Evolution's Rainbow. Joan Roughgarden
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swallows (Hirundo pyrrhonota) are perhaps our closest cousins when it comes to life in the big city.33 Swallows live as monogamous pairs in colonies. Their nests, which look like small pitchers, are arranged side by side. Up to five thousand birds form what amounts to a city of mud huts. Not all swallows live in the big city, though. Some live in small villages of twenty or so nests, and some males hang out outside of town without any nests.
Cliff swallow life includes many features of our own city life—a hot real estate market, trespassing, robbery, hanky-panky with the neighbors, plus presumably some compensations. Nonetheless, most observers of cliff swallow life emphasize the problems, perhaps because the birds live in the countryside, where the virtues of city living are underappreciated. The nests are packed in so densely that occasionally a bird is trapped inside and dies when a neighbor’s construction project blocks its entrance hole. Even worse, the droppings from a nest above may clog up and bury a nest below—swallows seriously need expertise in civil engineering. Swallows also have a major public health crisis. The colony density promotes the growth of bugs that harm the chicks and adults.
Birds occasionally “trespass” into one another’s nests by either barging in on the owner unannounced or following the owner in before he or she can turn around and block the entrance. Seventy-five percent of the trespassers are males. Of nest entries considered “successful,” 14 percent of the time, the trespasser stole grass used to line the nest; 9 percent of the time, a male trespasser forced himself on his neighbor’s wife; 7 percent of the time, the trespasser stole some still-wet mud before it had dried; 3 percent of the time, a female laid an egg, or transferred an egg, into her neighbor’s nest; 1 percent of the time, the trespasser tossed one of the neighbor’s eggs or chicks out the window; and in 0.3 percent of the cases, the trespasser evicted the owner. Enough intrigue for a new TV drama, “Cliff Swallow Vice.”
Females congregate in flocks while gathering mud and grass to make nests. “Surplus” males hang around mud holes waiting for females to alight. The males circle above and then pounce, “forcing” copulation as the pair flails about in the mud. Nonetheless, some males may be innocent of evil intent, traveling to a mud hole for regular ol’ mud, where they encounter a female who “elicits a forced copulation.” Also, “females did not always appear to struggle with the males attempting to copulate with them at a mud hole. Some females clearly allowed successful cloacal contact.”34 Indeed, 86 percent of the extra-pair copulations (EPCs) at the mud hole appeared to “achieve” cloacal contact.
What do husbands back at the nest do about all this? They are “suspected of dealing with the threat of cuckoldry by frequently copulating with their mates.” Indeed, “the male copulated with his mate virtually each time she returned to the nest,” leading to conjugal love “dozens” of time in a single morning.
Biologists have observed which males are “perpetrators” of illicit romance. Among thirty-eight male birds observed in EPCs, one male “committed” twelve copulations, another eleven, the next eight, and so forth. Thirty percent of the EPCs involved the top three, with the numbers trailing off to those who had only one dalliance apiece. Thus only a few males commonly “engaged in this behavior,” whereas most “did it” casually or not at all.
EPCs lead to extra-pair paternity, or EPP—that is, to eggs in the nest fathered by a male other than the male tending the nest. Females place eggs in one another’s nest, leading to varied egg maternity in the nest too. Extra-pair maternity, or EPM, refers to eggs mothered by females other than the female tending the nest.
Females either lay eggs in their neighbor’s nest or transfer eggs laid in their own nest to other nests by carrying them in their bills. Females transfer eggs primarily to nests nearby, within five nests of their own. The transfer typically happens when the recipient nest is left unattended. In several cases, though, a male nest owner allowed a neighboring female to enter his nest and lay an egg there while he was present. The female does not toss an egg already there to make room for hers; she simply adds her own egg to those already there. Around 15 percent of the nests wind up with one or more eggs with extra-pair maternity.
Biologists call transferring eggs “brood parasitism” the bird owning the recipient nest is called a “host” and the bird delivering the egg, a “parasite.” Host birds lay 71 percent fewer eggs than parasite birds, implying that parasites are somehow taking advantage of the hosts. However, the parasites are themselves often parasitized, as they leave their own nest unguarded.
Just as some males are more likely to “perpetrate” an EPC, some females are more likely to be brood parasites. In one study 29 percent of the females labeled as parasites laid eggs in two or more nests, whereas other females laid eggs only in their own nests. Females did not brood-parasitize to get out of housework. The females labeled as parasites contributed just as much parental care and raised as many offspring in their own nests as did their hosts. The advantage to brood parasites is simply leaving more eggs in the nests of other females, not in lowering the size of their own nest. Just as certain females were more likely to lay eggs in another’s nest, some females were more likely to receive the eggs of others. Also, females sometimes transfer baby nestlings who have already hatched.
When EPCs and brood parasitism are taken into account, 43 percent of the nests are estimated to contain an egg unrelated to one or both birds tending the nest. Clearly, cliff swallows have decoupled economic monogamy from reproductive monogamy.
Both EPPs and EPMs lead to eggs in the nest unrelated to one or both of the paired birds at the nest. EPPs and EPMs are not symmetric, however, because an EPP implies that one gamete was transferred, whereas an EPM implies the transfer of two gametes, one from the mother and one from the father. In fact, a female transferring an egg may not be intending to get a free ride, but rather may be transferring the egg to the father’s nest. So-called brood parasitism hasn’t been demonstrated to be competitive at all.
The appetite for seeing theft and deceit everywhere has blinded biologists to other interpretations of what’s going on. Swallows apparently have a distributed system for raising young. Throughout the colony the parenting workload is essentially parceled out to work teams of two adults apiece, which amounts to economic monogamy, even though the egg and gamete trading implies an absence of reproductive monogamy. Each team winds up tending about the same number of eggs and nestlings in its nest, and each clutch of nestlings contains offspring from the neighborhood. (For more on a system of distributed paternity in a closely related species, the tree swallow, see chapter 7.)
After fledging, the juvenile birds gather in flocks called crèches. The adults continue to feed their young for a few days after they have flown the nest by searching them out in these crèches and giving them food there. They can recognize their own offspring by listening for an individually distinctive signature in their calls. Some juveniles do not join the crèches, returning instead to the nests, much as an eighth grader might return to kindergarten for cookies. Called kleptoparasites, the juveniles block the nest entrance and intercept food destined for the baby nestlings inside. The parents “willingly” feed these juveniles. The adults never evict them as they routinely do other adults who trespass. Why? The parents are supposedly unable to recognize the juveniles as thieves and are duped into disgorging their food to someone other than their nestlings.
Why isn’t the food given the juveniles considered a voluntary donation by the adults? Calling juveniles at the nest kleptoparasites criminalizes these birds and implies that the adults are incapable of knowing their own best interests. Indeed, any swallow who doesn’t do what they’re supposed to is criminalized. The males in EPCs are “perpetrators” of copulations, the females are “elicitors” of copulations, and females who place eggs in other nests are “parasites.” Birds haven’t been corrupted by sex and violence in the movies; shouldn’t they be better behaved?
The plot thickens. Males sometimes copulate with other males at mud holes, described as “fights in the mud.” In an experiment in which stuffed models of birds were placed near a mud hole, 70 percent of the copulation attempts were directed by males to the male models. The interpretation was that the males were “mistaken” they were unable to distinguish the sex of the stuffed birds. Hmm . . .
Cliff