Older male sparrows don't put much of a fight when they hear a young male singing in their territory, it probably means that younger birds aren't considered much of a threat by other sparrows, a new study has said.
However, a male white-crowned sparrow will act much more aggressively if it hears a bird of the same age singing in a territory it claims as its own.
Angelika Possel, curator of Ohio State University's Borror Laboratory of Bioacoustics and the tetrapod division found this in a study conducted with Douglas Nelson, associate professor of evolution, ecology and organismal biology at Ohio State and director of the laboratory.
“These male sparrows assess an opponent's fighting ability based on age. And for a mature sparrow, a young male is just not going to scare them,” lead author Possel said.
The researchers conducted the study in a migratory population of white-crowned sparrows that nested in a state park in Bandon, Oregon from 2008 to 2011. They have been studying this population since 2005.
In this study, the scientists mapped out territories of 16 male white-crowned sparrows — eight of which had held territories at the park in previous years (identified by bands placed on their legs in previous years) and eight second-year males that had never held a territory there before. Researchers placed a loudspeaker within the birds' territories and played recordings that suggested either a second-year bird or an older, mature bird had invaded their territory.
Several measures determined how threatened the birds were by what they perceived as an incursion into their territories. If the male perceives the bird they hear as a greater threat, it will approach the loudspeaker more closely (to confront the rival), take more flights toward the speaker, and sing more songs.
Results showed that older birds didn't react as strongly when they heard a recording of a second-year bird than they did to one of an older male. In other words, when they heard the second-year male, they didn't approach the loudspeaker as closely, they didn't fly to the speaker as many times, and they didn't sing as often in response.
“Other research suggests that younger male birds are less successful at attracting females than older males. That means older males see these young birds as little threat to them and not worth a lot of attention,” Poesel said.
The results appear online in the journal Biology Letters.