When it comes to protecting their nests, reed warblers have something in common with Super Bowl hopefuls, the Pittsburgh Steelers: both believe the best offense is a great defense.  But whereas the Steelers must stop the Arizona Cardinals, the songbirds need to steel themselves against marauding cuckoos.

The reason: reed warbler nests are a favorite target of cuckoos, which are always on the lookout for places to lay their eggs and foist off the hard work of child rearing onto an unsuspecting dupe. But new research shows that reed warblers don't take such abuse lying down. Instead, they mount a mighty defense, mobbing, attacking and loudly scolding invading cuckoos until they give up and fly off.

Scientists report today in Current Biology that the warblers mob in ways that suggest they balance the risks of doing so—such as drawing the attention of potential predators in the process —with the likelihood that their nests will be hijacked by cuckoos.

Justin Welbergen and Nicholas Davies, zoologists at the University of Cambridge in England, studied the mobbing activity of reed warblers by placing wooden mannequins of cuckoos near 191 of the songbirds' nests in rural areas that were home to both species. They found that warblers were three to four times less likely to have cuckoos commandeer their nests if they mobbed the trespassers than if they did not use the defense strategy.

"We found that birds with 'riskier' nests were more likely to defend themselves by mobbing. In addition, we found that birds that nested in riskier locations [where cuckoos were more prevalent, for instance] were less likely to be parasitized if they mobbed," Welbergen says.

In places where parasitism was uncommon, however, mobbing appeared to attract rather than discourage the moochers, suggesting that in these areas the best defense was a weak offense. In these locations, the warblers shunned decoys of parrots, species that do not parasitize their nests—indicating that mobbing is a behavior that the birds do not use indiscriminately.

The researchers say the findings are further evidence of an arms, (wings?) race between reed warblers and cuckoos. Over time, the two species have evolved tit for tat to gain an advantage in their parasite–host relationship. Reed warblers, for example, learned to eject foreign cuckoo eggs from their nests; cuckoos, in turn, began laying eggs that resembled those of the smaller bird, reducing the chances of rejection.

Similarly, being mobbed raises the odds that a cuckoo will be sighted by a larger predator—so cuckoos have evolved to resemble sparrow hawks, thereby discouraging warblers from engaging in mobbing behavior.

"Thus, the emerging view is that hosts use a 'defense in depth strategy,' whereby they deploy sequential lines of defense in a co-evolutionary arms race with corresponding offensive lines of the parasite," Welbergen says.

Interestingly, Welbergen says, many reed warblers did not mob cuckoos even though their nests were in locations with a great degree of parasitism—perhaps because they were naive birds that had to refine their ability to discriminate cuckoos from other species, such as potentially lethal sparrow hawks, he says. "Further studies are needed," he notes, "to test whether reed warblers need to learn to mob cuckoos."

 Michael Sorenson, a Boston University bird biologist, says "it boggles the mind to think about how all of this subtle flexibility in behavior has evolved: What genetic changes [compared with nonparasitized birds] produce in reed warblers the ability to make these seemingly sophisticated responses to variation in parasitism risk?"

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