Some bluebirds find happiness on the cheating side of town

In his 1953 hit song, the late country singer-songwriter Hank Williams crooned “your cheatin’ heart,” which critics say was inspired by infidelity in his marriages. Some call it the “unofficial anthem” of country music.

It also could be a theme of sorts in the bird world. While scientists estimate that more than 90% of bird species are monogamous — one male and one female forming a “pair bond” to raise a brood — the sex lives of birds actually might rival a country song about unfaithful spouses. In other words, a lot of birds cheat on their mates.

For decades, faithful sexual partnerships among birds were thought to be the norm. But in the 1980s, studies by Patricia Gowaty, then at the University of Georgia, and others began to shatter that notion.

Take, for example, Eastern bluebirds. At this time of year, no sight can appear sweeter than a seemingly inseparable pair of bluebirds, male and female, feeding their babies in a nest box. Yet,

Gowaty, using genetic techniques to observe bluebird offspring in Georgia and South Carolina, found that 15% to 20% of chicks being raised by a bonded pair of bluebirds were not fathered by the male. (Male birds can’t distinguish their own babies from those sired by other males in a nest.)

Since then, other studies have shown that both males and females in “pair bond” relationships commonly cheat on each other. Gowaty found that of 180 monogamous bird species, only about 10% remain sexually faithful.

In his book, “The Sibley Guide to Birds and Behavior,” ornithologist David Sibley says it appears that sexual loyalty “may be the exception rather than the rule among birds.”

Why do birds cheat — or engage in “extra-pair copulation

(EMC),” as scientists call it? Researchers have suggested that female birds seek sexual dalliances with other males because of a natural urge to produce the best possible offspring. A male, on the other hand, may try to mate with other females to spread his genes as widely as possible.

In the sky

From David Dundee, Tellus Science Museum astronomer: The moon, new this weekend, will be a thin crescent by Monday night. Four planets are visible now: Venus, Mars, Jupiter and Saturn, all low in the east a few hours before sunrise. Mars will appear near Saturn on Monday before sunrise.

CREATIVE COMMONS Bluebirds are monogamous, meaning a male and a female form a “pair bond” to raise babies during the nesting season. Yet studies show “cheating” often occurs among birds in monogamous relationships.