Firefinch 



Assailed by the piercing, seeming- 

 ly nonstop demands of a wailing 

 newborn, what mother has not 

 wished, at least for a moment, that she 

 could outsource her childrearing to 

 some kind neighbor? Wouldn't it be nice 

 to fly off for a week, a month — forever — 

 to reclaim freedom, silence, and per- 

 haps even romance? Indigobirds are liv- 

 ing this fantasy. 



They are also providing biologists 

 with another good example of what Stu- 

 art Bearhop is describing among Euro- 

 pean blackcaps in his article, "Change in 

 the Air": the process of sympatric speci- 

 ation, in which one population of ani- 

 mals splits into two species in the same 

 geographic location. 



Ten closely related indigobird species, 

 all native to Africa, consistently lay their 

 eggs in the nests of other birds, mostly 

 firefinches. Then they take off — saving 

 themselves the hassle of foraging for food 

 for their young and freeing themselves to 

 breed again. The firefinches, meanwhile, 

 raise the indigobird young along with their 

 own chicks in the same nest. 



Why doesn't the firefinch parent kick 

 the intruders out of her nest? Even 

 among people, it would be the rare 

 neighbor who would put up with your 

 kids forever without making a squawk. 

 For one thing, when indigobird chicks 

 open their mouths to beg for food, the 

 firefinch mother sees an intricate pattern 

 of black, blue, yellow, and white stripes 

 and dots that, through natural selection, 

 have evolved to look indistinguishable 

 from those of her own chicks. The indigo- 

 bird chicks also mimic the movements, 

 and, in some species, the begging calls of 

 the host's own chicks. 



Yet surprisingly, our observations show 

 that firefinches are not always duped by 

 such evolutionary disguises. When food 

 is in short supply, firefinches may remove 

 the interloper's egg, which differs in size 

 from her own. But when food is plentiful, 

 firefinches raise the indigobird chick. The 

 apparent selflessness of the firefinch may 

 actually mask self-interest: evicting an in- 

 digobird egg may pose too much risk 



Song Lines 



by Robert B. Payne 

 and Michael D. Sorenson 



that the firefinch will damage her own 

 eggs in the process. 



Remarkably, it turns out that the un- 

 usual parenting behaviors of fire- 

 finches and indigobirds were responsi- 

 ble for the evolution of the current array 

 of indigobird species, each associated 

 with different hosts. Songs were the 

 key. Each new indigobird species got its 

 start when one or more females laid 

 their eggs, accidentally or opportunisti- 

 cally, in the nests of a novel host. Their 

 offspring, even in that first generation, 

 comprised a new, reproductively iso- 

 lated "song population" or "host race." 

 When the offspring reached sexual 

 maturity and began courtship, the males 

 of the new song population sang the 

 firefinch songs they had learned from 

 their foster father. Female indigobirds 

 chose the males that sang the songs the 

 females had heard as chicks, songs im- 

 printed in their memories. (As with most 

 other songbirds, the male indigobirds 

 sing, but the females do not.) 



The result was that birds living in 

 one area sorted themselves into mating 

 groups according to the songs they sang 

 and the kinds of finches that raised 

 them. Once the mating groups formed, 

 the stage was set for sympatric specia- 

 tion. Genetic data show that the ten dis- 

 tinct indigobird species that now occur 

 across Africa evolved not over millions of 

 years, as is typical for other bird species, 

 but over just tens of thousands of years. 

 Such rapid emergence of distinct species 

 is a remarkable example of both sym- 

 patric speciation and adaptive evolution. 



Could we re-create and experimentally 

 confirm the critical behavioral conditions 

 that led to new indigobird species? To do 

 so, we populated an aviary with a pair of 

 village indigobirds (Vidua chalybeata), 

 along with twelve pairs of Bengalese 

 finches (Lonchura striata) and twelve 

 pairs of red-billed firefinches (Lagonostic- 

 ta senegala). The birds chose mates from 

 among the available members of their 

 own species, and began to breed. 



As expected, the female indigobird in 



Indigobird 



our aviary laid each of her eggs in the 

 nest of a firefinch, her typical host, and 

 then mated again. Typically she would lay 

 one egg a day in a firefinch nest, then 

 move on to another firefinch nest after 

 one or two days of laying. We gingerly 

 relocated some of the indigobird eggs 

 we found in firefinch nests to the nests of 

 Bengalese finches, a "novel" host. 



Soon enough, the male indigobird 

 chicks were imitating the songs of their 

 new Bengalese finch parents. The crucial 

 test came when the young indigobirds 

 were ready to mate: we played a tape of 

 indigobirds singing firefinch songs 

 through one loudspeaker, and indigo- 

 birds singing Bengalese songs through a 

 second loudspeaker. The female indigo- 

 birds raised by Bengalese parents clearly 

 preferred the Bengalese songs, sexually 

 approaching the speaker. 



The same young indigobird mothers 

 also laid their eggs in nests of the novel 

 host, the Bengalese finch. The indigo- 

 birds made that choice even though 

 nests of the host species their own 

 mothers had intended to use — namely, 

 the nests of the firefinch — were available 

 in the same aviary. In a similar way, in- 

 digobirds in West Africa recently colo- 

 nized two new host species, setting off 

 the early stages of what could be more 

 indigobird speciation. 



As for the indigobird's traditional 

 host, the firefinch, we have observed a 

 surprising example of its "open-minded- 

 ness" toward alternative parenting 

 arrangements. An adventurous indigo- 

 bird climbed into a nest already occu- 

 pied by male and female firefinches. No 

 matter: the intruder snuggled in, laid her 

 own egg, and abandoned it to the good 

 graces of the firefinches, which then 

 took on foster duty — an instance of tol- 

 erance that seems altogether out of sync 

 with the commoner image of the natural 

 world as "red in tooth and claw." 



Robert B. Payne is a professor of zoology and 

 Curator of Birds in the Museum of Zoology at 

 the University of Michigan, in Ann Arbor. 

 Michael D. Sorenson is an associate profes- 

 sor of biology at Boston University. 



September 2006 NATURAL HISTORY 



41 



