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 brood competitive dynamics and often results in nestling mortality. The prevailing hypotheses contend that parents incubate early because asynchronous hatching provides fitness benefits to parents or surviving offspring. An alternative idea is that early incubation is the best of a bad job because of the costs of delaying incubation to the viability of first-laid eggs. To explore this, we examined the potential for microbial infection, and the relative effects of infection and suboptimal development temperatures on the viability of pearly-eyed thrasher (Margarops fuscatus) eggs. We exposed newly laid eggs for 5 days at either end of a tropical altitudinal gradient and cleaned shells of half the eggs to reduce microbial growth. Uncleaned eggs were infected more than were cleaned eggs, and infection was greater for eggs exposed at the cool, humid site than at the hot, less humid site. Parentally incubated eggs, however, were not infected, suggesting that incubation limits infection. The consequence of exposure to infection and high ambient temperatures was a dramatic reduction in viability; cleaned eggs held at the cool site had the highest hatching success, which was significantly greater than for uncleaned eggs at this site and for cleaned eggs held at the hot site. This provides the first evidence that microbes can infect unincubated eggs of a wild bird, and that infection and ambient temperature act independently to reduce hatching success. These factors could affect avian life-history strategies in diverse habitats.