Avian Mating Systems

Mating Systems

Is there Romance left in nature, or is nature governed by so many instinctual
selective devices that animals act more or less like machines run on a
computer program?

The study of mating as a whole has been revolutionized in the last few decades. Let me start the discussion of mating systems in birds with a quick review of relevant terminology.

Monagamy (90 percent of birds): Birds have one mate.

Polygamy: mating system where an individual may associate with many different
members of the other sex.

Polygyny: males pair with two or more females (2 percent).

Polyandry: Females pairing with several males. (1 percent) Found predominantly
in shore-birds. Often occurs in sexual reversal (one sex is larger than
another).

Polygynandry: Males pair with many females and females pair with several
males. Basically a mixed mating system. It's common if fishes but not with
birds. Its characteristic of tinamous, and flightless ratites.

Promiscuity: Indiscriminate sexual relations (its doubtfull that they
are indiscriminate, even though this is how it's defined). Males though
may be willing to copulate with as many males as possible.

Sexual Selection: Form of natural selection in which variation in fitness among individuals
is generated by different mating success.

Sexual selection can arise from two different methods

• 1) male rivalry: One male could deny access by other males to a female.
  For instance, territoriality would be a form of acquiring beneficial resources
  that would allow them better success in mating.
• 2) Female preference: if females prefer some character of males, as a
  form of better fitness, they can greatly effect breeding traits.

Darwin's notion of sexual selection

"Before we discuss further the question whether females select the more
attractive males or accept the first whom they may encounter, it will
be advisable briefly to consider the mental powers of birds. Their reason
is generaly, and perhaps justly, ranked as low; yet some facts could be
given leading to an opposite conclusion. Low powers of reasoning, however,
are compatible, as we see with mankind, with strong affections, acute
perception, and a taste for the beautifull and it is with these latter
qualities that we are here concerned."

Charles Darwin, 1871

The Descent of Man and Selection in Relation to Sex.

Wallaces response:

A young man, when courting, brushes or curls his hair, and has his moustache,
beard or whiskers in perfect order, and no doubt his sweetheart admires
them; ….

‘the permanence of female taste was ‘scarcely verifiable
in human experience’

Goddes, P., and Thomson, J.A. 1889

The Evolution of Sex

When the concepts of sexual selection and mate preference were coming to the forefront on scientific thought, they were hotly contested. One of the main reasons for the debate was that the men in that day and age could not agree that human females had any preference for other males. If human females had no preference, how could birds have a preference?

Some of the evidence came from extra-pair copulations, also called extra-pair
paternity.

Other evidence came from females of whom have a certain reperoire size that would choose males with a larger repertoire size (length of their call). Thus it is believed that the size of the song is a trait of male birds that has been shapped by female presence.

Its also important to show that males and sons have some sort of genetic
linkage. One is that bright-stripped males were linked to bright-stripped
sons.

Long tailed Widows in Africa: Males have very long tails. It’s
a classic study to see if females prefer longer tails. Thus, he had several
birds that were control. He also took some males and cut their tale, and
took other males and glued their tales. Turns out that Female long-tailed
widows greatly preferred males with longer tails.

Anders Molar did a study with Barn Swallows. He showed that female barn
swallows preferred to breed with males that had long streamers.

Polygyny threshold model. (Orians 1969): Why would a female ever choose
to be a secondary female in a males territory that already has a female,
instead of being the only female in another males territory. You can basically
look at fitness curves in relation to certain territories. There is a
cost to being the secondary female would have a curve shifted down. Thus,
how much better does a territory have to be to allow a female to choose
to be a secondary female on that territory.

If ‘A’ is the decrease in fitness induced on a female by being
the secondary female on a territory. ‘B’ is then the change
in territory quality of two curves.

EPP: environmental potential for polygyny: A high potential for polygeny
occurs if there is patchy distributions of resources so that one male
can obtain more than his fair share of resources.

LEKS: Aggregations of males that come together for no other reason than
to attract females. Its in leks that males can reaches extremes. The most
dominant male will sometimes get more than 50 percent of matings.

European RUFFS: Some males in a lek look very different. For example
the European Ruff an have a satellite male that looks almost albino. The
terminal males are darker. The satellite males exist only to get the females
that are inbetween male territories. (Photo by C.C. Doncaster).

Swallow-tail manikins: Engage in cooperative displays in leks. One group
in a single lek can have a dominant male. There are three helpers in the
group with the dominant. They will then engage in a marvelous leapfrog
dance. The idea is that maybe the helper males are just younger males
that need to learn the ropes from the older males. They are not closely
related though. Or maybe the next helper may become the next dominant.

New Guinea Birds of Paradise exibit the extremes of gaudiness.

Cotingas of central and south America. In addition to colored feathers,
birds have skin accessories. For instance, blood engourgment in the mouth
area become erect when blood enters the area.