Urban Bird Populations Research Articles

Reproduction management affects breeding ecology and reproduction costs in feral urban Pigeons (Columba livia)

Reproduction management of natural popsulations can have long-term consequences that have to be examined to avoid unwanted side effects. Management policies of urban Pigeons ( Columba livia Gmelin, 1789) include the set up of public Pigeon houses that aim at limiting hatching rate by egg removal. However, long-term consequences of this management method on the ecology of this species are still unknown. In this study we examined how egg removal affected egg-laying cycles of Pigeons by using a powerful method of time-series analysis, the wavelet method. We compared egg-laying cycles in Pigeon houses exposed to different management treatments and found that egg-laying cycles were shorter (4 weeks) in Pigeon houses with egg removal compared with control Pigeon houses without egg removal (11 weeks), suggesting that Pigeons respond to egg-removal pressure by multiplying reproduction attempts. Furthermore, we found that egg quality, an important index of female condition, was negatively affected by egg removal. This result suggests that the observed increase of egg production can lead to an increase of reproductive physiological costs and to a decrease of female condition. This study raises issues about potential consequences of such a management procedure on parasite resistance and health status of urban bird populations.

Do domestic cats impose an unsustainable harvest on urban bird populations?

We assessed the impact of domestic cats on population persistence of native and exotic urban bird populations using a model adjusted for habitat-specific catch rates, cat ownership and hunting activity data. GPS-derived home ranges of 32 cats and resource selection indices demonstrated the degree of penetration and preference for native vegetation fragments. Owners reported on prey brought back by 144 domestic cats in Dunedin, New Zealand, during 12 months. One third of cats never brought back prey, and 21% returned more than one item/month. Cats brought back a mean of 13.4 prey items/year (median = 4), with cats aged <1 year returning more prey than older cats. Birds were the most common prey, followed by rodents. Although cats penetrated adjacent vegetation fragments they did not catch more birds and preferred garden habitat, suggesting that predation pressure may be reduced in fragments. Cat home range size appears to be constrained by cat density while the number of birds caught depends on the density of available prey. Estimates of city-wide catch for six bird species were either more than total urban population size estimates or close to lower confidence intervals. Modelling of three species indicated low likelihood of population persistence with cat predation. The observed persistence of these prey species suggests a meta-population structure with urban populations acting as sinks with source populations located on the city fringe.

Differences in Patch Use Behavior Between an Urban and Rural Species: Effects of Distance from Shelter and Wing Molt-Gaps

It has been suggested that urban bird populations and communities are controlled by bottom-up mechanisms because predation costs are lower in urban than in non-urban habitats. We hypothesized that urban birds are less sensitive to variations in the cost of predation than non-urban birds. We predicted that the house sparrow, a widespread urban species, is less sensitive to variations in predation risk, while foraging, than its rural (less urban) congener, the Spanish sparrow. We quantified foraging behavior of these species, as affected by the proximity to shelter, in large outdoor aviaries. We then clipped feathers from the birds' wings to manipulate escape ability and increase predation risk. We predicted that birds experience increasing predation risk with increasing distance from shelter, and that reduced wing surface increases the birds' sensitivity to risk of predation with respect to distance from shelter. Both species displayed increasing giving-up densities in seed trays with increasing distance from shelter, indicating that foraging costs increase with distance from shelter. As predicted, the two species differed in their response to proximity of shelter: we concluded that house sparrows experienced a less pronounced increase in perceived predation cost with increasing distance from shelter than did Spanish sparrows. Contrary to our prediction, wing surface reduction had no effect on seed tray utilization. Therefore, it appears that, when feeding in patches at distances from shelter, as used in the present study, the cost of predation affects foraging and micro-habitat use in Spanish sparrows more than in house sparrows.

The effect of urbanisation on avian morphology and latitudinal gradients in body size

Urban areas occupy a large and growing proportion of the earth. Such sites exhibit distinctive characteristics relative to adjacent rural habitats, and many species have colonised and now successfully exploit urban habitats. The change in selection pressures as a result of urbanisation has led to trait divergence in some urban populations relative to their rural counterparts, but studies have generally been local in scale and the generality of differentiation thus remains unknown. The European blackbird Turdus merula is one of the commonest urban bird species in the Western Palearctic, but populations vary substantially in the length of time they have been urbanised. Here we investigate patterns of morphological variation in European blackbirds occupying 11 paired urban and rural habitats across much of the urbanised range of this species and spanning 25° of latitude. First, we assessed the extent to which urban and rural blackbirds are differentiated morphologically and the consistency of any differentiation across the range. Paired urban and rural Blackbird populations frequently exhibited significant morphological differences, but the magnitude and direction of differentiation was site dependent. We then investigated whether the nature of latitudinal gradients in body‐size differed between urban and rural populations, as predicted by differences in the climatic regimes of urban and rural areas. Blackbird body‐size exhibited strong latitudinal gradients, but their form did not differ significantly between urban and rural habitats. The latitudinal gradient in body size may be a consequence of Seebohm's rule, that more migratory populations occurring at high latitudes have longer wings. We conclude that while there can be substantial morphological variation between adjacent urban and rural bird populations, such differentiation may not apply across a species’ range. Locality specific differentiation of urban and rural blackbirds may arise if the selection pressures acting on blackbird morphology vary in an inconsistent manner between urban and rural habitats. Alternatively, phenotypic divergence could arise in a stochastic manner depending on the morphological traits of colonists, through founder effects.

The establishment of an urban bird population

1. Despite the accelerating global spread of urbanized habitats and its associated implications for wildlife and humans, surprisingly little is known about the biology of urban ecosystems. 2. Using data from a 60-year study period, this paper provides a detailed description of how the northern goshawk Accipiter gentilis L.--generally considered a shy forest species--colonized the city of Hamburg, Germany. Six non-mutually exclusive hypotheses are investigated regarding the environmental factors that may have triggered this invasion. 3. The spatio-temporal analysis of 2556 goshawk chance observations (extracted from a total data set of 1 174 493 bird observations; 1946-2003) showed that hawks regularly visited the city centre decades before the first successful breeding attempts were recorded. Many observations were made in parts of the city where territories were established in later years, demonstrating that these early visitors had encountered, but not used, potential nest sites. 4. Pioneer settlement coincided with: (i) an increase in (legal) hunting pressure on goshawks in nearby rural areas; (ii) an increase in avian prey abundance in the city; and (iii) a succession of severe winters in the Greater Hamburg area. On the other hand, there was no evidence to suggest that the early stages of the invasion were due to: (i) decreasing food availability in rural areas; (ii) major habitat changes in the city; or (iii) rural intraguild dynamics forcing hawks into urban refugia. While breeding numbers of a potential rural source population were at a long-term low when the city was colonized, prior to first settlement there was a sharp increase of goshawk chance observations in the city and its rural periphery. 5. The urban population expanded rapidly, and pair numbers began to stabilize after about 10 years. Ringing data (219 ringed nestlings from 70 urban broods; 1996-2000) demonstrated that most urban recruits had fledged in the city, but also confirmed considerable gene flow between urban and rural habitats. Analysis of chance observations (as raw data or as detrended time series) suggested a tight coupling of population dynamics inside and outside the city. 6. City-colonizations such as the one described here provide a valuable opportunity to study some fundamental aspects of population ecology on a scale at which detailed monitoring is logistically feasible. Furthermore, a good understanding of urban ecology has become essential for efficient wildlife conservation in modern, human-altered environments.

Cats about town: is predation by free‐ranging pet catsFelis catuslikely to affect urban bird populations?

Even though they are fed daily by their owners, free‐ranging pet catsFelis catusmay kill wild birds and, given their high densities (typically &gt; 200 cats/km2), it has been postulated that cat predation could be a significant negative factor affecting the dynamics of urban bird populations. In this study, we: (1) used questionnaire surveys in 10 sites within the city of Bristol, UK, to estimate cat density; (2) estimated the number of birds killed annually in five sites by asking cat owners to record prey animals returned home; and then (3) compared the number of birds killed with breeding density and productivity to estimate the potential impact of cat predation. In addition, we (4) compared the condition of those birds killed by cats versus those killed in collisions, e.g. window strikes. Mean (± sd) cat density was 348 ± 86 cats/km2(n = 10 sites); considering the eight species most commonly taken by cats, the mean ratios of adult birds/cats and juvenile birds/cats across the five sites were 1.17 ± 0.23 and 3.07 ± 0.74, respectively. Approximately 60% of the cats studied for up to 1 year at each site never returned any prey home; despite this, the estimated number of birds killed was large relative to their breeding density and productivity in many sites. Across species, cat‐killed birds were in significantly poorer condition than those killed following collisions; this is consistent with the notion that cat predation represents a compensatory rather than additive form of mortality. Interpretation of these results is, however, complicated by patterns of body mass regulation in passerines. The predation rates estimated in this study would suggest that cats were likely to have been a major cause of mortality for some species of birds. The effect of cat predation in urban landscapes therefore warrants further investigation. The potential limitations of the current study are discussed, along with suggestions for resolving them.

Garden bird feeding predicts the structure of urban avian assemblages

ABSTRACTHouseholds across the developed world cumulatively spend many millions of dollars annually on feeding garden birds. While beneficial effects on avian assemblages are frequently claimed, the relationships between levels of garden bird feeding and local avian populations are unknown. Using data from a large UK city, we show that both avian species richness and abundance vary across different socioeconomic neighbourhood types. We examined whether patterns in bird feeding could explain this variation. The density of bird feeding stations across the urban environment was strongly positively related to avian abundance, after controlling for differences in habitat availability. This effect was almost exclusively driven by the abundance of those species known to utilize garden feeding stations frequently. In contrast, the density of feeding stations had no effect on avian species richness. We also examined variation in the proportion of households in different communities that provide food for birds, a factor that is not correlated with feeder density. The prevalence of bird feeding across different neighbourhoods declined as socioeconomic deprivation increased, and increased with avian species richness and abundance. Our results suggest that the provision of supplementary food for birds by multiple landowners across a city can impact the status of urban bird populations. The potential for harnessing these actions for conservation needs to be explored.

Urban bird declines and the fear of cats

AbstractThe role of domestic cats Felis catus in the troubling, on‐going decline of many urban bird populations in the UK is controversial. Debate, in the UK and elsewhere, has centred on the level of avian mortality directly imposed by cats, and on whether this is principally compensatory (the ‘doomed surplus’ hypothesis) or additive (the ‘hapless survivor’ hypothesis). However, it is well established that predators also have indirect, sub‐lethal effects on their prey where life‐history responses to predation risk affect birth and death rates. Here, using a simple model combining cat predation on birds with a sub‐lethal (fear) effect of cat density on bird fecundity, we show that these sub‐lethal effects may be substantial for urban songbirds. When cat densities are as high as has been recorded in the UK, and even when predation mortality is low (e.g. &lt;1%), a small reduction in fecundity due to sub‐lethal effects (e.g. &lt;1 offspring year−1 cat−1) can result in marked decreases in bird abundances (up to 95%). Thus, low predation rates in urban areas do not necessarily equate with a correspondingly low impact of cats on birds. Sub‐lethal effects may depress bird populations to such an extent that low predation rates simply reflect low prey numbers.

Living in the city: Resource availability, predation, and bird population dynamics in urban areas

This article explores factors that shape population structure in novel environments that have received scant theoretical attention: cities. Urban bird populations exhibit higher densities and lower diversity. Some work suggests this may result from lower predation pressure and more predictable and abundant resources. These factors may lead to populations with few winners and many losers regarding access to food, body condition, and reproductive success. We explore these hypotheses with an individual-energy-based competition model with two phenotypes of differing foraging ability. We show that low frequency resource fluctuations favor strong competitors and vice versa. We show that low predation skews equilibrium populations in favor of weak competitors and vice versa. Increasing the time between resource pulses can thus shift population structure from weak to strong competitor dominance. Given recent evidence for more constant resource input and lower predation in urban areas, the model helps understand observed urban bird population structure.

Credit or debit? Resource input changes population dynamics of city‐slicker birds

The underlying evolutionary mechanisms of urban bird populations have hardly been studied. High food density and low predation risk serve to explain the global pattern of extremely high urban bird population densities. Both these bottom‐up and top‐down effects are paradoxical since the per capita amount of food is small due to competition, and domestic predator density is high in cities. The bottom‐up paradox can be resolved by taking into account the high food resource‐predictability in cities. Concerning the top‐down effect, recent studies suggest that at least when it comes to nest predation the effect of cats is minor. I suggest that the combination of high food predictability and low predation risk in cities alter bird foraging behaviour, which in turn affects population dynamics. In terms of density, the result is that bird populations exceed the carrying capacity of the urban environment, costing heavily on body condition and/or life span. Under such conditions the population should consist of a few winners and many losers. Only the winners have sufficient access to food resources and the opportunity to reproduce. The highly predictable continuous input of food in the urban environment allows them to “live on their credit”. They may trade off between offspring body condition and clutch size. In the lack of predation, the losers among the fledglings may survive for a relatively long period, getting just enough energy to survive. Though they may never become healthy enough to reproduce, they will have a major contribution to the observed population density. Results of several case studies seem to support the credit card hypothesis and suggest that it can serve as a general rule for the evolution of animal populations and communities in highly predictable human managed environments.

A citywide breeding bird survey for Washington, D.C.

‘DC Birdscape’ was initiated in 1993 to systematically count the birds occurring throughout Washington, D.C. during the breeding season. It involved a coordinated planning effort and partnership between the Audubon Naturalist Society, the National Park Service, and the National Biological Survey, and engaged the participation of more than 100 volunteers. A method for rapidly assessing the status of bird populations over a large area was developed and incorporated into a Geographic Information System to allow a multidimensional analysis of species presence and abundance across a variety of urban land use areas. A total of 91 species were observed, with an estimated total number of 115, making Washington, D.C. almost as ‘bird rich’ as nearby suburban counties. Data from the study clearly indicate that avian species are not randomly distributed throughout the Washington, D.C. metropolitan area, and show affinity, at least in part, to some of the most broadly recognized land use patterns that are commonly used to zone and classify urban areas under development schemes. This study represents a prototype that will allow efficient and economical monitoring of urban bird populations.

Relation of Park Size and Vegetation to Urban Bird Populations in Seattle, Washington

Most research directed toward determining the habitat needs of various birds has centered on natural ecosystems, while urban ecosystems have been largely ignored. However, with the rapid expansion of urban and suburban development and the associated modification of habitat, the importance of understanding the relationship of birdlife to urban habitats is evident. Urban parks, especially, provide much potential avian habitat, and offer an opportunity to study the relationships of bird communities to human-induced changes in habitat such as extensive vegetation modification, reduction in size of suitable habitats, disturbance associated with proximity to human populations, presence of buildings, etc. My purpose here is to examine some effects of changes in urban parks on bird populations in the Pacific northwest. The hypotheses to be tested are that 1) parks with large tracts of natural forest can support a diversity and abundance of birds comparable to those in natural forest areas, and that 2) modification of the forest and small size of parks are associated with reduced diversity and abundance of birds in urban parks.