| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Horizons Fairy Wren v. Cuckoo Among cuckoos, deadbeat parenting is a family tradition. A mother cuckoo’s idea of exemplary child care is selecting a songbird nest, dropping in an egg of her own when the owners aren’t looking, and then fluttering away for good. For unwitting adoptive parents, hosting a cuckoo can spell disaster. The cuckoo chick often hatches ahead of the natural brood—and to make matters worse, the interloper frequently rolls legitimate eggs and chicks right out of the nest. In defense, many birds have learned to spot cuckoo eggs and pitch them overboard. Yet once the freeloader hatches, the battle is generally lost. Most parent birds will keep feeding a cuckoo, even though it may look nothing like their own nestlings. In fact, some cuckoo fledglings may grow to be six times larger than their duped caretakers. Australia’s outback is home to a more cunning cuckoo than most. Horsfield’s bronze-cuckoo (Chrysococcyx basalis) lays its eggs within the saclike, woven-grass nests of superb fairy-wrens (Malurus cyaneus). From within the nest’s dim nterior, the cuckoo’s red-brown, speckled eggs appear nearly identical in color and size to the fairy wren’s own clutch. Unable to eject cuckoo eggs for fear of tossing their own offspring, fairy-wrens have taken this evolutionary arms race to the next level. Biologist Naomi Langmore of Australian National University, Canberra, and colleagues report in the journal Science that fairy-wrens have learned to recognize cuckoo chicks. Instead of feeding the monster that murderously ejected their own young, fairy-wrens simply abandon both nest and cuckoo and start a new family elsewhere. Wren mothers can recognize foreign chicks with considerable accuracy. Langmore and colleagues found that of the 29 parasitized nests they monitored, the fairy-wrens abandoned eleven, or roughly 40 percent. By comparison, parent birds deserted none of the 95 nests containing only wren chicks. “It’s a complete turnaround from thinking that the Australian host is way behind. Rather, they have already lost the race at the egg recognition stage, and moved on to chick recognition,” Langmore says. For some reason, fairy-wren mothers are far better at discerning cuckoo chicks than their mates. Males will continue to feed calling cuckoo chicks for many days after females have deserted the nest. Fairy-wrens use at least two cues to identify fledgling freeloaders. Mothers were most likely to abandon nests containing just one nestling, suggesting that they had learned this is a sign that a cuckoo chick had taken up residence and ejected their offspring. The second clue comes from the begging calls of cuckoo chicks, which sound slightly different than fairy-wren young. But the cuckoos are fighting back. Horsfield’s bronze-cuckoo chicks have learned to mimic the triple cheeps of fairy-wren begging calls. The impersonations are accurate enough to fool plenty of parents into feeding the fakers. Some scientists consider their response an example of evolution in action. The fairy-wren’s unique chickspotting skills, say scientists, probably emerged in response to heavy parasitism pressures. As cuckoos refined their egg camouflage, the fairy-wrens countered by stepping-up their own defense strategy. Nest abandonment only works because of Australia’s balmy climate. Fairy-wrens can raise up to three broods per nesting season—a luxury birds living elsewhere don’t have. “The benefits of rejecting chicks are quite high for fairy wrens because they have time to rear a new brood of their own. But in the northern hemisphere, birds have quite a short breeding season, and don’t have time to rear another brood,” Langmore says, so rejecting a chick offers no advantage. But unless cuckoos improve their own parenting skills, the battle between these two birds will continue to rage for the foreseeable future. Counting Coots While superb fairy wrens must overcome the shenanigans of another species, American coots (Fulica americana) face a tougher task—sorting their own eggs from those deposited on the sly by other coots. According to new research published in the journal Nature, coots may accomplish this feat by counting. In the study, biologist Bruce Lyon of the University of California at Santa Cruz tracked the contents of 417 wild coot nests in British Columbia, Canada. He found the “lay ‘em and leave ‘em” attitude epidemic among coots. Nearly 40 percent of nests were saddled with alien eggs, many laid by birds who hadn’t yet started their own nests. Sowing eggs far and wide is easy for coots. These squat gray waterbirds build their bulrush nests quite close to one another, which makes it easy to drop an egg here and there during a casual stroll. Cheating also makes sense from an energetic standpoint. “For coots, eggs are tremendously cheap,” Lyon says. In fact, parasitized nests hosted an average of three illegitimate eggs—the majority laid by next-door neighbors. Kicking odd eggs out is of the utmost importance to coots. A coot’s ability to winnow out foreign eggs spells the difference between life and death for her own young. Up to half of coot hatchlings die because their parents can’t supply enough food. Any strange chicks that survive do so at the expense of a parent’s blood offspring. If that weren’t bad enough, nest parasitism often causes birds to shortchange themselves of eggs. Many bird species stop laying when they feel enough eggs beneath them. But this approach would convince parasitized birds to shut off egg production before they’ve laid their natural limit. Coots may dodge these problems by running the numbers. Coots that rejected parasite eggs, Lyon found, laid average-sized clutches. By contrast, birds that accepted parasite eggs laid smaller clutches. This difference in strategy, Lyon argues, means the birds are always keeping track of how many of the eggs in the nest they consider their own. The birds that included parasite eggs in their counts just got fooled. Lyon found that birds may identify planted eggs by sight. “Each hen’s eggs are fairly distinctive. Close up, the eggs are lots of different colors, with brown and black and lavender spots.” While coots ejected only two percent of their own eggs (which were usually cracked), they successfully spotted up to a third of parasite eggs. Parasite eggs that were much darker or lighter, or of a different hue than host eggs, were the most likely to be rejected. These were deliberately walled up in the nest material where they wouldn’t hatch. But some birds have the bad luck to receive eggs similar in appearance to their own. Coots whose suspicions may have been raised because the egg tally didn’t add up, but can’t bring themselves to evict a well-camouflaged suspect egg, employ a sophisticated second-line defense. Instead of rejecting such eggs outright, coots banish them to the periphery of the clutch. There they remain throughout incubation, despite the regular turning and repositioning accorded to fully accepted eggs. Outlier eggs do hatch, but emerge several days later than the main brood. This strategy, Lyon says, allows coots to hedge their bets. Chicks from the main clutch will hatch first and are more likely to survive than younger, weaker suspect chicks. However, if a few older chicks do die, the nest-owners still have other young that might be theirs to raise. Lyon says coots have learned to count because they have compelling ecological reasons to do so. “Showing animals can count doesn’t show us why they count. The link was finding the ability in a wild animal and demonstrating why it mattered, and clutch size is a very good measure of Darwinian fitness,” Lyon says. In an accompanying article, animal counting expert Malte Andersson of the University of Goteborg, Sweden, writes that “There is suggestive evidence that counting may be important” for an animal to reproduce successfully when subjected to brood parasitism. But, he adds, “it is rarely as clear as here.” However, the study doesn’t convince nest parasitism expert Stephen Rothstein of the University of California, Santa Barbara. Rothstein points out the difference in clutch size between coots that rejected suspicious eggs and those that acceped them amounted to only one egg. To prove coots are counting, he would like to see the effect of adding many more foreign eggs to coot nests. Rothstein suggests a simpler explanation for the birds’ apparent mathematical ability. “Once a coot detects a parasitic egg or another hen near the nest, it could simply become unresponsive to its perception of current clutch size and continue laying its preprogrammed number.” Kathleen M. Wong is Senior Editor of California Wild. |