High testosterone levels are something of a mixed blessing for dark-eyed junco songbirds, say researchers from North Dakota State University, writing in the May issue of The American Naturalist. The team's study found that male birds with extra testosterone were more attractive to females and produced more offspring, but the offspring were smaller and more frequently neglected by the parents. The smaller offspring consequently had lower survival rates than their larger siblings.
The extra testosterone also made the male birds sing more sweetly and fly farther. These macho birds proved irresistible to older, more experienced female juncos, but that attractiveness carried some risks. The elevated testosterone levels increased activity, possibly attracting more predators, and made the male birds more susceptible to disease. "They had lower immune function and paid a cost with lower survival rates," said study leader Wendy Reed. Interestingly, the increased testosterone made the dark-eyed male juncos less attentive parents to their offspring as they made fewer nest visits, resulting in less food delivered and less time spent at the nest.
The study was based on observations carried out on more than 400 junco nests in the Appalachian Mountains of Virginia for nine breeding seasons. One group of birds received testosterone implants under the skin and a control group of birds received implants that were left empty. The implants were removed from the birds at the end of each breeding season.
Source: North Dakota State University
