Sex differences in foraging behaviour have been explained by size dimorphism and/or avoidance of inter-sexual competition for depletable resources. To distinguish between these 2 hypotheses, we examined how intrinsic factors (sex-related differences) and extrinsic factors (year differences) shape the foraging behaviour of size-dimorphic imperial shags Phalacrocorax atriceps albiventer breeding at New Island, Falkland Islands/Islas Malvinas. We deployed time-depth and compass loggers to male and female imperial shags over 3 consecutive chick-feeding seasons. Males and females partly overlapped in coastal foraging areas, which were used mainly for benthic diving. Males additionally used offshore areas over deep water for shallow pelagic diving, suggesting that spatial segregation is involved in the avoidance of inter-sexual competition for food. Stable isotope data suggested differences in prey composition between the sexes, with consistently higher trophic levels in males, as expected for their larger size. Males were 27% heavier than females and reached greater maximum dive depths (98.9 ± 5.3 m) than females (54.1 ± 2.9 m). However, contrary to predictions based on body size dimorphism, the median dive depths of males were similar to those of females. While females used mainly benthic diving, males were more flexible in their benthic and pelagic foraging behaviour. Females also carried out more dives per day in all years, and deeper and longer dives than males in one year. As dive parameters differed strongly among the years, our results suggest that body size dimorphism and the avoidance of inter-sexual competition for food are involved in the evolution of sex-related differences in foraging in this species.

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Summary 1.Condition-dependent expression of ornamental traits is a fundamental assumption of theories on the honesty of sexual signals, and it is widely assumed that condition-dependence is a signature feature of ornaments. 2. Some of the best understood condition-dependent traits are the striking carotenoid-based plumage signals of male birds, yet little is known about the many less conspicuous, less elaborate carotenoid-based plumage colours that often comprise large parts of the plumage. 3. We examined colour (reflectance) of carotenoid-based plumage in male greenfinches that were provided with naturalistic diets with relatively low and with enhanced lutein availability during their annual moult. Using a variety of objective colorimetrics, including physiological models of avian colour vision, we compared experimental effects and general condition-dependence on the contrasting bright yellow tail patch, the yellow-green breast as well as three duller, yellow-to olive-green patches (back, crown, rump). 4. Irrespective of the analysis method used, we found consistent and large diet effects on the reflectance of the tail, much weaker effects on the reflectance of the breast, and no significant effects on the other three plumage parts. Likewise, we found that only the colour of the tail was strongly associated with circulating (plasma) lutein concentration, as well as with general condition (body mass, haematocrit). 5. Our results suggest that, in accord with current theories on the signal honesty, the striking yellow tail patch of the male greenfinch appears to be particularly well-adapted to signal information on carotenoid availability and general condition of the male during moult.

Bird feathers are employed in a wide range of carbon and nitrogen isotope studies relating to diet and migration. Feathers are chemically inert with respect to carbon and nitrogen, after synthesis. It has always been assumed that feathers show isotope values characteristic of keratin, a fibrous structural protein from which they are formed. Little attention has been paid to other components of feathers such as melanin or carotenoids. Melanin is synthesized from tyrosine, which is depleted in both (13)C and (15)N. We compared isotope values of coeval black and white feathers in four different species. Black feather parts were in all cases significantly depleted in (13)C relative to white feather parts but in most species no clear trend was discernable for (15)N. We suggest that additional evaluation may be required to characterize the carbon and nitrogen isotope contribution of feather pigments like carotenoids. Care should be taken in future stable isotope studies when comparing differently coloured feathers.