Introduction to urban welfare ecology research

20 Dec 2019

Urban welfare ecology can be considered a subfield of welfare biology, which has been fostered as the study of living beings with respect to their positive and negative wellbeing. While welfare biology encompasses the wellbeing of all wild animals, urban welfare ecology would focus on the wild animals who live in urban, suburban, and industrial environments. There’s an increasing amount of information about the lives of animals in these environments, but very little of it focuses on the wellbeing of individuals. Because of this, information has to be drawn from secondary use of information in a variety of fields, especially urban ecology, and also animal welfare science, animal behavior, and population ecology, among others. For example, from population ecology we can learn about the size and ages of animals in populations, and the biological and environmental factors that may be driving them. Information about reproduction and survival at different ages and life stages can help us understand what the lives of animals are like in terms of their wellbeing.

The field of urban welfare ecology would study animals living in environments shared by humans. However, its aim would not be to just learn how to best achieve co-existence without doing harm. Rather, it would consist in understanding what the lives of urban animals are actually like, and what factors affect them positively or negatively with regard to their welfare. Applied urban welfare ecology would thus inform decisions about courses of action to help make their lives better and how to make the actions compatible with other aims of society so animals can benefit rather than be harmed by them.

Here, we’ll exemplify what early work in this field will be like, by giving an overview of what the lives of some common urban animals are like, particularly factors related to reproduction, death, and health. We’ll consider five features of urban animal populations and their potential to impact the wellbeing of the individuals in those populations. Then, we’ll consider how this kind of information can help us make better decisions about how to improve the lives of wild animals in urban environments. Finally, we’ll discuss some of the important gaps in our knowledge and what we consider promising areas for further study.

Although the majority of animals living in urban environments are reptiles, amphibians, and invertebrates like insects and arachnids, there is little information about their lives. We focus here not on the animals that are most representative but those about which there is a lot more information. They are birds, bats, and rodents like mice, squirrels, and woodchucks. Of the small the number of species we looked at, most tended to have high population density and small territories, long breeding seasons, high survival in winter, and low death rate due to predators. Health and stressors appear to be more variable.

 

Death rates

Many birds and mammals in urban environments have relatively low death rates. Pigeons, robins, blackbirds, field mice, fox squirrels, and woodchucks (groundhogs) are some examples.1 This can be seen in the number of juveniles who reach adulthood2 and the presence of aging animals in urban environments.3 The high number of juveniles who reach adulthood may be due in some cases to a combination of low predation and comparatively high mortality rates in adults due to motors vehicle accidents, especially in species where animals tend to disperse, such as fox squirrels.4

The presence of old, sick, and handicapped animals5 also suggests that predation risk is low for many populations of urban animals that have been studied, because predators generally target weak and sick individuals. Many animals in urban environments show low vigilance behaviors (scanning for predators) and low stress levels, which also indicate low predation risk.6 In squirrels, juveniles are more vigilant than adults, which seems to indicate that they learn from experience that they are in a low-threat environment. Low vigilance levels also increase the amount of time animals have for foraging, and can improve body condition and reproductive success.7

Winter survival rates also tend to be high, probably due to ease of finding shelter and food8 and even migratory birds like blackbirds and robins may spend the winter in urban areas.9 This is likely to be because there is sufficient shelter, particularly in buildings and other constructions, and because of the availability of food during the winter, which often includes food sources that come directly or indirectly from humans. Survival rates do still fluctuate by season and by year for at least some animal populations.10 In some cases, the fluctuations appear to be related to how much human-provided food is available.11

Nestling mortality and egg loss are usually due to predation, weather conditions, and intraspecific aggression.12 In some species of birds, the presence of supplementary food increases chick survival rates13 and may reduce aggression among chicks.14

High death rates are found in house sparrows, mainly from illness and predation. They are preyed upon by owls, sparrow hawks, and other corvids.15 The major cause of death from disease is avian malaria, either directly or because the weakness it causes makes it difficult for them to find food or spot predators.16

Death rates can increase in areas immediately around bird feeders if large numbers of birds around them attract corvids that prey on other birds and eggs.17 Supplemental food may also attract feral cats that attack birds and rodents.18

Another source of mortality is when supplemental food is present during only part of the year, or is insufficient during the breeding season. When supplementary food runs out, more animals die due to lack of food.19

Urban areas tend to provide good shelter from cold, but extreme heat can kill animals in a very short period of time. Bats in Australia are particularly susceptible to hyperthermia quickly followed by death.20

Although there are few studies about invertebrates, what we know about them in general tells us that, because of the reproductive strategies prevalent among them, consisting in having huge numbers of offspring, most of them will die shortly after they are hatched.

 

Reproduction rates

High reproduction rates are common in birds and small mammals in urban environments, including field mice, raccoons, and fox squirrels.21

A lot of the same factors that contribute to low death rates contribute to high rates of reproduction. The main ones are availability of food and shelter and low predation. Fox squirrels use buildings as a place to shelter and raise their young.22 Blackbirds and pigeons use building ledges, bridges and other structures for nesting,23 and some birds, like pigeons, don’t require specific nesting materials and can make use of whatever materials they find, including string and wire.24

Among blackbirds, pigeons, and several other bird species that have been observed in urban environments to have longer breeding seasons, some reproduce more than once per breeding season, and some have larger clutch sizes as well. Urban pigeons can breed all year long, although most of their reproduction is in the late spring and summer. Longer breeding seasons are also found in birds who don’t make use of human constructions for their nests, such as American robins.25

In the presence of human sources of food, intentional or unintentional, many small mammals and birds have been found to have higher body mass and produce more children with higher chances of survival.26 This may be because plentiful food supply allows breeding females to spend less time and energy on foraging, allowing for earlier incubation and better protection of eggs.27 However, lower survival rates can result when population density gets too high.28

Not all animals respond to more favorable living conditions by breeding earlier or more often. Some birds and small animals in urban environments with low predation risk may have slowed rates of physiological aging and a “slow” life strategy, with high investment in self-maintenance and low reproductive rates, as has been found for insular opossums who live in a lower stress environment due to lack of predation and as a result, they have been observed to age slowly, start reproducing later and have smaller litter sizes at all ages.29

Disturbances in communication, including mating calls, have been observed in some songbird and frog populations due to urban noise, although their responses are highly species-specific. It can cause increased vigilance30 and difficulties in parents communicating with each other and with their children, sometimes leading to reduced breeding behavior and fewer and less healthy offspring.31

 

Population density

Animals in urban environments generally have high population densities.32 This is likely because they have small territories,33 and, as we have seen, low rates of death due to abundant supplies of food and shelter and less predation. Restricted movement may be a contributing factor for some animals. For example, it’s difficult for field mice to move between habitat patches because of urban infrastructures.34 Birds are more able to move around and more likely to move throughout a city, but in the presence of food and shelter they tend to maintain relatively small territories where they are safer and can conserve energy.

Other urban animals also tend to have smaller territories and live in the same place year round. Like birds such as pigeons and blackbirds, field mice tend to be sedentary, staying in the same habitats throughout the year,35 and juveniles are more likely to stay near their parents.36 Animals who do disperse, like fox squirrels, are more likely to die from being hit by cars than by predation. Juvenile house sparrows disperse from their colonies and are more likely to die during this stage.37

Flying foxes (fruit bats) generally migrate depending on where there is available food, but fruit trees on the streets of some cities can provide year-round food for them, and some set up a permanent camp in urban areas.38

Densities of squirrels and pigeons fluctuate seasonally and annually,39 and often in response to food availability (usually from human sources).40 An abundance of food contributes to high population density, and very high population densities in small areas can attract predators, particularly birds, and lead to hyperpredation within a small area.41 High population density can also increase the spread of diseases.

 

Intraspecific aggression

Where population densities are higher, intraspecific aggression tends to increase, often when there is increased competition for food. This is seen among squirrels.42 It may also happen between species of birds when there is a high population density at the end of winter and migrants return to the area for breeding season.43

Aggression has been observed even in the presence of large food supplies, and may be due to competition for the highest quality food.

The presence of high quality supplemental food may decrease territorial behavior,44 although in some cases it can increase, apparently because the well-fed animals have more energy for defensive behaviors. For example, Carolina wrens who are provided with food spend more time singing to defend their territories.45

 

Indicators of health and stress

Although low stress due to less risk of predation is observed in many urban species,46 other features of the environment cause stress for some species. Increased vigilance is sometimes seen with higher levels of urban noise.47 Traffic noise has been found to increase stress levels in tree frogs and some birds become more vigilant in response to it. This may be because the noise can mask signs of the approach of predators and impede communication among birds.48

Artificial light, particularly at night, can affect hormone levels and sleep patterns of mammals and birds.49 Nocturnal insects have especially sensitive photoreceptors and they can become disoriented to the degree that their eating and reproductive habits are disturbed.50 Animals who are attracted to lights are at greater risk of being preyed upon.51

Intraspecific and interspecific conflict over food are sources of stress for some urban animals, particularly at very high population densities and at times when there is greater need for food, such as during breeding season and when migrants return to the city after the winter.

Although many animals thrive in the presence of human food,52 high amounts of human-provided bread, cakes, and other sweets have been found to have negative health effects on birds,53 mammals,54 and lizards.55

Another health risk related to supplemental feeding is due to disease transmission from feeders, which varies depending on the number of visitors, feeder type, and habitat.56 Disease transmission can be increased by sedentary habits and high population density.57 Other factors in the spread of disease and parasite infestations are high temperatures58 and polluted waters.59

Parasite infestations are common but vary in their severity. Field mice tend to carry heavy parasite loads and remain healthy.60 House sparrows are highly susceptible to diseases like avian malaria,61 which is caused by a parasite that is present in 75-100% of some populations even when they are not immunocompromised. It can become active as a result of stress, weakened immune system, or low food availability. The active disease results in anemia, weakness, depression, and lack of appetite. Some birds fall into a coma and die, and others remain chronically ill.62

Urban mammals and birds are subject to tick paralysis, a toxin carried by ticks that can paralyze and sometimes kill the animals, either directly or because the paralysis makes them unable to escape from predators.63 Young spectacled flying foxes (bats) in Australia are commonly orphaned when their parents die or become sick and unable to care for them.64

Foot deformities are common in pigeons in urban areas, possibly due to the wires, string, and human hair they use in building their nests. String or human hair can get tightly wound around their toes to the point that it cuts off circulation and can cause a toe to fall off.65

Because many urban animals live longer, common health problems of aging have been observed, including cancer and dental problems.66 Animals living with lifelong handicaps are more common due to low predation rates.

 

More work is needed in this promising research area

Most of the animals we looked at more closely had high resource availability and low predation risk, and on average had fairly long and healthy lives. There seem to be relatively high survival rates at all life stages even at high population densities, though when it gets high enough, population density will drive survival rates down again. This doesn’t mean, however, that the lives of these animals are easy, or that they contain little suffering. On the contrary, all the factors mentioned in our website section about the situation of animals in the wild affect these animals as well and can have a very negative impact on their wellbeing. What we have seen is just that some animals find themselves in a less harsh environment in urban areas.

We must note, however, that here we have only looked at the wellbeing of the studied animals themselves. It is also important to know how their presence affects other animals in their habitats, either positively or negatively. It may be that in a certain situation, we find very good results when we consider the lives of certain animals but the evaluation of the overall scenario is worse. This is crucial because the ultimate aim of gaining this knowledge is to help us make decisions about how to achieve the best possible overall scenario for animals. This might seem a daunting task, but we must also consider the possibility that some fairly simple decisions can be made. For example, policies aiming at reducing the populations of pigeons have been implemented in different urban areas. Pigeons and sparrows eat similar diets, and data from life history theory and population dynamics may imply pigeons have or potentially could have better lives than sparrows. If the impact of pigeons and sparrows on other animals were comparatively equal, then if policies to reduce pigeon populations led to an increase in sparrow populations, it would be negative in terms of the wellbeing of the animals.

In light of this, we can present some tentative ideas about questions that future research in this field could ask. They could include:

  • Which animals, among those typically living in urban areas, have slower life strategies (which means also low stress and better health), and what are their mortality rates, especially at the very beginnings of their lives?
  • What are the positive and negative impacts these animals have on the wellbeing of other animals?
  • What are the effects of their presence in an area in terms of preventing the presence of other animals with easier or harder lives?
  • How easy or difficult it is to have a positive impact (or to reduce our negative impact) in those animals?

Of course, there are other questions that could be relevant here. One is about on which animals more research is being done. Another is for which animals there is more interest in doing further research or in taking them into consideration in policy-making, which would make it easier to promote measures that could have a positive impact on those animals. Yet another one is how fruitful research concerning certain wild animals living in urban environments could be in terms of being applied to a variety of scenarios and of teaching useful lessons for other animals as well. We haven’t explored these questions in this first examination of the field, although they should be considered together with the previous considerations in designing an intelligent strategy to optimize the impact of our efforts for the sake of urban wild animals.


Notes

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2 Peterson, M. N.; Lopez, R. R.; Frank, P. A.; Porter, B. A. and Silvy, N. J. (2004) “Key deer fawn response to urbanization: is sustainable development possible?”, Wildlife Society Bulletin, 32, pp. 493-499. McCleery, R. A. (2008) “Reproduction, juvenile survival and retention in an urban fox squirrel population”, Urban Ecosystems, 12, pp. 177-184. Millsap, B. A. (2002) “Survival of Florida burrowing owls along an urban-development gradient”, Journal of Raptor Research, 36, pp. 3-10 [accessed on 28 November 2019].

3 Partecke, J.; Schwabl, I. & Gwinner, E. (2006) “Stress and the city: Urbanization and its effects on the stress physiology in European blackbirds”, Ecology, 87, pp. 1945-1952. Evans, K. L.; Gaston, K. J.; Sharp, S. P.; McGowan, A. & Hatchwell, B.J. (2009) “The effect of urbanisation on avian morphology and latitudinal gradients in body size”, Oikos, 118, pp. 251-259.

4 McCleery, R. A.; Lopez, R. R.; Silvy, N. J. & Gallant, D. L. (2008) “Fox squirrel survival in urban and rural environments” op. cit. McCleery, R. A. (2008) “Reproduction, juvenile survival and retention in an urban fox squirrel population”, op. cit. Millsap, B. A. (2002) “Survival of Florida burrowing owls along an urban-development gradient”, op. cit. Garnett, S.; Whybird, O. & Spencer, H. (1999) “Conservation status of the Spectacled Flying-fox Pteropus conspicillatus in Australia”, Australian Zoologist, 31, pp. 38-54 [accessed on 28 November 2019].

5 Gliwicz, J.; Goszczynski, J. & Luniak, M. (1994) “Characteristic features of animal populations under synurbization – the case of the blackbird and of the striped field mouse”, Memorabilia Zoologica, 49, pp. 237-244.

6 Blumstein, D.T. (2002) “Moving to suburbia: Ontogenetic and evolutionary consequences of life on predator-free islands”, Journal of Biogeography, 29, pp. 685-692. Lopez, R. R.; Silvy, N. J; Frank, P. A.; Jones, D. A.; Vieira, M. E. P. & Whisenant, S. W. (2003) “Survival, mortality, and life expectancy of Florida key deer”, op. cit. Adams, L. W.; Van Druff, L. W. & Luniak, M. (2005) “Managing urban habitats and wildlife”, in Braun, C. E. (ed.) Techniques for wildlife investigations and management, 7th ed., Bethesda: Wild Society, pp. 714-739. Coss, R. G. (1999) “Effects of relaxed natural selection on the evolution of behavior”, in Foster, S. A. & Endler, J. A. (eds.) Geographic variation in behavior: Perspectives on evolutionary mechanisms, Oxford: Oxford University Press, pp. 180-208. Etter, D. R.; Hollis, K. M.; Deelen, T. R. V.; Ludwig, D. R.; Chelsvig, J. E.; Anchor, C. L. & Warner, R. E. (2002) “Survival and movements of white-tailed deer in suburban Chicago, Illinois”, op. cit.

7 Lima, S. L. & Dill L. M. (1990) “Behavioral decisions made under the risk of predation: A review prospectus”, Canadian Journal of Zoology, 68, pp. 619-640. Brown, J. (1999) “Vigilance, patch use and habitat selection: Foraging under predation risk”, Evolutionary Ecology Research, 1, pp. 49-71.

8 Jokimaki, J. & Suhonen, J. (1998) “Distribution and habitat selection of wintering birds in urban environments”, Landscape and Urban Planning, 39, pp. 253-263.

9 Zúñiga-Vega, J. J.; Solano-Zavaleta, I.; Sáenz-Escobar, M. F. & Ramírez-Cruz, G. A. (2019) “Habitat traits that increase the probability of occupancy of migratory birds in an urban ecological reserve”, Acta Oecologica, 101. Partecke, J. & Gwinner, E. (2007) “Increased sedentariness in European blackbirds following urbanization: A consequence of local adaptation?”, Ecology, pp. 882-890. Jokimaki, J.; Suhonen, J.; Inki, K. & Jokinen, S. (1996) “Biogeographical comparison of winter bird assemblages in urban environments in Finland”, Journal of Biogeography,23, pp. 379-386.

10 McCleery, R. A.; Lopez, R. R.; Silvy, N. J. & Gallant, D. L. (2008) “Fox squirrel survival in urban and rural environments” op. cit. Snow, D. W. & Perrins, C. M. (1998) The birds of the Western Palaearctic, concise ed., Oxford: Oxford University Press.

11 Sol, D.; Santos, D. M. & Cuadrado, M. (2000) “Age-related feeding site selection in urban pigeons (Columba livia): Experimental evidence of the competition hypothesis”, Canadian Journal of Zoology, 78, pp. 144-149. Buijs, J. A. & Van Wijnen, J. H. (2001) “Survey of feral rock doves (Columba livia) in Amsterdam, a bird-human association”, Urban Ecosystems, 5, pp. 235-241. Morand-Ferron, J.; Lalande, E. & Giraldeau, L. A. (2009) “Large scale input matching by urban feral pigeons (Columba livia)”, Ethology, 115, pp. 707-712.

12 Roux, P. le; Kok, O. & Butler, H. (2013) “Broeigedrag en -sukses van tuinduiwe”, Suid-Afrikaanse Tydskrif vir Natuurwetenskap en Tegnologie, 32 [accessed on 24 October 2019].

13 Mock, D. W.; Lamey, T. C.; Williams, C. F. & Pelletier, A. (1987) “Flexibility in the development of heron sibling aggression: An intraspecific test of the prey-size hypothesis”, Animal Behaviour, 35, pp. 1386-1393. Bollinger, P. B.; Bollinger, E. K. & Malecki, R. A. (1990) “Tests of three hypotheses of hatching asynchrony in the common tern”, The Auk, 107, pp. 696-706. Gill, V. A. & Hatch, S. A. (2002) “Components of productivity in black-legged kittiwakes Rissa tridactyla: Response to supplemental feeding”, Journal of Avian Biology, 33, pp. 113-126 [accessed on 22 November 2019]. Gonzalez et al. 2006. Jansson, C.; Ekman, J. & Von Bromssen, A. (1981) “Winter mortality and food-supply in tits Parus spp.”, op. cit.

14 González, L. M.; Margalida, A.; Sánchez, R. & Oria, J. (2006) “Supplementary feeding as an effective tool for improving breeding success in the Spanish imperial eagle (Aquila adalberti)”, Biological Conservation, 129, pp. 477-486.

15 Bell C. P.; Baker, S. W.; Parkes, N. G.; Brook, M. D. & Chamberlain, D. E. (2010) “The role of the Eurasian Sparrowhawk (Accipiter nisus) in the decline of the house sparrow (Passer domesticus) in Britain”, The Auk, 127, pp. 411-420. Goszczynski, J.; Jablonski, P.; Lesinski, G. & Romanowski, J. (1993) “Variation in diet of Tawny Owl Strix aluco L. along an urbanization gradient”, Acta Ornithologica, 27, pp. 113-123.

16 Dadam, D.; Robinson, R. A.; Clements, A.; Peach, W. J.; Bennett, M.; Rowcliffe, J. M. & Cunningham, A. A. (2019) “Avian malaria-mediated population decline of a widespread iconic bird species”, Royal Society Open Science, 6 (7), pp. 182-197 [accessed on 19 August 2019].

17 Martinson, T. J. & Flaspohler, D. J. (2003) “Winter bird feeding and localized predation on simulated bark-dwelling arthropods”, Wildlife Society Bulletin,31, pp. 510-516.

18 Skórka, P.; Sierpowska, K.; Haidt, A.; Myczko, Ł.; Ekner-Grzyb, A.; Rosin, Z. M.; Kwieciński, Z.; Suchodolska, J.; Takacs, V.; Jankowiak, Ł.; Wasielewski, O.; Graclik, A.; Krawczyk, A. J.; Kasprzak, A.; Szwajkowski, P.; Wylegała, P.; Malecha, A. W.; Mizera, T. & Tryjanowski, P. (2016) “Habitat preferences of two sparrow species are modified by abundances of other birds in an urban environment”, Current Zoology, 62, pp. 357-368 [accessed on 28 October 2019].

19 Martinson, T. J. & Flaspohler, D. J. (2003) “Winter bird feeding and localized predation on simulated bark-dwelling arthropods”, op. cit.

20 Bittel, J. (2019) “A heat wave in Australia killed 23,000 spectacled flying foxes”, April 10, NRDC [accessed on 14 November 2019]. Welbergen, J. A.; Klose, S. M.; Markus, N. & Eby, P. (2008) “Climate change and the effects of temperature extremes on Australian flying foxes”, Proceedings of the Royal Society B: Biological Sciences, 275, pp. 419-425 [accessed on 14 October 2019].

21 Krebs, C. J. (2009 [1972]) Ecology: The Experimental Analysis of Distribution and Abundance, 6th ed., San Francisco: Benjamin-Cummings.

22 Ibid.

23 Savard, J. P. L. & Falls, J. B. (1981) “Influence of habitat structure on the nesting height of birds in urban areas” Canadian Journal of Zoology, 59, pp. 924-932. Hetmański, T.; Bocheński, M.; Tryjanowski, P. & Skórka, P. (2011) “The effect of habitat and number of inhabitants on the population sizes of feral pigeons around towns in northern Poland”, European Journal of Wildlife Research, 57, pp. 421-428 [accessed on 2 December 2019]. Przybylska, K.; Haidt, A.; Myczko, Ł.; Ekner-Grzyb, A.; Rosin, Z. M.; Kwieciński, Z.; Tryjanowski, P.; Suchodolska, J.; Takacs, V.; Jankowiak, Ł.; Tobółka, M.; Wasielewski, O.; Graclik, A.; Krawczyk, A. J.; Kasprzak, A.; Szwajkowski, P.; Wylegała, P.; Malecha, A. W.; Mizera, T. & Skórka, P. (2012) “Local and landscape-level factors affecting the density and distribution of the feral pigeon Columba livia var. domestica in an urban environment”, Ornithologica, 47, pp. 37-45. Sacchi, R.; Gentilli, A.; Razzetti, E. & Barbieri, F. (2002) “Effects of building features on density and flock distribution of feral pigeons Columba livia var. domestica in an urban environment”, Canadian Journal of Zoology, 80, pp. 48-54. Ali, S.; Rakha, B. A.; Iftikhar, H.; Nadeem, M. S. & Rafique, M. (2013) “Ecology of feral pigeon (Columba livia) in urban areas of Rawalpindi/Islamabad, Pakistan”, Pakistan Journal of Zoology, 45, pp. 1229-1234 [accessed on 3 December 2019]. Pike, M.; Spennemann, D. H. R. & Watson, M. J. (2017) “Building use by urban commensal avifauna in Melbourne CBD, Australia”, Emu: Austral Ornithology, 117, pp. 284-289.

24 Goodwin, D. (1960) “Comparative ecology of pigeons in inner London”, British Birds, 53, pp. 201-212. Roux, P. le; Kok, O. & Butler, H. (2013) “Broeigedrag en -sukses van tuinduiwe”, op. cit.

25 Morneau, F.; Lépine, C.; Décarie, R.; Villard, M.-A. & Desgranges, J.-L. (1995) “Reproduction of American robin (Turdus migratorius) in a suburban environment”, Landscape and Urban Planning, 32, pp. 55-62.

26 Robb, G. N.; McDonald, R. A.; Chamberlain, D. E. & Bearhop, S. (2008) “Food for thought: Supplementary feeding as a driver of ecological change in avian populations”, Frontiers in Ecology and the Environment, 6, pp. 476-484. Barba, E. L.; Gildelgado, J. A. & Monros, J. S. (1995) “The costs of being late: Consequences of delaying great tit Parus major first clutches”, Journal of Animal Ecology, 64, pp. 642-651 [accessed on 4 December 2019].

27 Bollinger, P. B.; Bollinger, E. K. & Malecki, R. A. (1990) “Tests of three hypotheses of hatching asynchrony in the common tern”, op. cit.

28 Jansson, C.; Ekman, J. & Von Bromssen, A. (1981) “Winter mortality and food-supply in tits Parus spp.”, op. cit.

29 Austad, S. N. (1993) “Retarded senescence in an insular population of Virginia opossums (Didelphis virginiana)”, Journal of Zoology, 229, pp. 695-708. Sepp, T.; McGraw, K. J.; Kaasik, A. & Giraudeau, M. (2017) “A review of urban impacts on avian life-history evolution: Does city living lead to slower pace of life?”, op. cit.

30 Owens, J. L.; Stec, C. L. & O’Hatnick, A. (2012) “The effects of extended exposure to traffic noise on parid social and risk-taking behavior”, Behavioural Processes, 91, pp. 61-69. Quinn, J. L.; Whittingham, M. J.; Butler, S. J. & Cresswell, W. (2006) “Noise, predation risk compensation and vigilance in the chaffinch Fringilla coelebs”, Journal of Avian Biology, 37, pp. 601-608. Zanette, L. Y.; White, A. F.; Allen, M. C. & Clinchy, M. (2011) “Perceived predation risk reduces the number of offspring songbirds produce per year”, Science, 334, pp. 1398-1401.

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35 Ibid.

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