Testosterone dynamics of migratory birds during stopover

Area of Science:

• Avian ecology
• Endocrinology
• Animal behavior

Background:

• Migratory birds must balance energy reserves with hormonal changes for breeding.
• Spring stopovers are critical periods for physiological preparation for reproduction.

Purpose of the Study:

• Investigate relationships between testosterone, body condition, sociality, territoriality, and fueling rates in Western Subalpine Warbler males during a trans-Saharan stopover.
• Examine how hormonal profiles and behaviors during migration relate to breeding readiness.

Main Methods:

• Collected blood samples to measure baseline and induced testosterone levels.
• Assessed body condition and fueling rates.
• Simulated territory intrusions to observe behavioral and hormonal responses.
• Analyzed data in relation to social density and stopover duration.

Main Results:

• Testosterone levels were highly variable, with some males showing breeding levels and others undetectable levels.
• Testosterone correlated with body condition, indicating an endocrine-energy link.
• Simulated territorial intrusions increased testosterone to breeding levels.
• Higher territorial male density was associated with lower testosterone, suggesting a 'dear enemy' effect.
• Decreasing testosterone correlated with increasing body condition, potentially due to stopover territoriality.

Conclusions:

• Hormonal preparation for breeding begins during spring stopovers and is linked to body condition.
• Stopover territoriality may modulate the negative impacts of high testosterone.
• Male-male social interactions during spring migration mirror those during the breeding season.