Stress Responses and Adaptability: How Birds Cope

Birds live busy lives filled with hidden dangers and plenty of hard work. Stress plays a major role in their survival as a mechanism for reacting to potentially dangerous conditions like the sudden appearance of a hawk or the loss of physical condition from long-term food shortages.

Birds react to stress through an endocrine response that releases stress hormones. These chemical messengers can be measured through body fluid and tissue samples, but even the casual observer can observe their results when startled birds burst into flight.

In this guide, we’ll take a closer look at some of the causes of stress in birds and how they respond and adapt to these natural and human-caused challenges.

Understanding Stress in Birds

Definition of Stress

We’ve all experienced stress, but before we delve deeper into the avian stress response, it would be wise to discuss what stress means in a biological context. Stress can be loosely defined as the physiological reaction to challenges and stimuli perceived as threats. Stress triggers an endocrine response in birds that causes both positive and negative physiological and behavioral responses.

Acute stress results from a sudden but short-lived threat or event, like being chased by a predator or frightened by a thunderclap. Chronic stress is a long-term reaction to situations like droughts or unusually cold seasons.

Common Stressors

Birds are sensitive to various stressors (conditions that cause stress), which may impact them in the short and long term. The following are typical examples:

Weather variables like heat and cold
Food shortages
Competition and aggressive social interactions
Physical exertion
The presence of threats like predators and humans
Disturbances like noise, traffic, and light pollution
Environmental degradation and chemical pollution
Reproductive efforts, i.e. nest construction and feeding chicks
Molting

Birds are sensitive to various stressors (conditions that cause stress), which may impact them in the short and long term. Zitting Cisticola feeding young chick

Hormone Responses to Stress

Role of Corticosterone

The adrenal glands produce corticosterone, a glucocorticoid stress hormone. It is secreted into the vascular system by instructions sent from the hypothalamus, a region of the brain. These hormones help birds cope with stress by altering their behaviors and physiological processes to help them adapt to the cause of stress.

Fight or Flight Response

When faced with a sudden threat, birds must act fast and take the actions most likely to ensure their survival. Most birds are no match for their predators, so they typically rely on their speed and agility to escape danger. However, a trapped bird, or one faced with an aggressive challenger or territorial intruder, may choose to fight and drive the threat away.

Fight or flight is initiated by the avian nervous system, which stimulates the release of endocrine system hormones epinephrine and norepinephrine from the adrenal glands. These hormones trigger rapid changes that help birds spring into action for a quick getaway or a fierce fight. Physiological changes include increased blood flow, pressure, and heart rate.

Behavioral Manifestations of Stress

Changes in Behavior

Different species and different individuals respond to stress in different ways. Acute stress may cause the fight or flight response described above, which puts an immediate hold on normal day-to-day behaviors like feeding and preening in favor of survival.

Long-term, chronic stress has less dramatic but equally important effects on bird survival by affecting breeding behaviors and foraging strategies and behaviors.

Stress and Migration

For birds, annual migration is a physically demanding behavior that involves intense activity, reduced food intake, and sometimes increased risk of predation and exposure to extreme weather events. However, birds must brave this seemingly stressful activity to escape the greater stress of surviving the winter and food shortages it brings.

Birds have evolved to cope with these stressors as part of their natural yearly cycle, and while levels vary between species, birds do not necessarily have high stress hormones during migration.

Long-term Adaptation to Stress

Physiological Adaptations

Rapid response to acute stressors like pursuit by a predator has obvious benefits for bird survival, but long-term exposure to chronic stress has quite the opposite effect. Potential results include lowered immune function, decreased fitness, and reduced breeding success.

Adaptability to stress varies between individuals and species, with some birds, like House Sparrows, showing a remarkable ability to adapt to new environments. These birds have adapted their behavioral responses to stress to such a degree that ‘wild’ birds may happily hop around your feet at a park or live within busy supermarkets.

Most birds cannot adapt to urban stressors like traffic, noise, and human activity, but many can adapt to other long-duration stressors. Resident birds that endure harsh winters manage environmental stressors by altering physiological processes like their basal metabolic rate.

Behavioral Coping strategies

Exposure to chronic stress can cause some marked behavioral changes in wild birds. Food shortages and resulting stress may be important departure cues for irregular, facultative migrants like the Pine Siskin or increase risk-taking behaviors in hungry Zebra Finches.

Some long-lived birds can also alter their breeding behaviors in response to stress and avoid nesting when subjected to chronic stress to preserve their health for future attempts.

Implications of Stress on Bird Health and Populations

Health Consequences

Long-term stress can negatively impact bird reproductive success and affect social interactions and feeding behaviors. Captive birds suffering from chronic stress may show some typical behavior changes like reduced eating, aggression or withdrawal, and harmful feather plucking.

Population-level Effects

Stress can significantly affect birds at individual and group levels, but some stressors act over wide areas and affect entire populations or even species. Birds with specific habitat requirements or limited distributions may be especially vulnerable to the impacts of acute or chronic stressors in their environments.

The ability to survive environmental stressors can impact species and population distributions. Over time, unpredictable events like droughts may result in local extinctions and recoveries but also contribute to rapid population-level changes through natural selection when survival is limited to individuals with favorable traits.

Food shortages and resulting stress may be important departure cues for irregular, facultative migrants like the Pine Siskin. Pine siskin perching in a pine tree

Research and Conservation Considerations

Studying Stress in Birds

Scientists can study the stress response in birds by quantifying the levels of the stress hormone corticosterone in their bloodstream. However, to collect a fresh blood sample, researchers must capture birds, which can affect their stress hormone levels.

Researchers can also measure corticosterone levels from wild birds by collecting tissue samples or with less invasive techniques like stool samples.

Feather sampling offers a promising new method for monitoring bird stress since plucked feathers do not reflect increased corticosterone levels from handling. However, these samples only provide a glimpse into the bird’s stress levels from the time when the feather was actively growing.

Conservation Strategies

So why would scientists want to catch birds or collect bodily samples to measure their stress levels?

One good reason is to gather information about the health of birds exposed to stressors in their environment. This data provides valuable current information on the health of individuals and populations but also helps conservationists and managers predict future trends since stressed birds can suffer decreased breeding success.

Feather sampling offers a promising new method for monitoring bird stress since plucked feathers do not reflect increased corticosterone levels from handling

Summary

Just like us, birds experience stress, which can be hard on their health in the long term. However, our feathered friends would not survive for long without the production of stress hormones like corticosterone from the endocrine system or the sudden sympathetic nervous system activation that triggers the release of epinephrine and norepinephrine.

Understanding and measuring the avian stress response provides a window into the relationship between birds and their environment. Looking at the bigger picture, stressed birds can also indicate stressed ecosystems since avians are such essential role players in natural systems worldwide.