each of two populations (for example, prey and predator) might evolve to maximize its efficiency in dealing with the other -- evolution of efficiency might then lead to an evolutionary arms race between predator and prey

an evolutionary arms race is a form of coevolution in which two species evolve escalating adaptations in response to each other

do evolutionary arms races actually happen?   if so, what stops the escalation?

brood parasites are species of birds that lay their eggs in other species' nests -- often reduce the host's own reproduction -- like predators and other kinds of parasites they take advantage of another species for their own benefit

brood parasitism has evolved independently in 6 phylogenetic families of birds . . .

cowbirds (family Icteridae [New World blackbirds and orioles], genus Molothrus)

5 parasitic species (plus one that is not), N & S America

including Brown-headed Cowbird, only brood parasite in North America

cuckoos (order Cuculiformes)

many parasitic species in Africa, Europe, and Asia

most New World cuckoos are not parasitic (some tropical species are) -- NC cuckoos build their own nests

indigobirds (family Estrildidae [Old World finches], genus Vidua)

all 8 species, Africa

also the cuckoo-finch (family Ploceidae [Old World sparrows], one species, Africa), the honeyguides (family Indicatoridae, all 5 species, Africa and Asia), and one duck (family Anatidae, one species, Argentina)

European Cuckoo (the legendary cuckoo) lays its eggs in nests of birds much smaller than itself

potential hosts make it difficult for a cuckoo to succeed -- hosts abandon nests that they know have been visited by a cuckoo about the time the host starts to lay eggs -- they also remove strange-looking eggs from their nests

female cuckoos, however, are sneaky! -- they make brief (10 second) visits to host nests, remove one host egg, lay one egg -- they watch hosts to determine the correct day to lay

in any one area about 4-6 species serve as primary hosts -- but each female cuckoo specializes on one host and lays eggs that more or less match that hosts' in coloration -- recent experiments have confirmed that egg coloration is inherited by female cuckoos (in birds females have a unique chromosome -- the opposite of the situation in mammals -- and this chromosome includes the loci affecting egg coloration)

cuckoo's egg hatches sooner than host's -- one-day-old cuckoo pushes the other eggs out of the nest -- host then feeds the young cuckoo until it is more than 5 times its own size and for almost twice as long as it would feed its own young!

hosts thus display an astounding contrast between subtlety in recognizing their own eggs and stupidity in accepting a young cuckoo!

host discrimination and cuckoo mimicry are a case of coevolution -- host species are less discriminating in rejecting strange eggs in areas without cuckoos (for example, Iceland) -- conversely, cuckoos using nondiscriminating host species (for example, Dunnocks) do not lay mimetic eggs

calculations show cuckoos and hosts might reach an equilibrium -- as hosts make finer discriminations between eggs they probably make more mistakes by throwing out their own eggs -- as long as cuckoos do not get too numerous (cuckoos parasitize only 1-10% of any one hosts' nests), hosts do best to take a little risk with cuckoo eggs because they avoid the risk of throwing out their own eggs -- when discrimination by hosts is limited, evolution of mimicry by cuckoos is also limited, so an equilibrium might result

sometimes hosts might win -- cuckoo hosts in England have changed in less than a century -- possibly because previous host species have become more discriminating, so cuckoos exploiting them have decreased even to extinction -- cuckoos adapting to new, naive host species increased

sometimes brood parasites might win -- in North America Brown-headed Cowbirds have reduced several species to the verge of extinction -- cowbirds originally associated with bison -- in the past century cowbirds have expanded their range following removal of forest and spread of cattle and they have begun to parasitize some new species -- several of these new hosts now are on the verge of extinction -- controversial large-scale efforts to control populations of cowbirds might be saving some of these species

can one species drive another species to extinction? . . . can one species even drive itself to extinction by driving an essential host or prey species to extinction?

failure of host parents to discriminate against nestling cuckoos (young cuckoos in the nest as opposed to cuckoos' eggs) is perplexing -- hosts fail to discriminate even in experiments with a young cuckoo placed beside the hosts' own young!

several observations are relevant to this failure to discriminate ...

• once the young cuckoo gets big enough to recognize easily it is too late in the season for the host to start over again (so the advantage of discrimination for the host is reduced -- but probably not zero)
• the young cuckoo throws out the hosts' other eggs so the host cannot make direct comparisons between its own young and the young cuckoo
• the hosts' gullibility has limits -- although the young cuckoo ends up both bigger and louder than a whole brood of the hosts' young, host parents do not feed it more than they would a normal brood of their own young

again a trade-off might limit the evolution of discrimination -- advantages of discrimination against young cuckoos for a minimal benefit on infrequent occasions might be more than offset by a slight increase in errors in response to the hosts' own nestlings on the more frequent occasions when no cuckoo is present

unlike cuckoos, young indigobirds in Africa have elaborate mimicry of the hosts' young -- both the bright markings inside the mouth and the calls -- are their hosts more discriminating about the young they feed than are the hosts of cuckoos? . . . we don't yet know

in other species of cuckoos, young cuckoos are closer in size to their hosts' young and are usually raised along with the hosts' young -- in these cases the hosts lose less from parastism -- they are correspondingly less discriminating about eggs -- parasites' eggs are correspondingly less mimetic -- another case in which mimicry matches discrimination

conclusion . . .

all evidence indicates that brood parasites and hosts have evolved escalating adaptations in response to each other -- coevolution has resulted in an evolutionary arms race

evidence suggests that in some situations host or parasite can "win" the evolutionary race and push the other species to extinction

hosts face trade-offs that limit their ability to respond to parasites -- in some cases these trade-offs might result in an evolutionary equilibrium of host and parasite

it even seems possible that oscillations might occur -- but there is no evidence from field studies yet