No, you say, he must mean prothonotary warbler, not promiscuous! No one knows if prothonotaries are promiscuous but, like most temperate zone warblers, they are likely to be as brazen as a black-throated blue. Most warblers don’t wobble a bit at mating with neighbors, so much so that much of their breeding season is not just targeted on producing babies in their own nests but on procreating with as many partners as possible.
DNA Fingerprinting, as exhibited daily on the Maury Povich Show, proves infidelity. Take the hooded warbler, whose average nest contains 33% of young not sired by their father! Who is being unfaithful—him or her? The Smithsonian Migratory Bird Center (SMBC) investigated this scandal. Initially, bets were on the males as the guilty partners, possibly because male biologists were the most verbose on the subject.
Male warblers had been observed 'mate guarding', constantly following their partners, to keep their wives safe from neighboring males with wandering eyes. Using this as a factual foundation, a house of cards was quickly produced. It was predicted that illegitimate youngsters would be most abundant when females were not fertile all at the same time (a female is fertile from about 5 days before she lays her first egg in a clutch to the day before she lays her last egg) because, they reasoned, the amorous male is freed from mate guarding and can flirt with nearby females when his mate is not fertile and his paternity no longer is in need of protection. In other words he has time on his hands.
As the idle male idea gained popularity, it was theorized that females would actively try to be fertile at the same time (synchrony) so that their partners would be preoccupied with mate guarding and have no time to fool around. In addition to keeping their husbands in line, females would gain the added benefit of avoiding the harassment of randy neighboring males. This is an example of how thought games and other forms of arm chair biology can lead one completely astray.
SMBC biologists uncovered the truth by carefully observing what was going on in the woods. Since the clandestine rendezvous of cheating and surreptitious spouses are difficult to observe, putting small radio transmitters on the birds was quite helpful in following their movements. One of the initial surprising discoveries was that a fertile female would actively try to seduce neighboring males by uttering a loud and distinctive call. Much like the Sirens of Greek mythology, males find this "song" irresistible. But if her sultry voice is not enough of an enticement, she will leave her own territory and foray into those of nearby males in her search of a romantic liaison The female's promiscuous behavior showed that mate guarding was not an effective way for a male to keep her to himself but, rather, an attempt to make the best of a bad situation.
But males were also unfaithful. They make sneaky forays onto neighboring territories, even if their own mates are fertile. So, synchrony was turned on its head: more "extra-pair" matings are to be expected when females are highly synchronous in their fertile period than we they are less synchronous.
Upon reflection, these results are not so surprising. A female is prone to stray from her own mate only if she is impressed with neighboring males in comparison to her own partner. Because song output reflects a male's general health, females probably are most attracted to males that sing a lot. After all, a male cannot sing and look for food at the same time so healthy males have more time to croon.
If neighboring males are in different stages of nesting, such comparisons are impossible because a male feeding babies has much less time to sing than one not so occupied. But males are comparable when at the same stage, which is precisely what synchronized breeding on the part of the females produces—lots of males all competing simultaneously in the same early nesting stage and under identical climatic conditions. Under this scenario males are quite likely to encounter fertile females by visiting neighboring territories and the reward of an illicit encounter outweighs the cost of discovery, and punishment, by the resident male.
There is great significance in what SMBC biologists have discovered. For example, if high synchrony promotes extra-pair behavior in the search for good genes, then temperate zone birds will be more prone to such promiscuous breeding than birds living in the tropics. Obviously, only in the temperate zone is nesting constrained by climate and the season short. When monogamy does rule and cuckoldry is rare, the sexes will tend to converge in their roles in reproduction because their reproduction is more truly a cooperative venture. Major differences between tropical and temperate zone birds based on female synchrony have been reviewed in "The Behavioral Ecology of Tropical Birds" (see references). Another major finding is that opportunities for promiscuous breeding are so important to most temperate birds that their choice of where to breed has been influenced.
Simply because there aren’t any neighbors, a male hooded warbler nesting in a small forest fragment cannot find other partners nor can a female choose from among several males. SMBC researchers observed male Hooded Warblers to leave these small forests and and fly over adjacent fields to the nearest woodland in search of females. But, since they could not often hear the female's "I'm ready" call notes, few were successful. Females never left their patch of woods because they had no way to evaluate males they could not hear sing.
The lesson is clear: birds do not avoid small patches of forest because they can’t raise young there but because, with extra-pair mating on their mind, they just don't have enough opportunity to be successfully promiscuous in such isolated places. Clearly, the promiscuous warbler has a lot to tell us!