Developmental asynchrony and onset of incubation among passerine birds in a mountain birch forest of Swedish Lapland

Abstract

The onset of incubation and the developmental asynchrony (presumably equivalent to the ensuing hatching spread) were investigated by transilluminating and photographing egg clutches of 14 passerine species in a mountain birch forest of Swedish Lapland. By comparing the visible developmental stages in the eggs (state of yolk swelling, appearance of the embryo and extra-embryonic membranes) with stages of known age obtained from artificially incubated eggs, the time interval between the youngest and oldest stages in each clutch was estimated. Most clutches were asynchronous. The degree of asynchrony ranged from 0.5 to about 2.0 days, with the majority of values ranging between 1 .0 and 1 .5 days. The pattern of developmental spread among the eggs indicated that a pronounced increase in incubation intensity occurred after the penultimate egg was laid or one day earlier. A minority of theclutches showed developmental synchrony. These were significantly more frequent in the coldest and most rainy of the investigated seasons.