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Avian clutch size
Clutch size refers to the number of eggs laid in a single brood by a nesting pair of birds. Several patterns of variation have been noted and the relationship between latitude and clutch size has been a topic of interest in avian reproduction and evolution. David Lack, an ornithologist and evolutionary biologist, was one of the first scientists to investigate thoroughly the effect of latitude on the number of eggs per nest. Since Lack's first paper in the mid-1940s, an extensive amount of research has been conducted in this area. Most bird species show a common pattern of increasing clutch size with increasing latitude. The proximate and ultimate causes for this pattern have been a subject of intense debate involving the ideas of group and individual selection.
== Food limitation and nest predation hypotheses ==
David Lack observed a direct relationship between latitude and avian clutch size. Birds near the equator laid approximately half as many eggs as those that resided in northern temperate habitats.〔 He observed an increasing clutch-size from the equator towards the poles (something he referred to as the “latitude trend”) for many passerine (perching) birds, near-passerine (tree-dwelling) birds and in various other groups: Strigiformes, Falconiformes, Ciconiiformes, Laridae, Ralliformes, Galliformes, Podicipedifomes, and Glareolidae, and in some Limicolae. He proposed the Food Limitation Hypothesis in an attempt to explain this unique pattern.〔 The hypothesis states that avian clutch size differences arise from differences in food availability.〔 Nature favours clutch sizes that correspond to the average maximum number of offspring that the parent can sustain given a limited food supply.〔 Thus, the shortage of food supply in tropical habitats (near the equator) limits avian clutch size.〔 Furthermore, the higher abundance of predators near the equator as compared to regions near the poles gave rise to the Nest Predation Hypothesis. High rates of nest predation may select for smaller clutches to reduce the parental investment in a single nesting attempt.〔 Moreover, larger clutches are more likely to be spotted by predators due to an increased rate of food delivery by the parent.〔 This increase in parental activity will increase the probability that predators will locate nests.〔 Various studies have been performed to find supporting evidence for these two hypotheses. One theoretical research study suggested that the latitudinal gradient in clutch size can be explained by the increasing seasonality of resources from the tropics to the poles by itself or in conjunction with a decreasing rate of predation and breeding season. However, field studies have provided little support for either of these hypotheses. It is clear that Lack's Food Limitation Hypothesis and the Nest Predation Hypothesis are plausible explanations for explaining the latitudinal variation in avian clutch size. However, further analysis is required as field studies have provided little support for these hypotheses.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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