Ivory Towers: The mating game (1)

WHEN males and females get together to mate, how do they select each other? Do males compete with each other, with the winner taking the female of his choice, or is it just a case of each chap selecting the best-looking available bird? This question is investigated in Non-random pairing in American kestrels: mate choice versus intra-sexual competition (Animal Behaviour Vol 44, November 1992) by Gary R Bortolotti and William M Iko of the University of Saskatchewan.

Falco sparverius, the American kestrel, is abundant in northern Saskatchewan, and the researchers were, over a three-year period, able to observe the mating habits of over 400 pairs in an attempt to distinguish between inter-sexual (mate choice) and intra-sexual (same-sex competition) effects on pairing behaviour.

Using size as a measure of competitive ability and condition (measured as mass divided by volume) as an index of quality, they looked for determinants of mate choice. 'Within mated pairs, there was no relationship between a bird's size and the condition of its mate,' they say. So the biggest males don't get the best females as would be predicted if intra-sexual competition was part of the process. But there was 'positive sorting by condition, suggesting both sexes used quality as a criterion in choosing mates'. Fit females attract fit males. 'Because females that did not obtain mates were in poorer condition than those that did, males may have rejected poor quality females.'

There is, however, a complicating factor: smaller males are better fliers and may be better able to find food. The researchers included in their figures an assessment of food abundance on each bird's territory (they assessed this by trapping small mammals with peanut butter). No correlation, however, was found between mate-choice and food availability.

The main conclusion was that males fight over territory, not women.