Hatching asynchrony in passerine birds: A consequence of conspicuous egg coloration?
Birds produce eggs of variable patterns and colors, ranging from cryptic splotching and streaking to conspicuous, unmarked, bright white or blue eggs. Why many birds with open-cup nests produce conspicuous eggs is puzzling because their bright colors might be expected to attract the attention of visual egg predators. Two recent hypotheses suggest that colorful eggs act either as a signal of female quality, thus encouraging males to increase parental care of nestlings, or as a blackmail mechanism to force males to contribute more during the egg stage through incubation or providing food to incubating females. In either scenario, females (or incubating males) would be expected to: 1) spend more time on the nest to conceal conspicuous eggs, resulting in shorter incubation periods, and 2) start incubating conspicuous eggs prior to clutch completion to hide them, resulting in asynchronous hatching. We investigated these predictions by collecting data on egg coloration, incubation periods, and hatching asynchrony from natural history accounts published in the Birds of North America Online. We restricted the analyses to North American species in the order Passeriformes (songbirds) that build open-cup nests.
When egg mass and family were controlled, we found no difference in incubation times between species with marked versus conspicuous eggs. These results did not support our first prediction that the length of the incubation period would be reduced in species with conspicuous eggs. Results of independent contrasts, however, showed that hatching asynchrony was related to egg color. Birds that produced conspicuous eggs were significantly more likely to exhibit hatching asynchrony than were birds that produced cryptic eggs, thereby supporting our second prediction. Historically, hatching asynchrony has been hypothesized to be the result of adaptive brood reduction in which females purposefully begin incubating eggs prior to clutch completion to set up a development hierarchy among their nestlings. If food availability is high, then all nestlings survive. If food availability is limited, however, the youngest, weakest nestlings die and the parents benefit by only investing in their most robust offspring. Empirical studies have provided inconsistent support for this brood reduction hypothesis. Our preliminary results suggest that hatching asynchrony may instead be a consequence of females accelerating the onset of incubation to reduce the predation risk of conspicuous eggs.