Because the barn swallow is very widespread and nests almost exclusively on man-made structures, it's been easy to study this bird for a very long time. One interesting finding is that Hirundo rustica's long tail streamers (outer edge tail feathers) are an excellent indicator of the birds' health and a predictor of breeding success.
Birds with the longest and most symmetrical tail streamers are the healthiest and most desirable mates. According to Cornell's Birds of North America, "Tail length tends to correlate with reproductive success, annual survival, propensity to engage in extra-pair copulation, parental effort, ability to withstand parasites, immunocompetence, and other measures of fitness."
In other words, if you're a barn swallow with a long symmetrical tail you're really healthy, you get to choose the best mate, and your nest will be very successful. You're also likely to be an older bird because tail length increases with age.
The down side is that long-tailed females are fickle. They always get the best mates but even when they're paired up they often "mess around" with un-mated long-tailed guys. "Thus long-tailed male barn swallows are cuckolded more often than their less attractive neighbors," says Frank B. Gill.
The longer the tail streamers, the better the bird. I'll be watching their tails now.
(photo by Cris Hamilton. Today’s Tenth Page is inspired by page 340 of Ornithology by Frank B. Gill.)