David C. Houston

Carotenoids and egg quality in the lesser black-backed gull Larus fuscus: a supplemental feeding study of maternal effects

Egg quality is a phenotype of, and can profoundly influence fitness in, both mother and offspring. However, the physiological mechanisms that underlie this maternal effect are poorly understood. Carotenoids are hypothesized to enhance antioxidant activity and immune function, and are responsible for the pigmentation of egg yolk. The proximate basis and consequences of this maternal investment, however, have not previously been studied in wild birds. In this supplemental feeding study of lesser black–backed gulls, Larus fuscus, carotenoid–fed females are shown to have increased integument pigmentation, higher plasma concentrations of carotenoids and antioxidant activity, and lower plasma concentrations of immunoglobulins (Igs) in comparison with controls. In turn, carotenoid–fed females produced eggs containing high carotenoid but low Ig concentrations (i.e. passive immunity), whereas control females produced eggs containing low carotenoid but high Ig concentrations. Within–clutch patterns of these resources varied over the laying sequence in a similar manner in both carotenoid–fed and control nests. Our results suggest that carotenoids could be one resource responsible for egg quality maternal effects in birds. We discuss the possible implications of carotenoid–mediated effects on phenotype for fitness in mothers and their offspring.

Why egg yolk is yellow.

We thank Charlotte Deerenberg, Pat Monaghan and Ruedi G. Nager for discussion and comments on this article. J.D.B. is supported by a scholarship from the Institute of Biomedical and Life Sciences, University of Glasgow.

Nutritional constraints on egg formation in the lesser black-backed gull : an experimental study

The use of supplementary feeding experiments to investigate the relationship between food supply and clutch and egg size in birds has yielded disparate results, some authors showing an advancement in laying date and/or increased clutch size in response to additional food whereas other studies have found no effect. Here we investigate the nutritional requirements of egg formation in lesser blackbacked gulls Larus fuscus L., through a series of feeding experiments providing additional food of different quality

WITHIN‐CLUTCH TRADE‐OFFS BETWEEN THE NUMBER AND QUALITY OF EGGS: EXPERIMENTAL MANIPULATIONS IN GULLS

We experimentally increased the number of eggs laid by Lesser Black-backed Gulls and examined the relationship between egg number and egg quality. Egg quality was measured in terms of egg composition and the probability that an egg would give rise to a fledged chick. In measuring the latter, we removed the potentially confounding effects (1) between parental quality and egg quality and (2) between increased egg production and parental condition, using a cross-fostering protocol in which eggs were reared by control parents. Lesser Black-backed Gulls were capable of producing, on average, almost three times the normal clutch of three eggs. Although egg mass did not fall below that of the last laid egg in normal clutches, as the clutch extended beyond three, experimental eggs contained relatively less lipid and relatively more water. Overall, the percentage of eggs that gave rise to fledged chicks (when reared singly by a foster parent) declined significantly with position in the experimental laying sequence; about two-thirds of the eggs laid at the beginning of the laying sequence gave rise to fledged chicks, whereas only about one-third of the last laid eggs did so. This was not due to any seasonal change in the capacity of foster parents to rear the chicks, and neither hatching nor fledging success of the eggs was related to their fresh mass. Thus, as more eggs are laid, important changes in egg composition occur that have, in themselves, substantial effects on offspring survival. This study provides clear evidence of a trade-off between egg number and egg quality.

Parental condition, brood sex ratio and differential young survival: an experimental study in gulls (Larus fuscus)

Abstract Empirical evidence is growing that the offspring sex ratio in birds can be biased in relation to the body condition of parents during breeding. The sex ratio bias may come about because (1) the actual production of the two sexes may be skewed and/or (2) there may be a sex bias in early nestling mortality contingent on parental condition. By manipulating parental condition and giving them a control brood to rear, thereby eliminating effects operating via the eggs, we examined the extent to which parental condition influences the post-hatching survival of male and female lesser black-backed gulls, Larus fuscus. We found that the pre-fledging survival of male chicks was strongly reduced in all-male broods reared by parents in poor condition. Pre-fledging survival of female chicks was, however, unaffected by parental condition or brood sex composition. Thus, independently of any production biases, sex differences in nestling mortality alone can bias the offspring sex ratio at fledging in relation to the prevailing rearing conditions. In other studies on gulls we have, however, also shown that females in poor condition at laying preferentially produce female eggs. Clearly a bias in fledging sex ratio can occur within the same species due to a combination of differential production and differential post-laying mortality; the latter can involve a differential effect of poor egg quality on male and female offspring, differential effects of brood sex composition on their survival and a difference in the capacity of parents to rear males and females. All of these processes need to be taken into account in attempting to understand offspring sex ratios.

Egg-laying capacity is limited by carotenoid pigment availability in wild gulls Larus fuscus.

In birds, experimentally increased egg production can reduce maternal condition, parenting ability and survival, and the quality of the eggs themselves. Such costs probably reflect resource limitation, but the identity of the resource(s) in question remains unclear. Carotenoids are antioxidants and immunomodulants that birds can only obtain in their diet. Trade–offs in the allocation of limiting carotenoids between somatic maintenance and egg production could therefore be an important factor underlying reproductive costs. We show that in wild lesser black–backed gulls, Larus fuscus, dietary carotenoid availability (i) constrained the capacity to re–lay following clutch removal; and (ii) affected the relationship between yolk mass and egg mass. However, whether carotenoids are limiting for egg production directly, by stimulating the synthesis or antioxidant protection of yolk precursors, or indirectly via effects on maternal health, requires further study.

Nutritional constraints on egg production in the blue tit : a supplementary feeding study

1. The influence of food supply on avian egg production has been investigated in many studies by the experimental provision of supplementary food. Most such studies, however, have considered food only in terms of energy supply. 2. In this paper we investigate the possibility that other nutrients may proximately constrain egg production. This was achieved through the experimental provisioning of breeding blue tits (Parus caeruleus L.) with two different qualities of supplementary food prior to and during the egg laying period. One group received a pure energy food (animal fat) and the other cooked eggs (on the assumption that this would contain all the necessary nutrients for egg formation). 3. Supplementary lipid and supplementary egg resulted in the same degree of laying enhancement compared with control birds. 4. The provision of supplementary egg resulted in a significant increase in egg volume ( 7%), whilst supplementary lipid had no significant effect on egg size. The eggs laid early in the laying sequence showed the greatest size increase, with eggs from the egg-fed treatment group being significantly larger than those in both the fat-fed and control groups. Clutch size declined through the season, but was unaffected by supplementary feeding. 5. We discuss the hypothesis that energy supply may influence the initiation of laying, whilst specific nutrients may proximately constrain egg production. The possible identity of these specific nutrients is considered.

Male Finches Selectively Pair with Fecund Females

Much attention has recently been focused on the factors determining mating success in animals. Most emphasis has been on variation in male characteristics, as females are often assumed to be the choosier sex. However, in monogamous species, securing a mate with high reproductive potential is important to both sexes. Here we show by means of manipulation experiments that male zebra finches selectively pair with females that, because of previous exposure to a high quality diet, will lay large clutches. This is the first demonstration that a female’s probability of obtaining a mate is directly linked to her egg production capacity independent of her age, experience or body size.

Egg Production Constraints and the Evolution of Avian Clutch Size

The aim of this study was to examine whether the capacity of female birds to produce high quality eggs could act as a constraint on the evolution of clutch size. By removal of the first laid egg, lesser black-backed gulls were experimentally induced to lay an additional egg. Their breeding success was then compared with a control group rearing the same clutch and brood size. The survival of chicks from the additional egg was only about a quarter of that of chicks from last laid eggs in the control group; it was found that this was largely an effect of reduced hatchling mass. The limited egg production capacity of females demonstrated by this experiment may in part explain discrepancies between predicted optimal and observed modal clutch sizes in birds.

The effect of social facilitation on foraging success in vultures: a modelling study

The status of many Gyps vulture populations are of acute conservation concern as several show marked and rapid decline. Vultures rely heavily on cues from conspecifics to locate carcasses via local enhancement. A simulation model is developed to explore the roles vulture and carcass densities play in this system, where information transfer plays a key role in locating food. We find a sigmoid relationship describing the probability of vultures finding food as a function of vulture density in the habitat. This relationship suggests a threshold density below which the foraging efficiency of the vulture population will drop rapidly towards zero. Management strategies should closely study this foraging system in order to maintain effective foraging densities.