This is a diary about bird sex. Before you get too excited, or perhaps repulsed, this diary is not about the act of copulation itself. In birds copulation is of extremely brief duration and particularly challenging because, unlike virtually all other animals with internal fertilization, birds don't have anything functionally equivalent to a penis. By chance, an excellent Daily Bucket about duck mating was published yesterday by OceanDiver and I urge you to all go read that immediately rather than wasting your time here. This diary is about the mating systems of birds and the number of social mates and genetic mates birds of various sexes may have.
At this point you may be scratching your head and asking what I mean by social mates and genetic mates. And why should I care about how many mates a bird has? Follow me below the orange bird pron rorschach for more.
A mating system is the pattern of mating within a species. Within our own species mating systems have been quite variable. Cultures in which men have multiple wives have been historically common and still persist in some parts of the world. Cultures in which wives have multiple husbands are less common but (caveat I'm no anthropologist and this is supposed to be about birds) but do exist/have existed.
Over the years naturalists/ornithologists had noticed that most birds formed pairs, built nests, and reared young as 'couples'. In other words most birds are monogamous. It turns out, that if you take a broad perspective mating proclivities within the animal kingdom, monogamy is somewhat unusual. Most animals don't pair up for any significant length of time. They meet, mate, and then go their separate ways. In many cases, for aquatic animals with external fertilization, the individuals don't even have to physically meet.
One clue for the prevalence of avian monogamy is that monogamy might have something to do with parental care. So what other kinds of animals care for their young? Mammals all care for their young. However monogamy is fairly rare in mammals. It occurs in some forest-dwelling primates such as marmosets and gibbons as well as in some social carnivores and some other critters. In these species males perform a lot of parental care. However in most mammals males perform little or no parental care and they are not monogamous.
Many fishes also have parental care. There is a lot of variation among fishes but the most typical pattern is one in which males care for the young. In sunfish and bass (freshwater bass in the sunfish family - bass is a term used to describe fishes in several families) males build nests and females approach the nest, mate and lay eggs in the nest and then leave the male to guard the eggs and baby fish once they hatch.
So why are birds so commonly monogamous? A rather simple idea is that monogamy is an evolutionary consequence of the high costs of reproduction in birds. In other words, it is freaking hard for birds to successfully rear offspring and it takes two parents, in most species, to do it successfully.
Are their non-monogamous birds? Most definitely. There is wide variation in mating systems. Aside from monogamous birds there are polygynous birds, polyandrous birds, and birds that are both polyandrous and polygynous at the same time. I'll explain what these terms mean below.
I'm going to provide a link to a diary I wrote on bird evolution and classification several years ago. If you are unfamiliar with major bird groups it may be helpful.
Polygyny This refers to situations in which males mate with multiple females. This is an extremely common mating system in mammals in which males often defend large home ranges containing multiple females. However it is comparatively rare in birds although there are quite a few familiar birds which are polygnynous. Two groups with many non-monogamous members are the Galliformes (chicken-like birds) and Anseriformes (ducks and their relatives). In both groups parental care by the female only is quite common. This is associated with chicks hatching at a late developmental stage and being able to provide some care for themselves immediately. To use a bit of ornithological jargon the chicks are precocial. Think about ducklings and chicks and how they are able to move about and feed themselves just about right away. Contrast that with the helpless state of hatchling songbirds which are described as altricial.
At the extreme end of this spectrum are the megapodes - turkey like birds from Australia that build huge mounds of vegetation and bury their eggs deep inside. The heat from decomposition incubates the eggs and neither parent provides further care. The egg hatches and the chick is on its own.
Brush Turkey - a type of megapode
An interesting observation about these non-monogamous birds is that usually the males are strikingly different in appearance from the females. Just think of roosters and hens for example. In the Galliformes males are almost always larger and more brightly plumaged than the females. In the Anseriformes you see striking differences among groups. Geese and swans are monogamous and the sexes are similar. Most ducks are not monogamous and the sexes are mostly very different in plumage.
Polygyny is also found in other types of birds. One of the most notable groups of birds with many polygynous species was featured last week in Dawn Chorus. This group is the Icteridae or black birds. In birds such as red-winged blackbirds males defend territories in which multiple females may nest. Note that in many icterid species males appear dramatically different than females.
Polyandry We also see a smaller number of birds in which the males care for the young and females do not provide parental care. This system is most commonly found in shorebirds, particularly species breeding in the arctic. Phalaropes are the most well known example of this type of mating system. Phalaropes essentially have the reverse mating system of a red-winged blackbird. Females will defend territories containing several males, each with a nest. The female will mate with each male and lay a clutch of eggs in his nest. The male will provide the bulk, if not all, of the parental care. Note that in phalaropes females are more brightly colored than the males.
The polyandrous systems in arctic breeding shorebirds are thought to be an evolutionary response to an extremely short breeding season with abundant resources. During the arctic summer the environment contains a lot of food but only for a very brief time. Unlike in temperate birds, females cannot rear multiple clutches sequentially. However females can increase their mating success by laying multiple clutches in quick succession and having the males rear them. The abundance of food means that one parent can successfully rear young.
Mixed Polyandry and Polygyny Some bird species have a mating system similar to that of the fishes described above. Males care for nests of eggs but in this case the eggs are laid by multiple females. Females move about and lay eggs in the nests of multiple males. This type of mating system is only found in ratites and tinamous which make up the earliest branch of the three major evolutionary lineages of living birds (the Galliformes and Anseriformes make up the second branch and all other birds make up the third).
Great Tinamou and Emu
You may be asking yourself at this point: what the heck is a ratite and a tinamou? Ratites are composed of the ostrich and its relatives the rheas, emus, kiwis, cassowaries and their extinct relatives. They are flightless birds of the southern hemisphere and are mostly very large. Tinamous are grouse like birds found in Central and South America which are closely related to the ratites. Tinamous can fly but are fairly secretive and spend most of their time on the ground. These birds are highly variable in mating system with monogamous (emus, kiwis, some tinamous) and polyandrous (cassowaries, some tinamous) species. The other species are the only terrestrial vertebrates with the polyandrous/polygynous fishlike mating system (ostrich, rheas, and most tinamous for which the details are known). In these species males brood the eggs of multiple females as described above. The most bizarre variation on this theme occurs in the rheas (South American birds resembling a small ostrich) in which females travel about in groups, display as a group to a male on a nest, mate with said male and leave him with a whole bunch of eggs and then move on to find another male. Apparently only a small proportion of males mate each year.
(Social) Monogamy The great majority of bird species form monogamous pair bonds of varying duration. The two members of the pair cooperate in nest building and care of the offspring. There has been much speculation as to why this social system is so widespread in birds but comparatively rare in other vertebrates. It seems likely that the combination of being endothermic (warm-blooded), laying eggs, and the need for flight have combined to make care of offspring particularly expensive in birds. Ectothermic (cold-blooded) animals such as fishes need a much lower energy intake and thus the need for feeding offspring during early stages is much reduced. The combination of live birth, the placenta, and the mammary gland appear to have strongly biased mammals towards female only parental care. Birds lay eggs which cannot be moved around so one parent is forced to be sedentary during brooding. Chicks need to undergo rapid development in most cases so that they can leave the nest as soon as possible. All of these factors appear to bias birds towards biparental care simply because one parent may not be able to do it alone.
Mating Systems and Sexual Selection
Above, I've made several references to species in which males and females differ in appearance. These differences are typically attributed to sexual selection. Sexual selection was an idea of Darwin's which he developed to explain differences between the sexes that were not directly related to producing offspring. He proposed that traits such as bright colors, increased size, weaponry, and elaborate displays evolved in one sex, usually males, in order to increase their success in competing for mates. Sexual selection can act in two ways: through direct competition among members of one sex and through mate choice by one sex on a trait in the other sex. The former is thought to be responsible for the evolution of weaponry such as horns and antlers as well as the increased size of males in many species. The latter is thought to be responsible for the evolution of sexual differences in color, ornamentation, and display behavior.
Ornamentation in the male booted racket-tail
A detailed discussion of sexual selection is beyond the scope of this diary. There are a lot of interesting questions to be addressed and someday I will write a diary or several specifically on sexual selection. Instead I will note that the more individuals of the opposite sex an individual can potentially mate with, the greater the potential for sexual selection. Observations of birds tend to bear this out. In polygynous species such as ducks and pheasants we see males that are often larger than females and have bright colors and striking ornaments. In a polygynous species typically a small number of males get most of the matings and thus reproductive competition is intense. In polyandrous species it is the females that are brighter and more colorful.
However there are some patterns that seem a bit odd. For example there are many birds, mostly if not all monogamous, in which both the male and the female are brightly colored and/or ornamented. Examples include puffins and some other auks, many parrots, many jays, toucans, and so on.
Other examples which cry out for explanation include the many bird species which are monogamous but in which males are brightly colored and females are not. Examples in North America include many finches and grosbeaks, warblers, and tanagers to name some of the most striking examples. In the next section we'll talk about why this is a 'problem' and some solutions.
Sexual Selection in Monogamous Birds
At first glance it seems that sexual selection should be weak in a monogamous species. If each individual only mates with one other individual then there shouldn't be much competition for mates. However the coloration of a scarlet tanager or a cardinal seems to indicate that sexual selection is operating.
One argument for sexual selection in monogamous birds has to do with the timing of reproduction in migratory species that defend breeding territories. Males arrive in the breeding area before females and set up territories. If the brightest males attract the first females to arrive they may have an advantage in terms of producing more total offspring over the breeding season. The brightest males may also end up with the healthiest, most vigorous females.
Another, comparatively recent discovery is that while most birds are socially monogamous they are not genetically monogamous. What this means is that most birds form monogamous social groups consisting of one male and one female. However, paternity analyses have revealed that very high levels of extra-pair copulations go on in many bird species. If male coloration attracts neighboring females to 'stray' then that is another way that sexual selection can be operating in monogamous birds.
The Polygyny Threshold
One question in territorial birds such as red-winged blackbirds is why females would 'settle' for joining a territory with another female already present. Why not find a male who doesn't have a mate. One idea is that the territory quality of certain males is so high that it is better to share it (and the male's parental care) than go elsewhere. This is obviously a very difficult thing to test and I don't know if it has ever been demonstrated.
Leks One of the most striking types of mating system is the lek. These are typically polygynous mating systems in which males defend a particular location which contains no resources for the females. In some cases males are concentrated in a particular area. In other cases (exploded leks) the males are distributed through the habitat but each only defends a very specific spot. Females visit the lek only to obtain matings and then rear the young elsewhere. In North America lekking only occurs (to my knowledge) in Galliform birds (grouse) but it occurs in other groups of birds elsewhere in the world. Lekking is a bit mysterious and has been the subject of intense study. Leks are cool things to see because lekking species often have really spectacular displays.