Bird Clutch Size
The number of eggs laid in a single brood (or nest) by one or more female birds.
Variation in the number of eggs laid in a single brood has fascinated researchers, especially ornithologists, since at least the 1940s. The number of eggs laid by a female varies enormously among taxonomic groups. Generally, it is constant and small, as in albatrosses and King Emperor penguins that lay one egg. In other species, clutch size is variable, and sometimes large, as in the partridge, which can lay between 14 and 20 eggs. The factors that are associated with the evolution of clutch size are, perhaps, one of the oldest topics in life history theory. To this end, several processes have been proposed that may influence selection on bird clutch size. Attempts to understand causes of avian clutch size variation range from intra- and interspecific studies, work on intrinsic (e.g., body mass, migratory behavior, nest type, diet) and extrinsic (e.g., latitude, temperature, precipitation, seasonality) potential drivers of such variation, to investigations of phylogenetic effects that may constrain clutch size variation. This entry provides a review of the information on bird clutch size, specifically from a perspective of the conflict of interest between parents and offspring over optimum clutch size.
David Lack and Clutch Size
The ornithologist David Lack was the first researcher to study the evolution of bird clutch size, in the 1940s (Lack 1947, 1948). Lack’s work was seminal, as it attempted to explain variation in clutch size in terms of individual selection, while many researchers at the time considered that clutch size evolved to allow a population or species to persist. Lack reasoned that a bird would lay the number of eggs that maximized the number of young that successfully left the nest (i.e., fledged). In altricial birds – those whose newly hatched young are immobile and require extensive nourishment – Lack argued that the ability of the parents to feed their offspring was the main factor influencing the number of fledged young. Lack’s pioneering theory, however, assumed a harmonious setting within bird families. The theory assumed, at least for a single parent, that selection acts on clutch size to maximize the number of fledglings per clutch. However, what is optimal for a single parent need not be so for both parents, nor for the young involved.
Conflicts of interest over the allocation of resources are a natural consequence of selection acting upon individuals that are not genetically identical. For instance, sexual reproduction leads to a situation in which parents and the offspring they produce are not genetically identical. Thus, natural selection could act in somewhat different ways on parents and their young. Robert Trivers, in the 1970s, explored the consequences and implications of this conflict in what is now known as the classical theory of parent-offspring conflict (Trivers 1974). Trivers’ notable work was based upon Hamilton’s rule (Hamilton 1964a, b) – another remarkable contribution that revolutionized our understanding of social evolution – to articulate a theory in which parental interests are challenged by offspring demanding more parental investment than is optimal from the parental point of view.
Briefly, Trivers’ theory of parent-offspring conflict can be understood with the following simple situation (which can be extended to include other sources of conflict). The scenario involves a single parent that raises one offspring per breeding season. This parent will face a trade-off between continuing to invest in the current young and keeping the resources to improve its own survival or to invest in future offspring. The trade-off could manifest itself as several potential combinations of these mentioned life-history components. For instance, resources should be invested in the current offspring if the benefits in terms of its fitness are greater than the costs to future offspring, measured as their fitness.
Suppose that offspring, somehow, can influence parental resource allocation. The offspring also faces a trade-off. This time, the trade-off is between resource being allocated to itself or to its future siblings. There is no doubt that the offspring benefits from receiving those resources. However, the costs, in terms of lost fitness to siblings, are substantial, because they all carry copies of the same genes. It is worth noting that the cost must be devalued by the relatedness between siblings. Therefore, the offspring will favor the diversion of resources towards itself when benefit > relatedness * cost, which is different from the criterion used by the parent (benefit > cost).
The “battleground” within which parent-offspring conflict takes place can take many forms. For instance, conflict can occur over the distribution of resources between current and future, unborn offspring, which is termed interbrood conflict. Another form of the conflict is that between members of the same brood, that is, intrabrood conflict. It is the latter that will be treated in more detail in the following section.
Clutch Size and Conflict
Most of what follows has been extensively discussed by Godfray and Parker (1991), thus readers should refer to that work for a more detailed discussion of the theory involving clutch size and conflict. For a more recent review of the topic, see Kilner and Hinde (2012). As the number of young in a clutch increases, the average fitness of the individual offspring is reduced. The main proposed mechanism underlying this decline in fitness is the competition between clutch members for limiting resources. Assuming this mechanism, clutch size is thus determined by the trade-off between the number and fitness of offspring in a clutch. What follows from this logic is that parents will be selected to produce a clutch size that maximizes the fitness returns of that clutch. The basic scenario, when all young have the same fitness, leads to the issue of augmenting the product of the number of young and average fitness. This result is known as the “Lack clutch size.”
From a parental perspective, it would be optimal if all offspring obtained their necessary share of resources, and not more, while not engaging in any type of intrabrood competition that could lead to a reduction in their fitness. Below, it is outlined how offspring competition within a brood generates conflict with the parent’s optimal “decision.” Then, an examination is presented on how competition between siblings, potentially, leads to selection on parents to reduce their clutch size. Chick hierarchies are discussed as a potential adaptation to reduce the negative effects of sibling competition. Finally, siblicide, possibly the most severe form of sibling conflict – when one sibling deliberately kills another – is discussed.
Intrabrood Sibling Conflict and Clutch Fitness
Bird sibling conflict and its evolution have most often been studied in the context of intrabrood competition between young and the evolution of begging. An assumption of how begging mediates the conflict between siblings is that increased begging translates into increased resources. In some bird species, it seems that parents allow the young to decide feeding priorities. Young frequently jostle among themselves to be closer to the parent when it arrives with food. However, in other species, parents seem to completely ignore begging, and feed, instead, the largest chick first and more frequently.
While increasing begging will, generally, lead to more individual benefits to the offspring producing the solicitation, begging can also incur costs. These costs can be experienced solely by the individual, when the costs are mainly metabolic, or they might be shared with brood mates, as with the attraction of predators that could be lured by the average levels of begging from the brood (Skutch 1949). Theoretical models predict that as clutch size increases, the reduction in parental fitness will also increase, both when costs are experienced individually or shared among brood mates. The extent of the reduction in fitness is dependent on clutch size and also on how the costs are spread among the brood. But, the main message is that competition among offspring will lead to a reduction in the fitness of the clutch.
Sibling Competition and Selection on Clutch Size
Sibling competition, as discussed above, potentially leads to a reduction in offspring fitness. Therefore, sibling conflict may act as a selective pressure that will lead to a reduction in clutch size. The underlying mechanism is that in reduced clutches, there is more resource available per capita. Theoretical models predict that forms of sibling conflict (namely the shared or individual costs) will usually reduce clutch size in the order of 20–30 %, which is substantial, when compared to a scenario of no sibling competition.
Offspring do not all have the same competitive abilities. Several bird species begin to incubate their eggs before clutch completion, leading to an asynchrony in hatching, which leads to a hierarchy in chick size. The resulting differences in chick size are likely to be associated with different competitive abilities. Sibling hierarchies can thus influence resource distribution and competitive solicitation. The study of sibling hierarchies has been of much interest to evolutionary biologists, with as much as eight different hypotheses proposed as explanations for the evolution of the asymmetry between young. One proposition is that the hierarchy is an adaptation by the parent to reduce sibling competition. The underlying reasoning is that the hierarchy leads to a preemptive division of resources, which minimizes fitness costs raised by competition.
Offspring killing one another in a brood is a well-known process that influences brood size in birds (Mock 2004). Siblicide is common among raptors and also in cranes, gannets and boobies, herons, pelicans, penguins, and skuas. When siblicide occurs in a species, it may be either facultative or obligate. The former may occur when, for instance, environmental conditions are unfavorable, and parents cannot provide enough resources to fledge successfully more than one young. In species with obligate siblicide, on the other hand, parents may lay a second or third egg as an insurance against the failure of the first egg.
This chapter has discussed that clutch size is often a product of several intrinsic and extrinsic factors that shape selective pressures on the number of eggs laid by female birds. Individual parents and offspring have different fitness optima, due to their nonidentical genetic constitution. This difference may lead to a conflict between each individual’s interests, and the consequences of the conflict can be detrimental to both parts involved. While offspring have little – or no – influence on the determination of the number of eggs that will be laid, competition between members of the same brood has important evolutionary implications for clutch size.
- Kilner, R. M., & Hinde, C. A. (2012). Parent-offspring conflict. In N. J. Royle, P. T. Smiseth, & M. Kölliker (Eds.), The evolution of parental care (pp. 119–132). Oxford, UK: Oxford University Press.Google Scholar
- Mock, D. W. (2004). More than kin and less than kind. Cambridge, MA: Harvard University Press.Google Scholar