Sex-specific maternal effect on egg mass,laying order,and sibling competition in the Bengalese finch (<Emphasis Type="Italic">Lonchura striata</Emphasis> var. <Emphasis Type="Italic">domestica</Emphasis>)

Maternal effects, such as investment in eggs, have profound effects on offspring fitness. Mothers are expected to skew their investment depending on the laying order and sex when unequal sibling competition occurs within a brood because of sex-specific vulnerability and age hierarchy caused by asynchronous hatching. The Bengalese finch hatches asynchronously and shows a moderate reversed sexual size dimorphism. However, contrary to commonly accepted assumptions of size-dependent vulnerability, the smaller sex (male) is more vulnerable to developmental stress caused by sibling competition. We investigated whether maternal investment would be biased by the position in laying order and the sex of eggs, and also explored the possible differences in growth patterns depending on sex, laying order, and age hierarchy by observing chicks fostered to experimentally manipulated broods where brood composition was controlled and age hierarchy was more enhanced than in natural breeding conditions. We found that overall patterns of maternal investment favored the disadvantageous sectors of sibling competition, i.e., eggs of later laying order and sons over those of early laying order and daughters. We also examined the effect of laying order on adult body size and sex differences in growth patterns. When reared in the subordinate age hierarchy, females could compensate for the deficit of decreased growth rate by taking longer to mature, whereas males could not. We suggest that this sex-specific growth pattern could be the cause of sex differences in vulnerability to early developmental stress.     

Early ontogenetic effects on song quality in the Bengalese finch (<Emphasis Type="Italic">Lonchura striata</Emphasis> var. <Emphasis Type="Italic">domestica</Emphasis>): laying order,sibling competition,and song syntax

Birdsong differs from other sexual traits in that the acquisition process involves learning. Especially in close-ended learning species like the Bengalese finch, conditions experienced during the critical song-learning period can have a profound influence on song quality. Therefore, to understand song evolution from a life-history perspective, we investigated early ontogenetic effects on song quality. In particular, we focused on maternal effects and sibling competition. In asynchronously hatching bird species, the age hierarchy among nestlings affects physical development due to competition for food; mothers may influence this competition by adjusting their investment in each egg according to its sequence in the laying order. To independently assess these effects, chicks of the Bengalese finch were cross-fostered so that the age hierarchies formed in fostered broods were independent of the laying order. Our results indicate that song quality partially reflects early ontogenetic conditions, whereas song duration and note-type repertoire were independent of either laying order or age hierarchy. The syntactical complexity of note order declined over the laying sequence. This finding suggests that the song learning ability is influenced by within-clutch variation in maternal investment toward eggs. Considering that song syntactical complexity is subject to female preference in the Bengalese finch, it is likely that maternal resource allocation strategies play a role in song evolution.