Breeding strategies of birds
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Breeding strategies of birds
Summary
Birds are known for their complex breeding behaviour, which may include territorial defence, courtship, mating, nestbuilding, brooding and care of young. In this article, various types of breeding behaviour and their relationship to appearance, life history, behaviour and environmental factors are discussed.
Thick-billed murre birds nesting on edge of cliff Jupiterimages Corporation 2008
Breeding systems in birds
Breeding behaviour varies greatly between different species, and is monogamous or - more rarely - polygamous. Polygamous breeding behaviour may be further subdivided into polygyny, polyandry and promiscuity, although the distinction between the first two and the last are not always clear. Brood parasitism may possibly be included as a further category, but this is discussed in a separate article.
Monogamy
Common loon nesting on water's edge Jupiterimages Corporation 2008
In this mating system, which is practised by some 90 percent of birds, one male and one female form a pair bond that persists at least until their chicks leave the nest. In some species, the bond lasts beyond the clutch, and may continue for years or even for life. In many migratory species, males and females leave for wintering grounds separately, but return to the same breeding ground and mates the next season. In these species - such as many storks, which use the same nest year after year - nest fidelity may be as strong or stronger as fidelity to a mate.
Ornithologists often use human terms, such as divorce, to describe phenomena associated with monogamy in birds. Divorce may follow inability of a pair to breed successfully, or failure of one of the pair to return to the breeding grounds within several days to several weeks of the other. Following the death of a partner, a bird will find another - though this may only happen in the following season. The commonly held but erroneous belief that swans mate exclusively for life is probably based on the observation of semi-domestic birds, which may not have access to new mates. Infidelity does occur - although it is rare in some species and common in others.
Monogamous birds share nesting duties such as building of nests, incubation of eggs and care of young, although males and females of some species may perform different tasks. In many eagles, for instance, incubation is done mostly or exclusively by the female, while the male provides most or all of the food.
In monogamous systems, most birds mate, leading to the preservation of genetic diversity. Gene selection is limited, although not eliminated, as fit parents are likely to rear more offspring than unfit ones. Chicks requiring long-term or intensive care are much more likely to survive if parental duties are split.
Polygyny
Masked weaver as depicted on Rwandan stamp Jupiterimages Corporation 2008
In polygynous systems, a single male mates with several females. Typically, the male courts and mates with females serially, so that at any given time during the breeding season, the consorts of the male may range from females who are laying eggs to ones whose chicks are fledging.
While some authors use the term polygyny to describe any mating system involving a single male and more than one female, it is often reserved for systems in which the male is to some extent involved in promoting the survival of his offspring. Male masked weavers (Ploceus velatus), for instance, do not participate in incubation or feeding of chicks, but they build nests and defend their territory. Where male participation in reproduction consists solely of courting and inseminating as many females as possible, the term promiscuity is often preferred.
Polygyny is rarer than monogamy, but much more common than polyandry. In this mating system relatively few males get the opportunity to breed. It promotes selection for favourable male characteristics, but may decrease genetic diversity.
Polyandry
African jacana Jupiterimages Corporation 2008
Polyandry is the reverse of polygyny, where a single female mates with several males. This is a relatively rare breeding strategy, although it is common in a few groups, including ratites and charadriiform shorebirds. As with polygyny, some breeding systems known as polyandrous are, more strictly speaking, better referred to as promiscuous.
One of the most classic examples of a polyandrous bird is the African jacana (Actophilornis africanus). In this charadriiform species, males build the nests (in which the female lays several eggs before moving on to the next nest), incubate the eggs and care for the young. Although the female takes no part in rearing her offspring, she defends the territory against intruders.
Only approximately one percent of birds are polyandrous. That this phenomenon is found at all may be due to the fact that, while female birds do invest much more per egg than males do per sperm, the inequality is not nearly as great as in mammals, where long gestation and periods and the dependence of young on milk tend to limit the number of progeny a female can produce in a season, as well as greatly increase her investment in each of her young.
In birds, the absolute necessity of the maternal presence ends very early, when the egg is laid. Once this has been done, either parent could - at least in theory - take over care of the progeny. By mating with several males and leaving the bulk of parental care to them, she remains free to continue laying eggs throughout the breeding season instead of limiting her production to the maximum number of eggs that she is capable of incubating at one time. Like polygyny, polyandry decreases genetic diversity, but favours selection for beneficial genes - in this case especially those that favour female breeding capacity.
Promiscuity
Hummingbird Jupiterimages Corporation 2008
In promiscuous systems, birds meet solely to mate, and one gender (usually - but not always - the female) is responsible for all post-fertilisation reproductive duties. Mating is, however, not random: Finding mates involve courtship displays and territorial defence by (usually) males, and birds capable of impressing the opposite sex on these counts are much more likely to reproduce.
Hummingbirds are typical of birds that mate promiscuously: The brightly coloured males are conspicuously different in appearance from the much drabber females, and are highly territorial. Males perform elaborate courtship displays to attract females. After the birds have mated, the female builds a nest in which she lays one or two eggs. She incubates and rears the chicks alone, with the male playing no part, except possibly to allow females he has mated with access to food in his territory.
The female grey phalarope (Phalaropius fulicarius) is a rare example of a bird that mates promiscuously with one male after another and remains with each just long enough to lay a clutch of eggs in his nest.
Cooperative breeding
Group of seagulls in nest Jupiterimages Corporation 2008
In this system, a monogamous breeding pair is assisted by several other birds, which may participate in caring for the chicks. These may be juveniles from previous broods, or unrelated, usually young birds that had been unable to find partners. By participating in group activities, birds may gain valuable experience for future use when they get the opportunity to mate. This system is adopted by many members of the order Passeriformes.
A very unusual type of cooperative breeding is seen in eclectus parrots (Eclectus roratus), where a breeding pair is assisted by several unrelated males. In addition, the breeding male ranges widely for food, and may mate with other females. The female, who remains close to the nest, is fed by the males. She sometimes mates with other males, which may occasionally include helpers. This system is known as polygynandry, or combined polygyny and polyandry. Eclectus females are also known to choose the gender of their chicks, although the reasons and mechanisms are not known.
Breeding systems and sexual dimorphism
Pheasant pair; male on the right Jupiterimages Corporation
In most bird species, males and females are more or less alike in size and general appearance. In some, however, the sexes are somewhat different, and in a small but notable minority males and females are so different in appearance that they may be mistaken for members of totally different species. These differences may involve size, colouration and ornamentation, including wattles and specially adapted display feathers, and are known as sexual dimorphism. Those involving colouration specifically are sometimes referred to as sexual dichromatism.
Many birds species develop relatively striking breeding plumage at the start of the breeding season, in contrast to drab eclipse plumage present otherwise. Sometimes these changes may affect other parts of the body as well. When these changes affect one sex only, it is usually the male.
Generally speaking, sexes tend to be more or less alike in monogamous species and different in polygamous species: In doves, for instance, monogamous species are typically cryptically coloured, while the males of closely related but polygamous species are frequently as brightly coloured as parrots and the females as cryptic as those of most other doves. Some extreme examples of sexually dimorphic features include the enormous, shimmering tail of the male peafowl (Pavo cristatus) and the bright colours and often highly elaborate display feathers of hummingbirds and birds of paradise. Among African birds, some striking examples include whydahs, bishops, most sunbirds and many weavers.
In monogamous species, most individuals get the opportunity to mate, whereas males often need to compete vigorously for females in polygamous species. This is especially true of promiscuous species, where, reproductively speaking, males are basically flying sperm banks. It is not surprising that these species often show the greatest degree of sexual dimorphism.
The beautifully coloured, long-tailed male common pheasant (Phasianus colchicus) lives only half as long on average as the drab female. N Roos
Desirability does, however, come at a price: Both striking colouring and the burden of elaborate display feathers make males more vulnerable to predation. The beautifully coloured, long-tailed male common pheasant (Phasianus colchicus), for instance, lives only half as long on average as the drab female. In evolutionary terms, however, a bird that lives only one season and leaves many descendants is more valuable than one who survives for many years but hardly ever gets a chance to breed. In some species, cryptic colouring in winter to some extent ameliorates the negative effects of being conspicuous in the breeding season.
Large male size not only gives polygamous birds an edge over rivals in competition for territories, but may also help them survive unfavourable conditions. In some migratory species, males leave for the breeding grounds before females in order to secure favourable territories. This often means that they arrive before the end of winter, when smaller birds may succumb to cold.
Polyandrous females tend to be larger and brighter in colour than males, but these differences are not as striking as the examples mentioned above. Population sizes are far more dependent on the number of egg layers than that of sperm providers, and the loss of females to predators is therefore more costly than that of males. Generally speaking, the more expendable a bird is, the more beautiful it can afford to be.
Some forms of sexual dimorphism are not fully understood. In birds of prey - especially large ones, such as eagles - females may be up to a third larger than males. This phenomenon is seen in active predators as far apart on the avian family tree as eagles, owls and falcons, but not in scavengers like vultures, which are closely related to eagles.
In these monogamous birds, females initially remain at the nest, protecting the eggs and young while males forage for food. Males generally hunt smaller, swifter prey than the slower but more powerful females, and their smaller but more frequent contributions may be more convenient during the nesting period than occasional large catches. After a few weeks, the female resumes hunting, and the pair thus provides the chick or chicks with food. The differences in size may also reduce competition for food and optimise hunting opportunities.
Another puzzling example of sexual dimorphism is seen in the eclectus parrot, where the sexes differ markedly in colour, but both are brightly coloured: In males, the predominant colour is bright green, while red and blue dominate in the even brighter females. Eclectus parrots participate in cooperative breeding, with groups consisting of the monogamous breeding pair and mostly young male helpers. Although there is no shortage of demand for female partners, it is thought that their striking appearance assists females in competing for rare and desirable nest cavity.
Such is the importance of the nest site that males and females have adopted entirely different lifestyles: The female always remains at the nest site to defend it against interlopers, while the male forages widely for food for himself and his family. Presumably the male's green feathers make him less visible against the leafy background of the jungle, while the female's brilliant colouring acts as a deterrent to would-be occupiers of the nesting cavity. She does not need camouflage, as she is able to retreat to the nest when threatened.
Factors affecting breeding systems
Emperor penguins with chicks Jupiterimages Corporation 2008
The feasibility of particular breeding systems may vary according to environmental and genetic circumstances. Generally speaking, breeding systems tend to involve two parents when chicks require intensive or long-term care, and when resources are scarce, while the opposite conditions favour promiscuity. The ability of some individuals to obtain and defend favourable breeding territories is conducive to polygamy. Environmental disturbance that forces birds to nest close together is conducive to brood parasitism.
Chicks vary from the super-precocial young of megapodes, which receive no parental care at all and are able to hunt and fly shortly after hatching, to the well-known precocial young of fowl and waterfowl, to semi-altricial or the naked, highly altricial young of many other birds.
Birds who produce precocial young that require little or short-term care are much more likely to be polygamous than those whose young are altricial, and therefore need intensive care in the first weeks of life, or those whose young require long-term care. Birds that occupy extreme environmental niches also tend to be monogamous: In emperor penguins (Aptenodytes forsteri), for instance, only careful division of labour and extended, specialised care of the egg and chick make it possible for these birds to breed successfully in the extreme cold of Antarctica.
The genetics underlying breeding systems in birds are not well known, but studies of the ruff (Philomachus pugnax), a polygamous charadriiform bird from an order where most members are monogamous or polyandrous, may cast some light on the phenomenon. The genetics underlying the unique breeding system of this highly sexually dimorphic bird is discussed in the information box below.
