Clutch sizes among birds vary from one or two eggs (albatrosses, penguins, hummingbirds, and doves) to as many as 20 among some nonpasserines such as ducks and geese. Among birds, two distinct reproductive tactics are evident:
nidicolous and nidifugous. Nidicolous chicks are altricial, hatching out pink and featherless with their eyes closed; such chicks require considerable parental care and stay in the nest for some time before they are fledged. Most passerines are nidicolous (=nest loving). In sharp contrast, chicks of nidifugous (=nest fugitives) nonpasserine species are precocial, hatching out with their eyes open and covered with down, fully capable of feeding themselves. In nidifugous species, parental care consists largely of protecting the chicks from predation and teaching them where to feed. A pair of nidicolous blue tits (Parus caeruleus) successfully fledged all the young from a brood of 18 chicks.
Another dichotomy among birds is determinate versus indeterminate layers. Determinate layers have been genetically programmed to lay a fixed number of eggs and cannot replace lost eggs, whereas indeterminate layers usually can lay almost as many eggs as necessary to fill out their clutch, replacing lost eggs as needed. Chickens are a good example (some white leghorn strains lay more than 200 eggs per year). In a classic experiment with a yellow-shafted flicker woodpecker that normally lays a clutch of seven or eight eggs, a researcher removed eggs as fast as a female laid them, leaving one 'nest' egg behind so that the female would not abandon her nest. This female laid 61 eggs over a period of 63 days! Presumably, a female bird feels ('counts'?) the eggs underneath her with her incubating brood patch and some tactile feedback is translated into hormonal changes that terminate egg laying and cause the female bird to become broody and incubate.
Lots of data have now been accumulated demonstrating optimal clutch sizes in birds. These elegant studies show that compared with very small and very large clutches, clutches of intermediate size leave proportionately more offspring that survive to breed in the next generation. This is an excellent example of stabilizing selection. Young birds from large clutches leave the nest at a lighter weight and have a substantially reduced post-fledging survivorship. The optimal clutch apparently represents the number of young for which parents can, on average, provide just enough food. Evidence exists for this in a population of great tits Parus major, which varied their average clutch size from eight to twelve over a 17-year period, apparently in response to crowding and resulting changes in the density of their major food, caterpillars. Nestling great tits from larger clutches weigh considerably less than those in smaller clutches.
In European starlings, Sturnus vulgaris, clutch size varies from two to eight; modal clutch size is four or five eggs, varying seasonally. Although the total number of chicks actually fledged per nest increases monotonically with clutch size, mortality among chicks from large clutches during the first 3 months of life is heavy. As a result, large clutches do not provide a greater return than smaller clutches and a clutch of five eggs constitutes an apparent optimum.
Clutch size in English chimney swifts, Apus apus, varies from one to three (rarely four). In these swifts, differential mortality among chicks from clutches of different sizes takes place in the nest before fledging. Swifts capture insect food on the wing and aerial feeding is much better during sunny summers than in cloudy ones. Most chicks from clutches of two leave the nest in sunny years, but only one survives in cloudy years, and the optimal clutch size shifts from three to two. Number of young leaving nest is a function of clutch size in these swifts in England where the success with more than one egg clearly depends on the weather. A polymorphism in clutch size persists with mean fledging success over all years being nearly identical (about 1.7 chicks) for clutches of both two and three.
Seabirds have a long pre-reproductive period (4-5 years) and relatively small clutch sizes (one to three chicks). Albatrosses do not become sexually mature until they are 5 years old and then they lay only a single egg-Wynne-Edwards interpreted this as an optimal clutch that produces a net number of young just replacing the parents during their lifetime of reproduction. As such, his 'balanced mortality' explanation involves group selection because individual birds do not necessarily raise as many chicks as are possible but rather produce only as many as are required to replace themselves. Clearly, a 'cheater' that produced more offspring would soon swamp the gene pool. Ashmole took issue with this interpretation, arguing that albatrosses can successfully raise only one chick. A chick addition experiment performed on the laysan albatross Diomedea immutabilis verified this: an extra chick was added to each of 18 nests a few days after hatching. These 18 nests with two chicks were compared with 18 natural 'control' nests with just one chick. After 3.5 months, only five chicks survived from the 36 in experimental nests, whereas 12 of the 18 chicks from single-chick nests were still alive. Parents could not find enough food to feed two chicks and most starved to death.
Even within the same widely ranging species, many birds and some mammals produce larger clutches (or litters) at higher latitudes than they do at lower latitudes. Such latitudinal increases in clutch size are widespread and have intrigued many population ecologists because of their general occurrence. The following several hypotheses, which are not mutually exclusive, are among those that have been proposed to explain latitudinal gradients in avian clutch sizes.
Daylength hypothesis. During the late spring and summer, days are longer at higher latitudes than at lower latitudes. Diurnal birds therefore have more daylight hours in which to gather food and thus are able to feed larger numbers of young. However, clutch and litter sizes also increase with latitude in nocturnal birds and mammals, which clearly have a shorter period for foraging.
Prey diversity hypothesis. Due to the very high diversity of insects at lower latitudes, tropical foragers are more confused, less able to form search images, and as a result, are less efficient at foraging than their temperate counterparts.
Spring bloom or competition hypothesis. Many temperate-zone birds are migratory, whereas few tropical birds migrate. During spring months at temperate latitudes, there is a great surge of primary production and insects dependent on these sources of matter and energy rapidly increase in numbers. Winter losses of both resident and migratory birds are often heavy so that spring populations may be relatively small. Hence, returning individuals find themselves in a competitive vacuum with abundant food and relatively little competition for it. In the Tropics, wintering migrants ensure that competition is keen all year long, whereas in temperate zones competition is distinctly reduced during spring months. Thus, because birds at higher latitudes can gather more food per unit time, they can raise larger numbers of offspring to an age at which young can fend for themselves.
Nest predation hypothesis. Tropical habitats house proportionately more predators, both individuals and species, than do temperate ones. Nest failure due to nest predation is extremely frequent in the tropics. Many nest predators locate bird nests by watching and following the parents. Because parents must make more trips to the nest if they have a large clutch, larger clutches should suffer heavier losses than smaller ones. In support of the hypothesis, hole-nesting birds, which are relatively free of nest predators, do not show as great an increase in clutch size with latitude as birds that do not nest in holes. Moreover, on tropical islands known to support fewer predators than adjacent mainland areas, birds tend to have larger clutches than they do in mainland populations.
Predators have been implicated as a factor in the evolution of clutch size even in avian species that do not feed their young (nidifugous birds). In Alaskan semipal-mated sandpipers, Calidris pusilla, ordinary clutches of four eggs fledge an average of 1.74 chicks; but clutches artificially raised to five fledge only one chick, primarily due to heightened predation.
Hazards of migration. Latitudinal gradients in clutch or litter size could also be influenced by the tradeoff between expectation of future offspring and optimal current investment in reproduction. If hazards of migration or overwintering at high latitudes inevitably result in greater mortality, expectation of life and residual reproductive value will both be reduced at higher latitudes. This in turn would favor an increased effort in current reproduction, and hence larger clutches.
An observation somewhat difficult to reconcile with above hypotheses is that clutch size often increases with altitude. Neither day length, insect diversity, migratory tendencies, competition, nor predation need necessarily vary altitudinally. Climatic uncertainty, both instability and/or unpredictability, could well result in reduced competition at higher elevations. Optimal clutch size generally involves a compromise between conflicting demands of predator avoidance, competitive ability, and clutch size.
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