Life history theory suggests that animals should balance their current investment in young against their chances to reproduce in the future. One fundamental prediction from the theory is that long-lived species should be restrictive in any increase of their current investment. It has been suggested that long-lived species, therefore, have evolved a fixed level of investment in young in order to maximize their own adult survival. However, recent experimental studies have shown that long-lived seabirds have a flexible reproductive performance and adjust their effort in raising young, both according to their own body condition and to the need of the chicks. In this study, we present a model of the optimal balance between reproductive effort and adult survival for long-lived birds breeding in a stochastic environment. During poor breeding conditions, maximum fitness is achieved either by not breeding at all, or by abandoning the brood. Beyond a certain threshold in breeding conditions, there is a steep increase in reproductive effort and an equally steep decrease in adult survival. The model is applied to two hypothetical long-lived seabirds differing in their potential fecundity. For the genotype with a potentially high fecundity, the model predicts a high threshold for breeding (i.e., breeding conditions need to be very good for the species to attempt breeding); above the threshold, the value of reproduction in terms of fitness is high. For the genotype with potentially low fecundity, the model predicts a low threshold for breeding, and the value of reproduction in terms of fitness is low. By increasing clutch size in the model, we examine the optimal response of the two genotypes to an experimental brood size manipulation. For both genotypes, the model predicts that the threshold for breeding is lower among controls than among enlarged broods, giving a range of possible outcomes of the experiment depending on breeding conditions. The few studies on brood enlargements in long-lived species carried out so far may support the predictions from the model.

On the cost of reproduction in long-lived birds: the influence of environmental variability

We evaluate the cost‐of‐reproduction hypothesis in the burying beetle Nicrophorus orbicollis and examine how the importance of this trade‐off changes as females age (i.e., the terminal‐investment hypothesis). These beetles breed on small vertebrate carcasses, which serve as a food resource for them and their offspring. Consistent with the cost‐of‐reproduction hypothesis, females manipulated to overproduce offspring suffered a reduction in fecundity and life span when compared to controls, although all reproducing females had reduced life spans compared to nonbreeding females. Older females produced larger broods and allocated less of the carcass to their own body mass and a greater proportion to offspring than did younger females. Resource allocation to offspring increased with age. Females given larger carcasses invested more in current reproduction and less in future reproduction than did females given smaller carcasses. Our results provide unconfounded support for both the cost‐of‐reproductio...

https://www.jstor.org/stable/10.1086/605963

Cost of reproduction, resource quality, and terminal investment in a burying beetle.

1. In birds with bi-parental care, handicapping is often assumed to decrease the amount of parental care of the handicapped partner. We discuss how handicapping could alter the shape of the handicapped bird's survival-effort curve (theoretical curve relating the survival of a parent to its effort) and show that the optimal response could yield a decrease, no response or even an increase in effort of the handicapped bird. 2. Male or female great tits Parus major (L.) were handicapped during the nestling period by clipping a number of feathers in order to study the effects on parental care and body condition. 3. Handicapped males significantly decreased their feeding rates, while handicapped females did not. Condition of handicapped females significantly deteriorated, while condition of handicapped males did not change during the experiment. Females with a handicapped partner fully compensated for their partner's decrease in work rate, while males with a handicapped partner did not show any compensation and even tended to decrease their feeding rates. 4. Using an inverse optimality approach, we reconstructed the theoretical curve relating the survival of a parent to its effort on the basis of the experimental effects. The handicapped male's survival-effort curve appeared to be slightly steeper than that of handicapped females. This suggests that handicapped males suffer more from an increase in effort than handicapped females.

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Differential response by males and females to manipulation of partner contribution in the great tit (Parus major)

How do caring parents respond to mate loss? Differential response by males and females

1. Age-related patterns of reproduction in birds are often interpreted in terms of changing reproductive value with age: as residual reproductive value decreases with age birds have to increase their effort at younger age. In contrast, the ‘state-based’ approach to life history postulates that not age per se determines reproductive decisions, but the ‘state’, reflecting condition or quality that may differ considerably between individuals of the same age. In this respect, we investigated the stability of body conditon, as well as individual quality differences in common terns Sterna hirundo and outline the role of parental quality and parental effort in reproductive performance. For lifetime identification we marked terns with passive transponders. Quality and effort were derived from body condition, which we monitored during the whole breeding season and every year for a period of up to 5 years.



2. Parameters of body condition and reproduction varied significantly between individuals, whereas intra-individual variation over years was very low. Hence, body condition is very stable and differs considerable between birds.



3. Among the factors number of chicks, year and parental body condition, the latter was the only significant factor influencing chick growth rates. Furthermore, high growth rates of chicks were correlated with good female condition, and breeding success was positively correlated with body condition of both parents during incubation and chick-rearing.



4. Parental effort increased significantly with increasing breeding success. The amount of expended effort depended on condition during incubation: mass loss was correlated with incubation mass, but not with mass during chick-rearing.



5. Age was not an important factor in terms of condition or quality and also in terms of reproduction. The age independent condition reflects bird quality, enabling high quality birds to invest high parental effort resulting in high reproductive success over years, yet without negative consequences on future reproduction.



6. We conclude that in the common tern condition is a stable and age-independent individual feature with substantial effects on reproduction. Hence, our data strongly support the ‘state-based’ approach to life history. The high constancy in condition within and great differences between birds may also result in differences in lifetime reproductive success and fitness.

Effects of parental quality and effort on the reproduction of common terns

The fitness of a parent in an altricial bird species is likely to be a function of the proportion of resources allocated to offspring production in relation to the amount spent on its own survival. Here we report an experiment on the Antarctic petrel in which we manipulated the costs of rearing an offspring by placing small lead loads on the legs of one parent. The bird could then either decrease its own body reserves or reduce the food load to the chick. The manipulated birds decreased their food load and increased the feeding interval, compared with unmanipulated birds. Consequently, the rate of chick loss increased. No significant difference was found between the body weights of experimental and control birds during the experiment.

Regulation of parental effort in a long-lived seabird an experimental manipulation of the cost of reproduction in the antarctic petrel, Thalassoica antarctica

Hans Christian Pedersen

Papers

Regulation of parental effort in a long-lived seabird an experimental manipulation of the cost of reproduction in the antarctic petrel, Thalassoica antarctica