Science Notebook

-- Rob Stein

Better Breeders Court Danger

Male songbirds given extra testosterone are more likely to father offspring than their peers, but they are also more likely to die during the breeding season, say researchers who studied dark-eyed juncos in western Virginia.

The researchers concluded that the same behaviors that made them more successful in mating put them at higher risk. The hormone appeared to enhance the male juncos' songs and courtship activity and got the attention of female juncos, but it also made them more visible to predators.

The extra testosterone also enabled the birds to escape immediate dangers, but it weakened their immunity and lowered fat reserves, which made them more susceptible to disease.

The researchers, led by Wendy Reed of North Dakota State University, monitored more than 400 nests from 1993 to 2000 after implanting about half the birds with small tubes with crystalline testosterone. The study was published in this month's American Naturalist.

In the short term, the treated birds produced more progeny. But while the extra hormone made them sing better, fly farther and attract more females, it also made their offspring smaller.

Juncos typically form long-term pairs. Those given extra testosterone had no greater success in pairing off with females but were significantly more likely to mate through what researchers call "extra-pair fertilizations." The treated juncos were more attractive to females, but they had poorer avian parenting skills.

-- Marc Kaufman

Better Breeders Court Danger

Male songbirds given extra testosterone are more likely to father offspring than their peers, but they are also more likely to die during the breeding season, say researchers who studied dark-eyed juncos in western Virginia.

The researchers concluded that the same behaviors that made them more successful in mating put them at higher risk. The hormone appeared to enhance the male juncos' songs and courtship activity and got the attention of female juncos, but it also made them more visible to predators.

The extra testosterone also enabled the birds to escape immediate dangers, but it weakened their immunity and lowered fat reserves, which made them more susceptible to disease.

The researchers, led by Wendy Reed of North Dakota State University, monitored more than 400 nests from 1993 to 2000 after implanting about half the birds with small tubes with crystalline testosterone. The study was published in this month's American Naturalist.

In the short term, the treated birds produced more progeny. But while the extra hormone made them sing better, fly farther and attract more females, it also made their offspring smaller.

Juncos typically form long-term pairs. Those given extra testosterone had no greater success in pairing off with females but were significantly more likely to mate through what researchers call "extra-pair fertilizations." The treated juncos were more attractive to females, but they had poorer avian parenting skills.

-- Marc Kaufman