The great tit (Parus major) is a small and colorful bird that is widely distributed across Europe and Asia. It is also one of the most successful urban adapters, being able to colonize and thrive in human-modified habitats such as parks, gardens, and cities.
However, living in urban environments can also pose challenges and stressors for wildlife, such as noise, pollution, predation, and competition.
A new study by researchers from Lund University in Sweden has investigated how urbanization affects the stress levels of great tits, by measuring the concentration of a stress hormone called corticosterone in their tail feathers.
The study, published in Conservation Physiology, has revealed that urban great tits are actually less stressed than their countryside cousins, suggesting that they have adapted well to their new surroundings.
How to measure stress in birds
Stress is a physiological response to environmental challenges or threats that can affect the survival and reproduction of animals. Stress can be measured by various indicators, such as behavior, physiology, or morphology, as per Phys.org.
One of the most common indicators of stress in birds is corticosterone, a hormone that is released into the bloodstream when birds perceive or experience stress.
Corticosterone can have both positive and negative effects on birds, depending on the intensity and duration of stress.
In the short term, corticosterone can help birds cope with stress by mobilizing energy and enhancing alertness.
In the long term, however, chronic exposure to corticosterone can impair immune function, growth, reproduction, and cognition.
Traditionally, corticosterone levels in birds are measured by taking blood samples, which can be invasive and stressful for the birds themselves.
Moreover, blood samples only reflect the corticosterone levels at the moment of capture, which may not represent the average or chronic stress levels of the birds.
Therefore, alternative methods of measuring corticosterone are needed, such as using feathers.
Feathers are made of keratin, a protein that incorporates hormones and other substances from the blood during feather growth.
By analyzing the corticosterone levels in feathers, researchers can estimate the average or chronic stress levels of birds over a period of time that corresponds to the duration of feather growth.
This method is less invasive and more informative than blood sampling, as it can capture the cumulative effects of stress on birds.
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How urbanization affects stress in great tits
The researchers collected tail feathers from 188 adult and juvenile great tits from 12 sites in southern Sweden, ranging from urban to rural habitats, as per Lund University.
They analyzed the corticosterone levels in the feathers using a technique called enzyme immunoassay, which detects and quantifies hormones based on their binding to specific antibodies.
The researchers found that urban great tits had significantly lower corticosterone levels than rural great tits, both in adults and juveniles.
This means that urban great tits experienced less stress than rural great tits during the period of feather growth, which lasted for about two to three weeks in late summer and early autumn.
The researchers also found that corticosterone levels varied among urban sites, depending on the degree of urbanization and the availability of natural habitats.
Great tits from sites with more green spaces, such as parks and gardens, had lower corticosterone levels than those from sites with less green spaces, such as industrial areas and city centers.
The researchers suggested that urban great tits may benefit from factors such as higher food availability, lower predation risk, and milder microclimate, which can reduce stress and enhance survival and reproduction.
They also proposed that urban great tits may have evolved behavioral or physiological adaptations to cope with urban stressors, such as noise, pollution, and competition
A new perspective on urban wildlife
The study has important implications for our understanding and conservation of urban wildlife.
It reveals that urbanization can have different effects on stress levels of different species, depending on their ecological traits and evolutionary history.
It also shows that not all urban habitats are equally stressful for wildlife, and that green spaces can provide refuges and resources for urban animals.
The study also suggested that urban great tits may serve as a model species for studying the effects of urbanization on wildlife physiology and behavior.
By comparing urban and rural populations of great tits, researchers can identify the genetic and environmental factors that influence their stress responses and adaptations to urban environments.
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