Microbial infection affects egg viability and incubation behavior in a tropical passerine

Many avian species initiate incubation before clutch completion,which causes eggs to hatch asynchronously. This influences broodcompetitive dynamics and often results in nestling mortality.The prevailing hypotheses contend that parents incubate earlybecause asynchronous hatching provides fitness benefits to parentsor surviving offspring. An alternative idea is that early incubationis the best of a bad job because of the costs of delaying incubationto the viability of first-laid eggs. To explore this, we examinedthe potential for microbial infection, and the relative effectsof infection and suboptimal development temperatures on theviability of pearly-eyed thrasher (Margarops fuscatus) eggs.We exposed newly laid eggs for 5 days at either end of a tropicalaltitudinal gradient and cleaned shells of half the eggs toreduce microbial growth. Uncleaned eggs were infected more thanwere cleaned eggs, and infection was greater for eggs exposedat the cool, humid site than at the hot, less humid site. Parentallyincubated eggs, however, were not infected, suggesting thatincubation limits infection. The consequence of exposure toinfection and high ambient temperatures was a dramatic reductionin viability; cleaned eggs held at the cool site had the highesthatching success, which was significantly greater than for uncleanedeggs at this site and for cleaned eggs held at the hot site.This provides the first evidence that microbes can infect unincubatedeggs of a wild bird, and that infection and ambient temperatureact independently to reduce hatching success. These factorscould affect avian life-history strategies in diverse habitats.