1. The altered ecological and environmental conditions in towns and cities strongly affect demographic traits of urban animal populations, for example avian reproductive success is often reduced. Previous work suggests that this is partly driven by low insect availability during the breeding season, but robust experimental evidence that supports this food limitation hypothesis is not yet available.2. We tested core predictions of the food limitation hypothesis using a controlled experiment that provided supplementary insect food (nutritionally enhanced mealworms supplied daily to meet 40%-50% of each supplemented brood's food requirements) to great tit nestlings in urban and forest habitats.3. We measured parental provisioning rates and estimated the amount of supplementary food consumed by control and experimental nestlings, and assessed their body size and survival rates. 4. Provisioning rates were similar across habitats and control and supplemented broods, but supplemented (and not control) broods consumed large quantities of supplementary food. As predicted by the food limitation hypothesis we found that nestlings in (a) urban control broods had smaller body size and nestling survival rates than those in forest control broods; (b) forest supplemented and control broods had similar body size and survival rates; (c) urban supplemented nestlings had larger body size and survival rates than those in urban control broods; and crucially (d) urban supplemented broods had similar body size and survival rates to nestlings in forest control broods. 5. Our results provide rare experimental support for the strong negative effects of food limitation during the nestling rearing period on urban birds' breeding success.Furthermore, the fact that supplementary food almost completely eliminated habitat differences in survival rate and nestling body size suggest that urban stressors other than food shortage contributed relatively little to the reduced avian breeding success. Finally, given the impacts of the amount of supplementary food that we provided and taking clutch size differences into account, our results suggest that urban insect populations in our study system would need to be increased by a factor of at least 2.5 for urban and forest great tits to have similar reproductive success.

Summary 1.The functional significance of intra-and interspecific structural variations in the flight feathers of birds is poorly understood. Here, a phylogenetic comparative analysis of four structural features (rachis width, barb and barbule density and porosity) of proximal and distal primary feathers of 137 European bird species was conducted. 2. Flight type (flapping and soaring, flapping and gliding, continuous flapping or passerine type), habitat (terrestrial, riparian or aquatic), wing characteristics (wing area, S and aspect ratio, AR) and moult strategy were all found to affect feather structure to some extent. Species characterized by low wing-beat frequency flight (soaring and gliding) have broader feather rachises (shafts) and feather vanes with lower barb density than birds associated with more active flapping modes of flight. However, the effect of flying mode on rachis width disappeared after controlling for S and AR, suggesting that rachis width is primarily determined by wing morphology. 3. Rachis width and feather vane density are likely related to differences in force distribution across the wingspan during different flight modes. An increase in shaft diameter, barb density and porosity from the proximal to distal wing feathers was found and was highest in species with flapping flight indicating that aerodynamic forces are more biased towards the distal feathers in flapping flyers than in soarers and gliders. 4. Habitat affected barb and barbule density, which was greatest in aquatic species, and within this group, barb density was greater in divers than non-divers, suggesting that the need for water repellency and resistance to water penetration may influence feather structure. However, we found little support for the importance of porosity in water repellency and water penetration, because porosity was similar in aquatic, riparian and terrestrial species and among the aquatic birds (divers and non-divers). We also found that barb density was affected by moult pattern. 5. Our results have broad implications for the understanding of the selection pressures driving flight feather functional morphology. Specifically, the large sample size relative to any previous studies has emphasized that the morphology of flight feathers is the result of a suite of selection pressures. As well as routine flight needs, constraints during moulting, habitat (particularly aquatic) and migratory requirements also affect flight feather morphology. Identifying the exact nature of these trade-offs will perhaps inform the reconstruction of the flying modes of extinct birds.

The ubiquitous activity of humans is a fundamental feature of urban environments affecting local wildlife in several ways. Testing the influence of human disturbance would ideally need experimental approach, however, in cities, this is challenging at relevant spatial and temporal scales. Thus, to better understand the ecological effects of human activity, we exploited the opportunity that the city-wide lockdowns due to the COVID-19 pandemic provided during the spring of 2020. We assessed changes in reproductive success of great tits (Parus major) at two urban habitats affected strikingly differently by the ‘anthropause’, and at an unaffected forest site. Our results do not support that urban great tits benefited from reduced human mobility during the lockdown. First, at one of our urban sites, the strongly (− 44%) reduced human disturbance in 2020 (compared to a long-term reference period) did not increase birds’ reproductive output relative to the forest habitat where human disturbance was low in all years. Second, in the other urban habitat, recreational human activity considerably increased (+ 40%) during the lockdown and this was associated with strongly reduced nestling body size compared to the pre-COVID reference year. Analyses of other environmental factors (meteorological conditions, lockdown-induced changes in air pollution) suggest that these are not likely to explain our results. Our study supports that intensified human disturbance can have adverse fitness consequences in urban populations. It also highlights that a few months of ‘anthropause’ is not enough to counterweight the detrimental impacts of urbanization on local wildlife populations.

BACKGROUNDThere has been a long history of anecdotal reports in the field of natural history and comparative (evolutionary) animal behavior. Although, at the time of writing there is an open call for researchers of animal behavior by one of the oldest journal of the field "BEHAVIOR" to report "anecdotal evidence of unique behavior" (Kret and Roth, 2020), nowadays we see a decreasing trend of reporting anecdotes in scientific journals (Ramsay and Teichroeb, 2019). We do not dispute the relevance of publishing rare and novel behaviors or events, as they can be important drivers for future research, but we would like to draw attention to the fact that these reports should follow some standards and authors should be careful in avoiding over-interpretations.An example of possible over-interpretation is a recently published article (Fayet et al., 2020a) that also received a lot of media hype (e.g., 79 news outlets at the time of writing; for more details see Altmetric, 2020). The authors reported on two separate occasions (one accidental field observation and one recorded on an 11 s long video) when two individuals of the Atlantic puffin (Fratercula arctica) were seen picking up or holding a stick in their beak, which then touched their body. These two cases were reported as an "Evidence of tool use in a seabird" (Fayet et al., 2020a).This publication was followed by at least three commentaries [Auersperg et al., 2020;Farrar, 2020;von Bayern et al., 2020; and for further discussion see also Recommendation of the Farrar (2020) commentary by Dechaume-Moncharmont (2020)] that provided partly supportive or alternative views on the original report. The present authors share some of the doubts presented earlier but in this contribution we use the above case as an example to point out the problems with such anecdotal observations in general, and suggest ways to improve the information exchange among researchers.

The plumage of birds plays an essential role in thermal insulation and influences the heat tolerance of birds. These plumage functions are mainly determined by the number and the density of feathers, but it is unclear how feather density responds to environmental changes in wild populations. In urban birds, both high temperature and limited food could generate changes in plumage traits. To investigate the effect of urbanization, we compared the number of feathers in nestlings between urban and forest Great Tits Parus major using a novel non-invasive method. We showed that urban nestlings have fewer feathers than forest nestlings at 6-9 days old. Although the density of feathers was slightly higher in urban nestlings, this was the result of the smaller size of their feather tracts. We suggest that the reduced feather number may be the result of either adaptation to higher urban temperatures, constrained feather development due to limited optimal nestling-food sources in urban environments, or both. Concentrating body feathers in a reduced tract area may also help birds to adapt to higher urban temperatures because this can increase the relative size of bare body surfaces which may facilitate heat dissipation. We suggest several possibilities for future studies that would help to disentangle the underlying mechanisms responsible for the observed patterns.