Maternal effects, in which differences in parental state cause differences in offspring fitness, are important in trade-offs influencing an individual's optimal reproductive strategy. In zebra finches (Taeniopygia guttata) we manipulated the nutritional state for four weeks before the start of breeding through protein supplementation. Zebra finches were kept on identical diets during the rest of the experiment. We then tested the effects of maternal state on offspring size, survival and fecundity. In order to separate the effects of maternal state occurring through egg production, incubation and chick-rearing, we used a cross-fostering experiment. We show that a protein-rich diet prior to laying improved maternal body weight prior to breeding compared with birds on a protein-poor diet. Poorer maternal state prior to breeding gave rise to offspring with lower fecundity than offspring from birds in a better nutritional state. Maternal state is thought to affect the conditions developing offspring experience through the bird's ability to produce and incubate eggs. Male and female embryos differed in their responses to conditions at different developmental stages. This shows that embryonic developmental conditions and sex differences in vulnerability to these conditions need to be incorporated into future models of selection, life-history evolution and sex-ratio theory.

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The division of labour in parental care between the two sexes varies between and within species. In birds, parents have been shown to invest more into egg production and nestling care when paired with an attractive rather than an unattractive mate, as predicted by the differential allocation hypothesis. Here we investigate variation in the female's and male's share of incubation behaviour, a vital, and costly, period of parental care during which the embryo is vulnerable to perturbations in developmental conditions. We manipulated the attractiveness of male zebra finches Taeniopygia guttata , using red or green leg-rings. To simulate their natural social environment we allowed them to breed in outdoor aviaries. All males within an aviary were given the same coloured ring to avoid ring-colour related assortative mating. Males within a colony, however, were still expected to show some variation in attractiveness with the earliest laying females possibly pairing with the most attractive males. Indeed we found that both factors played a role in explaining female incubation effort. Among females mated to red ringed males, earlier laying females contributed significantly more to incubation than late laying females, but no such pattern was found in females mated to green ringed males. Overall, there were no differences in the level of incubation provided by both parents between treatment groups, suggesting some compensation within the pair. Hatching success was correlated with a pair's total incubation effort. These results suggest that variation in the division of parental care between the sexes is in agreement with both increased effort of females mated with attractive males, and females compensating for the reduced effort of attractive males seeking further mating opportunities. These two factors can act at the same time in natural populations and both should be considered when explaining variation in division of labour between the sexes.

1The Differential Allocation Hypothesis (DAH) predicts that an individual should vary its 2 reproductive investment depending on the attractiveness of its mate. A generalised version 3 of the DAH also makes explicit that investment can be positive, i.e. higher for the offspring 4 of attractive males which are also predicted to be of higher quality, or negative, i.e. higher 5 for offspring of unattractive males thus compensating for inheriting poor paternal genes for 6 example. Moreover, investment can be allocated by the father as well as the mother. Few 7 studies have quantified both parental investment across reproductive stages and effects on 8 offspring survival and fecundity. Here, we tested the DAH by using red leg rings to increase 9 the attractiveness of male zebra finches Taeniopygia guttata and green leg rings to decrease 10 their attractiveness. All males within an aviary were given the same coloured ring to control 11 for assortative mating between treatments. Eggs were cross-fostered between and within 12 treatments to allow the differentiation of effects of egg investment and nestling-rearing 13 investment. Brood and clutch sizes were standardized. Both positive and negative changes 14 in investment were observed: Eggs from the red ringed group had higher yolk to albumen 15 ratios than eggs from green-ringed fathers. Cross-fostering revealed that nestlings from eggs 16 laid and incubated by red-ringed parents had higher hatching weights than those in the 17 green-ringed group. Both parents in the green-ringed group fed nestlings more frequently 18 than red-ringed parents. Ring colour was merely an experimental manipulation of male 19 attractiveness; so as red and green ringed males should be of the same quality on average, 20we might expect additional investment to result in elevated offspring quality. Offspring

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