Editorial Feature

Comparing the Quality of Life of Urban and Forest Great Tits

Due to its deep modification of biotic and abiotic elements compared to natural habitats, the urban environment, which is extending every year with human population expansion, causes problems for bird survival.

great tit, urban, population

Image Credit: Davide Bonora/Shutterstock.com

Urbanization causes habitat fragmentation and a major reduction in vegetative cover, as well as increased lighting duration and sound levels, a rise in local temperature, and chronic exposure to a variety of hazardous chemical substances.

In metropolitan individuals, physiological and genetic changes are also documented, and various studies have indicated environmental pressures on individual health and survival.

The first goal of recent research published in Birds was to see if the urban environment has a filtering effect on the quality of life of the birds living there.

The study’s second goal was to see if there were any reproductive consequences in terms of physical condition and telomere length by collecting longitudinal data from birds captured in the winter and recaptured throughout the breeding season. Researchers expected a considerable decrease in the physical condition and telomere length in the urban location than in the forest site due to higher reproductive limitations in the city.

Methodology

This research, which took place between February and June 2019, is part of long-term research on Great Tits populations living in artificial nest boxes along an urbanization gradient in Strasbourg, France.

The Great Tit is a popular passerine that breeds well in artificial nest boxes, making reproduction and adult captures easier to track. Furthermore, this species is an urban adaptor, able to survive in both human and natural environments.

Between April and June, Great Tits breed and lay an average of 7.5 eggs in the city and 9 eggs in the forest, which are incubated for about 13 days. A forest population residing in La Wantzenau forest within 12-20 km of Strasbourg was tracked, as was an urban population living in various urban parks and tree-lined walkways in the central region of Strasbourg.

Individuals from both sites were captured twice: first in the winter after the birds had dispersed but before breeding and again during the breeding season.

Results and Discussion

Regardless of the season or location, the age of collected birds differed significantly by sex. Females were taken at a younger age than males. None of the evaluated variables were significant when it came to body conditions (Table 1).

Table 1. Result of statistical analyses of morphological and genomic parameters between urban and forest birds and between seasons (winter and reproduction). Source: Saulnier, et al., 2022

  Age (n = 161) Body Size Index (n = 161) Body Condition (n = 161) RTL (n = 151)
Variable Chi² df p F df p F df p F df p
Site 0.156 1, 154 0.693 45.66 1, 153 <0.001 0.6002 1, 149 0.440 0.548 1, 142 0.461
Sex 61.35 1, 154 <0.001 39.97 1, 153 <0.001 1.6297 1, 149 0.203 0.395 1, 142 0.531
Season 0.400 1, 154 0.527 4.46 1, 153 0.042 3.9457 1, 149 0.051 0.623 1, 142 0.432
Age       5.82 1, 153 0.017 1.1800 1, 149 0.280 0.812 1, 142 0.370
Body condition                   0.577 1, 142 0.450
Site × Age                        
Site × Season       4.18 1, 153 0.048       4.525 1, 142 0.037
Site × Sex                        

 

A substantial connection between the season and the site was discovered for the body size index (Table 1, Figure 1).

The body size of birds between sites across seasons. Means and standard errors are represented. Different letters represent significant differences between seasons for the same site and bars represent the difference between sites for the same season.

Figure 1. The body size of birds between sites across seasons. Means and standard errors are represented. Different letters represent significant differences between seasons for the same site and bars represent the difference between sites for the same season. Image Credit: Source: Saulnier, et al., 2022

For RTL, the interplay between the season and the location was equally important (Figure 2). Winter telomeres in urban birds were longer than telomeres in urban birds taken during reproduction and winter telomeres in forest birds.

The relative telomere length of birds between sites across seasons. Means and standard errors are represented. Different letters represent significant differences between seasons for the same site and bars represent the difference between sites for the same season.

Figure 2. The relative telomere length of birds between sites across seasons. Means and standard errors are represented. Different letters represent significant differences between seasons for the same site and bars represent the difference between sites for the same season. Image Credit: Source: Saulnier, et al., 2022

Finally, researchers looked at differences in the physical condition and RTL of birds caught over both seasons. Between winter and reproduction, none of the variables studied substantially explained changes in body condition or RTL (Table 2).

Table 2. Result of statistical analyses of morphological and genomic parameters changes between seasons for re-caught birds. Source: Saulnier, et al., 2022

  Body Condition (n = 27) RTL (n = 23)
Variable F df p F df p
Site 0.048 1, 21 0.829 0.161 1, 16 0.694
Sex 0.523 1, 21 0.478 1.105 1, 16 0.310
Age 2.287 1, 21 0.146 0.069 1, 16 0.796
Fledging number 0.850 1, 21 0.367 0.689 1, 16 0.419
Hatching date 0.111 1, 21 0.742 2.778 1, 16 0.115
RTL Winter       0.527 1, 16 0.479

 

There were no variations in body condition between sites or seasons. Due to the high energetic demands associated with reproduction, bodily condition before breeding is linked to breeding success.

Nonetheless, investigations on urbanization in different bird species, particularly in adult birds, have not consistently found a difference in bodily condition. Anthropogenic food, which is more predictable and abundant, compensates for the loss of prey in cities.

Human-provided diets, on the other hand, are frequently of lower quality than natural foods, often providing insufficient protein, and can result in physiological deficits.

The findings also show a favorable link between body size and avian age, regardless of size or season. This connection could be the consequence of feather quality and the molt stage of the birds, as the body size index is partly linked with wing size.

In a study of common blackbirds, a relationship between wing lengths and age was discovered: younger blackbirds tended to have shorter wings than older birds.

Feather grade and length tend to rise between the post-fledging partial molt and the post-breeding adult molts for a variety of passerine species, particularly Great Tits. However, there was no change in age distribution between sites or seasons, implying that smaller breeders exist regardless of the age of the birds.

To fully comprehend the links between telomere length, reproductive costs, and weather in the city, it would be fascinating to conduct this experiment over several years with varying weather conditions.

Bolder birds had lower maximum corticosterone concentrations, stronger antioxidant capacity, and lower concentrations of reactive oxygen metabolites, resulting in decreased oxidative stress, according to certain research. The fact that urban birds have longer telomeres in the winter could be related to the selection of birds with a stronger ability for self-maintenance or a lower metabolic cost, which affects telomeres.

Similarly, trap feeders were placed near artificial food sources, which are an important element of urban birds’ winter diet. It is possible that larger individuals, who have higher energy needs, prefer these areas more than smaller individuals, increasing their chances of being caught.

Conclusion

Finally, the findings show morphological and genetic differences between winter individuals and breeders, but the topic of the city’s faltering role remains unanswered. If researchers consider that a smaller size offers an advantage in an urban setting and that smaller persons are of higher quality, researchers can conclude that the urban environment does indeed filter individual quality.

The results for telomere length throughout the reproduction period, on the other hand, do not support the filter hypothesis. These findings point to a greater cost of reproduction in the metropolis that is influenced by weather conditions.

Long-term research is needed to better comprehend the selection mechanisms that occur in cities, as well as the dynamics connected with inter-annual weather fluctuations.

Journal Reference:

Saulnier, A., Bleu, J., Lemonnier, G., Uhlrich, P., Zahn, S., Massemin, S. (2022) Does the Urban Environment Act as a Filter on the Individual Quality of Birds? Birds, 3(1), pp. 84-98. Available Online: https://www.mdpi.com/2673-6004/3/1/7/htm.

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