormones. The first is associated with "allostatic load," or the "current and predicted energetic demands facing an organism." Several studies have investigated whether the allostatic loads of urban birds are different from those found in rural counterparts. There are a variety of factors that can influence allostatic load, including an individual's exposure to certain disturbances, its perception of the environment, and its ability to access and utilize resources. To put that in more specific terms, each animal's response will vary depending on the number of house cats it has to dodge, ambient noise levels in its territory, whether it is able to supplement its diet at a bird feeder, and so on. Each of these factors may, in turn, be influenced by species identity, sex, and life history stage (for instance, whether the animal is currently breeding or not). Perhaps unsurprisingly, the handful of studies that have been conducted on this to date have yielded conflicting results; thus, it is hard to draw any general conclusions at this stage of the game.
The second question commonly asked by researchers is whether urban birds respond differently to "acute" challenges--those that go above and beyond the daily background dose of disturbance that animals are used to experiencing. According to Bonier, endocrine responses "to acute perturbations...can reveal how the urban environment shapes the responsiveness of the HPA axis or selects for particular endocrine phenotypes." Variations in these responses may explain why some urban birds differ from their rural counterparts in terms of flight initiation distance, neophobia, use of foraging innovations, and boldness.
(Florida scrub-jay, Aphelocoma coerulescens. Suburban jays have lower baseline glucocorticoid levels than nonurban jays, and this difference may partly be driven by high levels of supplemental food available in more anthropogenic habitats.)
Finally, Bonier examines work exploring differences in reproduction between urban and non-urban birds. As with allostatic load, there are many factors other than location, per se, that can influence this aspect of the endocrine system--including migratory behavior, the availability of resources such as food and nesting materials, and microclimate. Urban environments might be particularly hard on animals of certain age (and therefore reproductive) categories, which could alter the extent to which different individuals invest in breeding efforts. Such demographic responses to urbanization may be the key to successful reproduction in cities, both for particular individuals and entire species. Thus, Bonier hypothesizes that, in order to thrive in urban environments, animals may need to be "physiologically flexible enough to adjust their phenology and life history in response to urban conditions." Indeed, though there have only been a few urban reproduction studies to date, their results indicate that variations between urban and nonurban birds may stem from plasticity rather than heritable responses.
After sifting through the evidence, Bonier concludes that the "nascent field of urban endocrine ecology has not yet revealed any strong, consistent patterns." Further, although there have been a handful of observational studies asking how endocrine traits vary according to urbanization levels, there is a need for experimental studies that will more clearly demonstrate why the observed patterns occur. Of the many potential questions that can be asked, the author feels it would be particularly interesting to focus on whether endocrine traits "differentiate species that do and do not tolerate urban habitats"--specifically, to ask whether "certain endocrine traits differ among urban avoiders, urban adapters, and urban exploiters." Consistent variations in particular hormones would facilitate their use as proxies for fitness, potentially enabling researchers to more quickly characterize the health of local bird populations.
(European blackbird, Turdus merula. In the wild, levels of luteinizing hormone are similar in urban and nonurban female birds, but are higher in urban males than in nonurban individuals. These same patterns do not hold true for nestlings of these species raised in captivity, indicating that endocrine responses in these birds may be plastic, depending strongly on environmental conditions.)
Bonier also points out the need to generate enough data to permit meta-analyses and comparative studies. The current trend of studying one species in a single urban-nonurban site pairing (e.g. house sparrows, Passer domesticus, in and near Phoenix, AZ) makes it difficult to draw general conclusions about the effects of urbanization around the world. For one thing, variations in resident birds could be driven by habitat variables, such as humidity or local rainfall, that are influenced by degree of urbanization, rather than by urbanization itself. So far, there is only one study in which one or more species has been examined across multiple cities; there are also few projects examining multiple species in a single place. Multi-city studies will help researchers determine whether the effects of urbanization differ according to larger landscape and environmental patterns--for instance, because of a city's location in a desert environment (as in Phoenix) rather than a tropical one (such as Miami).
Other major suggestions include coupling field and captive studies and exploring different data collection techniques that may more accurately measure endocrine responses to urban challenges. Bonier also emphasizes the importance of placing endocrinological patterns within a broader context;
this can be done by utilizing "classical ecology" quantitative tools to measure things such as population density, degree of urbanization, and animal behavior. Together, these data should allow the field of urban endocrine ecology "to make important advances in our understanding of how rapidly expanding urbanization will impact organisms worldwide."
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Bonier, F. 2012. Hormones in the city: endocrine ecology of urban birds. Hormones and Behavior 61:763-772.
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