A baby bird going “cheep, cheep” as its mommy flies home to the nest may not mystify the casual observer. Little tyke’s hungry, wants some fresh worm. What’s not to understand?
An original thinker can see the mystery in the mundane, however, and some very original thinkers are asking complex questions about those cheeps.
Perhaps the biggest question has been, Is this a crooked system? For example, might the babe be making that racket to con its parents out of more than its fair share of worm? If so, are its parents devious in return?
Or is this one of the so-called honest signaling systems, in which the cost of making all that fuss keeps the chick and its parents communicating accurately?
These ideas about chick-parent behavior grew out of the study of sexy animal parts, such as a peacock’s tail. Such splendid ornaments clearly signal, “Hey, sweetie, be mine.” The relative splendor or tattiness of that tail seems to give females an honest signal as to whether the male’s a prime catch or a loser. Most birds can’t cheat and grow a finer tail than they deserve, biologists find.
Biologists have now set out to see if what they’ve learned about sexual signaling applies to the calls of begging offspring, not just in birds but in insects, mammals, and even plants. The answers concern the evolution of communication: how signals develop, interact with others, and then spread or fade away. All in all, there’s been a remarkably large amount of research on whether or not a baby bird cheeps the truth.
Conflict of interest
In the 1960s and 1970s, genetic studies caught up with the family troubles in “King Lear.” Theorists proposed that the modest differences in genes between siblings and between a youngster and either of its parents invite conflicts of interest. A parent, equally kin in genetic terms to each child, might tend to invest equally in the children. Yet each of those little darlings would benefit from sabotaging that equal distribution.
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Also from that era came the companion idea that an honest signal between animals, including such anatomical billboards as a peacock’s tail, has to cost the signaler something. If any old bird could afford to grow an eye-popper of a tail, then abundant cheating would destroy the value of the signal.
These ideas suggested that babies in the nest have an incentive to manipulate their parents into provisioning them magnificently, even if the effort leaves the parent too worn out to tend to another chick or next season’s young. Such conflicts in interest might drive the offspring toward evolving deceptive signals. However, the cost of the begging itself might keep false cheepers in check.
Baby-bird honesty raises three basic questions, according to a 1997 landmark review by Rebecca Kilner and Rufus Johnstone, both of the University of Cambridge in England. First, they asked, does a youngster’s need influence the intensity of begging? Next, do parents respond in proportion to begging intensity? Finally, how much does begging cost?
At that time, they found the evidence mixed for each question.
The answer to the first question now seems to be yes, according to Marty Leonard of Dalhousie University in Halifax, Nova Scotia. She has reviewed recent findings while editing a book on begging, due out next year.
Tests in the mid-1990s, for example, showed that pigeon nestlings that had been recently fed didn’t spend as much time begging as those that researchers had stinted. Likewise, food-deprived yellow-headed blackbirds called at a faster rate and called longer when they’d missed a meal than when they’d been fed on schedule.
Eggs beg too, it seems. Little noises come from white pelican eggs, and if the temperature falls just before hatching, the egg noises can speed up. Just like begging for food, these chirps for warmth intensify as the egg chills.
Chirping after hatching earns a chick a snuggle under a nice warm parent, so researchers propose that rapid chirps from eggs get them warmed, too.
For the second question–whether parents respond to a rising frenzy of begging–some experiments have indicated that the answer also may be yes, Leonard says. Pigeons dole out food to their brood in proportion to the youngsters’ begging. Furthermore, tests in canary nests found that parents give more nourishment to food-deprived chicks.
One study of red-winged blackbirds failed to find any effect from intensified chick begging, but a more recent experiment did. In that test, reported in 1998 by Julie E. Burford and her colleagues at Beloit (Wis.) College, red-winged blackbirds delivered extra food to their nestlings when researchers enhanced the begging cacophony by broadcasting recorded chick calls for 5 minutes. However when researchers played white noise for 5 minutes, the parents didn’t rush in any more food than when the nestlings squawked unassisted.
Tracking responses to different kinds of calls, Leonard and her collaborator Andrew G. Horn of Dalhousie University compared the power of begging squeaks from tree swallow nestlings that had been deprived of a meal or had been recently fed. The hungrier nestlings called more rapidly and frequently. Parents tended to make the first offer of food to a fake nestling near a speaker broadcasting the deprived-chick’s call rather than one putting out a better-fed chick’s call. The parents also made more total attempts to feed the fake chick with the deprivation call, the researchers reported in the January Behavioral Ecology and Sociobiology.
The researchers did a second experiment to work out just what made that hungrier chick’s call so compelling. After electronically tweaking the calls in various ways and broadcasting them to parents, Leonard and her colleagues concluded that faster and more-frequent calling won extra attention from parents. Just getting louder didn’t work.
In a different comparison of parental response, Nicola Saino of the University of Milan in Italy and her coworkers examined begging–this time from real barn swallow nestlings–linked to short-term or long-term troubles. The scientists created a short-term need by keeping otherwise healthy nestlings from getting a meal. For a longer-term problem, the researchers challenged a chick’s immune system by injecting a foreign substance.
Parents tended to favor chicks with either difficulty, giving them a bigger share of the food than they gave less troubled siblings. Nestlings with neither short-term nor long-term troubles had to make do with less parental attention, the researchers reported in the December 2000 American Naturalist.
Feeding responses aren’t just a bird thing. The meerkat, a kind of African mongoose that raises pups cooperatively within a group, also seems to succumb to the power of begging, Marta B. Manser of the University of Pennsylvania in Philadelphia reported in the November 2000 Behavioral Ecology and Sociobiology.
Meerkat pups don’t stay in a nest but tag along after an older group member for hours at a time. As the older forager explores, the trailing pup or pups call repeatedly. Should the lead meerkat locate a tasty scorpion or other invertebrate, a nearby pup switches calls, bleating faster, louder, and with a higher frequency. Watching meerkats and experimenting with recorded calls convinced Manser that pups pleading louder and more intensely got more of the bounty than less vocal youngsters did.
The last of the three questions–whether begging takes a toll on offspring–has proved to be the messiest issue. Leonard says that the metabolic demand of begging has proved relatively modest–less than 1 percent of the energy budget.
But researchers wonder whether begging incurs other costs. It might raise such a ruckus that it provokes attacks by predators.
Field experiments haven’t revealed clear-cut results. For instance, predators molested fake western bluebird nests at ground level more often when the nests were broadcasting begging calls than when they were silent. Yet when the researchers put the nests in trees, they found no clear difference in attacks on noisy and quiet nests.
Good ways to whine
Additional questions are emerging. For example, can begging animals learn especially good ways to whine, much as babies discover the most effective ways to manipulate their parents?
The answer to that one may be an emphatic yes, according to Hilla Kedar’s experiments hand-rearing house sparrows. She and her colleagues fed chicks only when they begged at a certain intensity, and the nestling house sparrows needed only a few hours to find the begging levels that got them the most goodies.
Kedar, who’s at Tel Aviv University in Israel, described the work in the September 2000 Proceedings of the Royal Society of London B.
Another issue is what to make of infants that squawk even when there’s no parent nearby. That’s the case for at least 15 percent of the begging calls from the southern grey shrike, noted Amber E. Budden at University of Wales in Bangor in the May Behavioral Ecology and Sociobiology.
Nestling barn owls have all-chick squawkfests, too. Alex Roulin of the University of Bern in Switzerland proposed last year that with no parents around, the young owls appear to settle who’ll get the goods when Mom or Dad does arrive. In Roulin’s observations, at the actual dishing out of food, a nestling refrained from vocalizing if it had been fed more recently than a sib. Once that needy case was fed, however, the formerly restrained nestling started yakking.
Other researchers are looking at a variety of ways in which one chick influences the begging of its siblings.
Fights as well as begging can influence the parents’ food appropriation, reported Bonnie J. Ploger of Hamline University in St. Paul, Minn., at the Animal Behavior meeting last July.
Her research team put Plexiglas dividers into notoriously raucous egret nests so all the chicks could see each other and get food from their parents but couldn’t touch each other. A hand warmer was tucked into each chick’s partition to prevent chills. To ease the disruption they caused, the researchers took down the entire assemblage at the end of each research day so parents could warm the nest at night.
Egret parents brought home the same total amount of food regardless of whether or not the researchers had partitioned the nests. The partitioning did change the food distribution, however. The alpha chick, the oldest and brawniest, got the biggest share of food when it was free to beat up its siblings, but it snagged only a bottom-ranker’s measure when partitions stopped the fights.
Without those fights, the second-ranked chick rocketed to the top of the food chain. Its mother preferentially gave it food, but its father showed no such favoritism. Ploger proposes that female egrets may have a built-in susceptibility to begging from chick number two.
A fundamental question has emerged from recent work: What’s a begging signal? Many of the experiments to date have relied on sounds, but creatures communicate over various channels.
One is sight. When patches on the head of a young Western grebe flush red, parents seem more likely to feed the chick. Warbler parents, too, seem to select chicks for feeding according to the color. The hue of the chicks’ gaping mouths varies with each young bird’s immune condition.
Larval burying beetles also beg visually, making a waving motion when their parents appear. When parent beetles lay eggs, they provide a dead animal to nourish the young. As the eggs hatch and larvae grow, the parents feed their brood with regurgitated carcass.
When a female dart-poison frog hops up to the little pool where one of her offspring is growing, the tadpole usually starts swimming around. Mom lays eggs in the pool, which the tadpole gobbles. Does the tadpole’s swimming a couple of laps signal either its need or worthiness for food?
Other senses may prove important, too. Baby ageienid spiders stroke their mother’s mouthparts, and she regurgitates food for them. Is that tactile begging? And as seeds form in plants, they synthesize hormones that start the flow of resources from maternal tissue. Is this begging by chemistry?
For inquiring minds, that initial “cheep, cheep” is proving very rich.