Date: Fri, 11 Apr 2003 16:23:07 -0400 Reply-To: Maryland Birds & Birding Sender: Maryland Birds & Birding From: Philip Webre Subject: costs of gaudiness Mime-Version: 1.0 Content-Type: text/plain; charset=Windows-874 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline Sexual selection=20 The cost of showing off Apr 10th 2003=20 From The Economist print edition Bird populations with flashier males are at higher risk of extinction CONSPICUOUS consumption has its price, at least in the avian world. For = decades, biologists have presumed that the gaudy plumage sported by the = males of many bird species must exact a toll. Indeed, one of the best = theoretical explanations for the evolution of such plumage positively = requires that it exact a toll: according to this model, the costs of = gaudiness are an unfakeable signal to the opposite sex that an individual = is otherwise fit and healthy. And there is a fair body of evidence to = support the idea that males do, indeed, suffer for their art by leading = more perilous and shorter lives. A new twist to this tale has just been reported by Paul Doherty, of = Colorado State University, and his colleagues. They have shown, in a paper = recently published in the Proceedings of the National Academy of Sciences, = that it is not only males who suffer the costs of luxuriant display. = Instead, populations that have gaudy males seem to be at greater risk of = local extinction than those of their dowdier cousins. Dr Doherty studied how bird populations have changed in the continental = United States and southern Canada over the course of 21 years, from 1975 = to 1996. The data came from the North American Breeding Bird Survey (BBS), = which is carried out each year by an army of volunteer bird-watchers who = travel along predetermined routes, stopping at intervals and recording all = the species that they can detect. Dr Doherty and his team combed the data collected by the BBS to find out = how populations of particular species had changed over the years. They = picked 153 species in which males and females sport different-coloured = plumage (in such cases the male is always the gaudier sex); and 185 = species for which male and female plumage looks identical=97at least, that = is, to the human eye. The apparent absence of a species from a site during the three minutes = that an observer from the BBS spends recording data at each place does not = necessarily mean that it is not really there. This is because more = colourful birds may just be easier to spot than dull ones. Statistical = techniques must be used to correct these problems and estimate how often a = recorded absence from a site of a species previously found there is a = genuine absence. But once the number-crunching has been done, it turns out = that populations of species with gaudy males are 23% more likely to become = extinct than are those of their more modestly feathered counterparts. Exactly why this should be the case remains unclear. Although gaudy males = are at individual risk, that need not necessarily harm a population's = prospects=97particularly if, as is often the case in such sexually = dimorphic birds, the male contributes little to the actual raising of the = young. One possibility is that the drop in genetic diversity that happens = if most females are mating with the same handful of attractive males means = that the whole population becomes vulnerable to new diseases. The good news for bird-watchers is that local extinction caused in this = way is generally temporary. Biology abhors a vacuum as much as physics, = and the ecological niches vacated by such extinctions are filled by = migrants from elsewhere. The bad news, Dr Doherty fears, is that as = natural habitat becomes fragmented by human activity, that might not be so = easy in future. Being dull may thus be a good response to human interventio= n. =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D To leave the MDOsprey list, send e-mail to listserv@home.ease.lsoft.com with the following message in line 1: signoff mdosprey =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D =========================================================================