Energy Requirements for Growth in Relation to Sexual Size Dimorphism in Marsh Harrier Circus aeruginosus Nestlings

Abstract

Food consumption was measured in six female and seven male hand-raised marsh harrier (Circus aeruginosus) nestlings. Females consumed on average 4,321 g and males consumed 3,571 g of food during the nestling stage from 0 to 36 d. Total consumption until 56 d was 6,960 g and 5,822 g for females and males, respectively. On the basis of Fisher's sex ratio theory, this food intake ratio of 0.46 (intake male/[intake male + female]) would explain the observed male-biased fledging sex ratio of 55% males in marsh harrier broods. Growth, gross energy intake, and metabolizable energy intake were measured, along with metabolism of the nestlings, enabling us to determine energy allocation. The assimilation quotient (Q = 0.72) did not differ systematically between the sexes. Differences in metabolic rates between males and females at 15 and 30 d of age were fully attributable to the difference in body mass. Sexual size dimorphism in marsh harriers (female body mass around 60 d of age is 1.28 times greater than male mass) did not fully explain the difference in food intake between male and female nestlings: an analysis of energy requirements for growth and body mass in 16 avian species shows that energy intake was less than proportional to the average body mass at release. The data presented in this study are in agreement with Fisher's theory of inverse proportionality between the sex-specific ratios of energy requirements for growth and of offspring numbers in the marsh harrier population.

Hatching sex ratio and sex specific chick mortality in common terns Sterna hirundo

“…Sex-specific parental input may take place in sexually-dimorphic avian species, in which one sex is larger than the other. Though the larger sex tends to be cheaper to raise per unit mass than the smaller sex (Krijgsveld et al 1998), larger size presumably demands a larger energy input (investment) during the chick rearing period (Anderson et al 1993;Weimerskirch et al 2000;Daunt et al 2001). Consistent results with Fisher's (1930) predictions apparently come from male-biased nestling mortality found among bird species where males are larger than females (e.g.…”

Male-biased brood sex ratio depresses average phenotypic quality of barn swallow nestlings under experimentally harsh conditions

“…Albeit no obvious sexual diVerences emerged in terms of morphology and immunity at most ages (see Saino et al 2002c for similar Wndings at age 12 days), sex-related diVerences in such parameters as susceptibility to parasites, hormone proWles or metabolic rates may translate into diVerences in resource demands by males compared to females in order to attain similar phenotypic values (e.g. Potti and Merino 1996;Krijgsveld et al 1998;Teather and Weatherhead 1998;Tschirren et al 2003). Diverse mechanisms of sex-related variation in susceptibility to harsh nest conditions may result in reduced phenotypic values of all the nestlings in enlarged broods if the parents do not adequately raise their food provisioning rates and partly compensate for the inherent disadvantage of their male oVspring by subtracting resources to females.…”

Parental care and adaptive brood sex ratio manipulation in birds

“…However, it need not always be true that the cost of producing offspring differs substantially between the larger and the smaller sex. For example, in nestling birds, sex dimorphism in body mass overestimates sex differences in energy requirements (Krijgsveld et al 1998). Conversely, males and females may be similar in size, but one sex may still be more costly to produce.…”