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Møller (1) found that experimental lengthening the tail of barn swallow males led to earlier mating and therefore an increase in offspring. Since he observed no significant differences in intensity of male-male aggressive encounters among males at the nesting sites he attributed the early mating solely to female choice. This released a flood of anthropomorphic speculations on what the female has in mind when she chooses a mate.
However, the earlier mating might be merely the result of use of the elongated tail as a status symbol in competition for foraging territories (100-500m from the nesting sites). Such use of display features would save energy otherwise used in physical fighting and competitive racing. The same could be said for the white spots on top of the tail as positively related to elongation of the tail. White spots are commonly displayed by birds in competition for food reserves.
Møller (2) does indeed carefully describe competitive flights of barn swallows in groups from which females are attracted to the nesting territories, though Møller does not consider any possible relationship between the area of such flights and reproductive resources. Such a relationship is clearly evident with the dragonfly, Perithemis tenera, in which groups of males perform competitive flights in areas from which females are later attracted (3). This resembles “trysting” behavior commonly observed among insects.
The male white-tailed skimmer dragonfliy, Plathemis lydia, displays the white upper surface of the abdomen toward other males competing for reproductive resources. Artificial blackening of the white surface diminishes territorial success of the males. This experimental elimination of the white feature results in greater expenditure of energy on the part of the blackened males (3). Observation would reveal the more successful in competition among these males (the white ones) would show less competitive intensity than the less successful (the black ones).
Among the barn swallows, optimal territory with plentiful food may influence the female in choice of a mate. Under natural conditions, Syrphidae flies form an important fraction of the food (2). These weak-flying yellowish flies that hover and perch on flowers may determine survival of the first brood of swallows in the cold spring weather. This may well account for the earlier mating and reproductive success of the males with the elongated tails. Regarding male features that attract females at the nesting sites, candidates might be the bright yellow gaping billl set off by the surrounding reddish plumage. The male displays these features toward the female at the nest site. The resplendency of the plumage may be conditioned by the adequate nutrition the competitively advantaged male has received in his foraging territory near the end of his prenuptial molt.
1. A.P. Møller (1992). Nature 357, 238-240,
2. A.P. Møller (1994). Sexual Selection and the Barn Swallow. Oxford University Press.
3. M.E. Jacobs (1955). Ecology 36, 566-586.
