VARIATION IN EXTINCTION RISK AMONG BIRDS - CHANCE OR EVOLUTIONARY PREDISPOSITION

Collar et al. (1994) estimate that of the 9672 extant species of bird, 1111 are threatened by extinction. Here, we test whether these threat ened species are simply a random sample of birds, or whether there is something about their biology that predisposes them to extinction. We ask three specific questions. First, is extinction risk randomly distr ibuted among families? Second, which families, if any, contain more, o r less, threatened species than would be expected by chance? Third, is variation between taxa in extinction risk associated with variation i n either body size or fecundity? Extinction risk is not randomly distr ibuted among families. The families which contain significantly more t hreatened species than expected are the parrots (Psittacidae), pheasan ts and allies (Phasianidae), albatrosses and allies (Procellariidae), rails (Rallidae), cranes (Gruidae), cracids (Cracidae), megapodes (Meg apodidae) and pigeons (Columbidae). The only family which contains sig nificantly fewer threatened species than expected is the woodpeckers ( Picidae). Extinction risk is also not distributed randomly with respec t to fecundity or body size. Once phylogeny has been controlled for, i ncreases in extinction risk are independently associated with increase s in body size and decreases in fecundity. We suggest that this is bec ause low rates of fecundity, which evolved many tens of millions of ye ars ago, predisposed certain lineages to extinction. Low-fecundity pop ulations take longer to recover if they are reduced to small sizes and are, therefore, more likely to go extinct if an external force causes an increase in the rate of mortality, thereby perturbing the natural balance between fecundity and mortality.