Human‐mediated habitat transformation is increasingly evident around the world. Yet, how this transformation influences species’ niche width and overlap remains unclear. On the one hand, human‐mediated habitat transformation promotes increased species similarity through trait‐based filtering, and an increased prevalence of generalist species with broad niches, resulting in functional homogenization. On the other hand, species that colonize transformed habitats could use empty niches, resulting in decreased species similarity and an expansion of assemblage‐level niche space. Here we explore these two alternatives in eight highly diverse passerine assembles in natural, rural and urban habitats in south and southwest China, a rapidly developing region of the world. Based on stable isotopes, we found that species’ niche width increased from natural to human‐made habitats, but there were no differences in niche overlap among habitats. Therefore, we found evidence for niche expansion, with generalists appearing to use empty niches created by human habitat modification, and with assemblages being comprised of complementary species. Further research is needed to determine whether increased between‐ or within‐individual niche variation is the main driver of niche expansion in transformed habitats.

The pace‐of‐life hypothesis predicts no impact of urbanization on stress responses. Accordingly, several studies have been inconsistent in showing differences in breath rate (BR), a proxy of acute stress responses to handling in passerines, between rural and urban areas. However, this evidence is limited to a single bird species and a limited geographic region (SW Europe). No study addressed whether this pattern is also apparent in other species or regions, such as in tropical environments, or whether it is dependent on the level of diet specialization, given that diet restriction and change influence stress responses. Here, we tested whether there were differences in BR between habitats and diet groups using eight highly diverse passerine assemblages experiencing different levels of anthropogenic disturbance (i.e., natural, rural, and urban locations) in SW China. We predicted that insectivores and herbivores (frugivores, nectarivores, and seed‐eating species) would show higher BR than omnivores. We also predicted no differences in BR among habitat types. BR was a moderately repeatable trait, which showed a negative relationship with body mass and a positive relationship with the time of the day. We also recorded a relatively strong phylogenetic bias in the expression of this trait. Confirming our predictions, our results showed no differences in BR among natural, rural, and urban locations. Similarly, within species, there were no differences in BR between rural and urban locations. However, we also found that herbivores showed higher BR than omnivores. Overall, our results provide support to the pace‐of‐life hypothesis, but suggest acute stress responses can be diet‐mediated, which may help to explain the marked decline of specialized trophic guilds around the world in response to anthropogenic disturbance.

Species in transformed habitats, frequently labeled as environmental generalists, tend to show broader niches than species in natural habitats. However, how population niche expansion translates into changes in the niches of individual organisms remains unclear, particularly in the context of habitat transformation. Niche expansion could be a product of individuals having broader niches, greater distances among individuals’ niches, or a combination of both processes. This would challenge the traditional conceptions on niche dynamics, which emphasize the role played by individual specialization (IS). Here, using stable isotopes, we computed total niche width (TNW), its within- and between-individual components (WIC and BIC), and IS (the ratio WIC/TNW), in 13 populations of 6 bird species and 8 populations of 3 frog species in natural and transformed habitats. We confirmed that species had broader niche width in transformed than in natural habitats, yet population niche expansion across habitats was mainly a product of increased distance between individuals. Within each habitat type, increases in TNW were linked to increases in WIC for all habitat types, while relationships between TNW and BIC were found in transformed but not in natural habitats. Hence, both increased individual niche width and increased distance among individuals were apparent within habitats, particularly in transformed ones, where increases in WIC dominated. Neither across or within habitats was niche expansion associated with increasing IS. Therefore, our results overturn traditional conceptions associated with the niche variation hypothesis and illustrate that niche expansion is not invariably associated with increased IS, because the distance between individual’s niches (BIC) can increase, as well as the breadth of those niches (WIC).

Vocalizations that signal predation risk such as alarm calls provide crucial information for the survival of group-living individuals. However, alarm calling may attract the predator’s attention and, to avoid this cost, animals can opt for alternative strategies to indicate danger, such as ‘adaptive silence’, which is the cessation of vocalizations. We investigate here whether abrupt contact call cessation would provoke alarm responses, or would reinforce the signal given by an alarm call. In an aviary setting, we conducted playback experiments with a group-living passerine, the Swinhoe’s white-eye, Zosterops simplex. We found that birds did not respond to a sudden call cessation, nor did they have a stronger response to alarm calls followed by silence than to alarm calls followed by contact calls. Confirming previous work investigating contact call rate, it appears that in this species contact calls encode information about social factors but not environmental conditions.

Heavy metal pollution is widespread in China, particularly in its mining regions. Mercury (Hg) concentrations in birds from Guizhou Province were recently reported to be above adverse effect levels, even in nonmining areas. We sampled birds to investigate whether Hg might be a threat near lead (Pb), zinc (Zn), and tin (Sn) mines in Guangxi Zhuang Autonomous Region. We measured concentrations of 8 metals/metalloids in feathers of 627 resident birds representing 60 species sampled across 14 sites on 5 rivers. We found that Hg concentrations (mean ± standard deviation, 1.27 ± 2.02 ppm) were lower than in the recent Guizhou study. Mercury had the fewest correlations with other metals/metalloids; the mined metals (Pb, Sn, Zn) had the most such correlations. Levels of cadmium (Cd; 1.12 ± 2.64 ppm), arsenic (As; 4.78 ± 17.11 ppm), and Pb (17.18 ± 24.49 ppm) were closer to thresholds of adverse effects or relatively high compared to other studies. With the exception of a few hot spots for Hg, Cd, and As near mines, metal/metalloid levels were fairly evenly distributed among sites, consistent with the regional occurrence of mineable ore deposits. It appears that Hg is not threatening to all avian species in China, although it may be problematic for some species near Hg mines. In addition to Hg, however, other metals/metalloids may pose wildlife health hazards. Environ Toxicol Chem 2020;39:371-380.