Relationship between reproductive hormones and migration distance in a polygynous songbird, the Red‑winged Blackbird (\u3ci\u3eAgelaius phoeniceus\u3c/i\u3e)

Many bird species migrate to southern overwintering locations to avoid harsh conditions at their breeding grounds, but at the cost of an energetically demanding migration that may delay their spring reproductive development. Previous work on the relationship between migration distance and reproductive readiness has primarily focused on early season baseline testosterone in both males and females. However, for females, testosterone alone may not be the appropriate measurement of reproductive development. Estradiol, a metabolite of testosterone that is essential for breeding behaviors and reproduction, should also be measured. Furthermore, baseline testosterone varies throughout the day and may change due to social interactions that occurred prior to sampling. Injection of gonadotropin-releasing hormone (GnRH) elicits an individual’s maximum potential testosterone production, minimizing daily and social variation. We explored relationships between migration distance and reproductive status after arrival to the breeding ground in Red-winged Blackbirds (Agelaius phoeniceus). We predicted that individuals that travel a shorter distance will have higher levels of reproductive hormones upon arrival given they are able to invest less in migration and more in reproduction. This is important because individuals that breed earlier often have higher reproductive success. In females, we measured baseline estradiol and testosterone. In males, we assessed baseline and GnRH-induced testosterone. Hormone values were related to migration distance, estimated by stable isotope analysis of claw samples collected before breeding began in eastern North Dakota. We found that males with shorter inferred migration distances have higher baseline testosterone upon arrival. However, inferred migration distance was not correlated with GnRH-induced testosterone. Female inferred migration distance was not correlated with baseline testosterone, but it was correlated with baseline estradiol. Females with higher testosterone had lower estradiol, suggesting that testosterone in females is not a reliable indicator of estradiol levels, thus readiness to breed. Our observations suggest that baseline hormone levels were related to migration distance, but baseline testosterone alone may not provide a complete assessment of a male or female’s preparedness to breed following spring migration