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Male and female offspring can differ in their susceptibility to pre-natal (e.g. egg quality) and post-natal (e.g. sib-sib competition) conditions, and parents can therefore increase their individual fitness by adjusting these maternal effects according to offspring sex. In birds, egg mass and laying/hatching order are the main determinants of offspring viability, but these effects can act differently on each sex. In a previous study, relatively large last-laid (c-)eggs of yellow-legged gulls (Larus michahellis) were more likely to carry a female embryo. This suggests compensatory allocation of maternal resources to daughters from c-eggs, which suffer reduced viability. In the present study, we supplemented yellow-legged gulls with food during the laying period to experimentally test whether their nutritional conditions were responsible for the observed covariation between c-egg sex and mass. As predicted, food supplementation enhanced female c-eggs' mass more than that of male c-eggs. Thus, this experiment indicates that mothers strategically allocated their resources to c-eggs, possibly in order to compensate for the larger susceptibility of daughters to hatching (and laying) order. The results also suggested that mothers decided on resource allocation depending on the sex of already ovulated c-eggs, rather than ovulating ova of either sex depending on food availability.

Females may invest more in reproduction if they acquire mates of high phenotypic quality, because offspring sired by preferred partners may be fitter than offspring sired by non-preferred ones. In this study, we tested the differential maternal allocation hypothesis in the freshwater crayfish, Austropotamobius italicus, by means of a pairing experiment aimed at evaluating the effects of specific male traits (body size, chelae size and chelae asymmetry) on female primary reproductive effort. Our results showed that females laid larger but fewer eggs for relatively small-sized, large-clawed males, and smaller but more numerous eggs for relatively large-sized, small-clawed males. Chelae asymmetry had no effects on female reproductive investment. While the ultimate consequences of this pattern of female allocation remain unclear, females were nevertheless able to adjust their primary reproductive effort in relation to mate characteristics in a species where inter-male competition and sexual coercion may mask or obscure their sexual preferences. In addition, our results suggest that female allocation may differentially affect male characters, thus promoting a trade-off between the expression of different male traits.

During an 8-year study, the clutches most frequently laid by Night Herons Nycticorax nycticorax and by Little Egrets Egretta garzetta were 1-egg smaller than the most productive ones, and this contrasts with the idea that natural selection maximises reproduction. The productivity of the largest clutches in these two herons seems to be proximately limited by their inability to hatch successfully clutches above four and six eggs, respectively. Major losses seem to involve hatching failures due to the parents' inability to incubate clutches above a certain size, or to early chick mortality due to competitive aggression among siblings. Clutch size and fledgling success were stable or slightly increased during the season, an unusual trend that may be linked to a late peak in food availability. Among the hypotheses put forward to explain the discrepancy between most frequent and most productive clutches in birds, the evidence for Night Herons and Little Egrets contrasts with the predictions of the “predation”, the “bad years”, and the “insurance” hypotheses. A clutch size modification experiment, the first one accomplished on a ciconiiform species, was conducted for 3 years, and its results contrasted with the predictions of the “individual optimum” hypothesis. The limited information now available suggest that the “incubation capacity” and the “adaptation in progress” hypotheses may be applicable to these herons, who find particularly abundant food resources in the rice fields of this study area [ABSTRACT FROM PUBLISHER]