On spring mornings, the subalpine tundra on Chilkat Pass can be a noisy, busy place. Male ptarmigan scream, squawk and make a strange variety of noises as they chase intruders off their breeding territories. When fights break out, the feathers fly as these small birds peck away at each other.

The female ptarmigan watching these battles from the sidelines might appear like passive observers, but they are probably getting exactly what they want -- monogamous partners that will defend their young. As for the male ptarmigan, they would no doubt prefer to have a harem -- if they could get away with it.

Susan Hannon, now a biology professor at the University of Alberta, has studied willow ptarmigan on Chilkat Pass for 13 breeding seasons. She describes their mating system as a classic "battle of the sexes."

More than 90 percent of bird species are monogamous, meaning that they mate with one partner. But monogamy is not as common in species like ptarmigan whose young mature quickly. To make it even more baffling, ptarmigan are grouse, which are usually polygynous, meaning the males mate with more than one female.

Male willow ptarmigan are the only ptarmigan that defend both females and their young. When the female ptarmigan is fertile, the male stands watch over her, and continues to protect her until the young leave the nest.

The male might be protecting the female from predators such as gyrfalcons, or he might be fending off a totally different sort of predator: other male ptarmigan.

Whatever the reason, this system clearly benefits the female ptarmigan as she is more likely to survive and raise her young with a little help from a protective male.

Hannon also looked at how this strategy benefits male ptarmigan as each bird will pursue the strategy that gives it the best chance of passing on its genes.

"Both males and females try to produce as many chicks as possible, so there are different strategies for each sex. It is not really a case of animals trying to perpetuate their species. Natural selection really works at the level of the individual," she says.

About 10 percent of male willow ptarmigan are polygynous and breed with more than one female. These birds tend to be the kings of the tundra, meaning that they are the biggest and the oldest of the male ptarmigan and have the biggest territories.

Males with bigger territories have a better chance of breeding with more than one female, mainly because the prominent female cannot be everywhere at once. Female ptarmigan will aggressively chase away other females who come too close to their breeding territories, possibly to the dismay of their male partners.

When a female ptarmigan tries to chase another female interloper off her territory, Hannon has observed male ptarmigan trying to intervene, sometimes chasing off their original partners so that they can get a chance with the new female.

To test whether males would choose polygyny if given the chance, Hannon removed some males from their breeding territories, thus freeing up extra females. Sure enough, the males in the surrounding area quickly moved in on the newly available females. Also, in years when more females are around, more male ptarmigan are polygynous.

So why don't more male ptarmigan pursue a polygynous lifestyle, if it means that they could pass on more of their genes? For one, being polygnous can be exhausting work as the males use up a lot of energy running back and forth between female partners, trying to guard them all.

And they must contend with the combative females, who clearly prefer monogamy. Females have another reason for not wanting to share their territories: they want all the food available from that patch of tundra for themselves.

Female ptarmigan return to their wintering grounds in spring with very little in the way of fat reserves. They spend about a third of their time feeding in the breeding territory, trying to store up enough energy to produce a clutch of eight to 10 eggs.

Hannon expects that monogamy has won out because ptarmigan are very vulnerable to predation on the tundra. Their nests are on the ground, where they are easy targets for gyrfalcons, golden eagles and other predators. Also, most years there are not a lot of extra female ptarmigan for males to choose from anyway.

Hannon became interested in ptarmigan while banding geese for the Canadian Wildlife Service in the western arctic. The cabin where she was staying just happened to be in a field with lots of ptarmigan around.

"They are incredibly interesting birds, and they are quite tame so they are easy to observe. When you have them all colour-banded and know which birds should be where, you can tell what they are doing," she says.

For more information on ptarmigan, contact the Canadian Wildlife Service in Whitehorse.