The Cuckolded Swallow

Swallow, Hirundo rustica

The aerial acrobatics of the swallow (Hirundo rustica) are something to behold this time of year. The midges and flies that they feed on are in abundance in the summer months, and H. rustica is certainly kitted out to take advantage, with its streamlined wings and long tails. These tails mark them out from similar relatives such swifts and sand martins, as well as the blue-black back and wings, white undersides, red face patch and black breast band (1). It is probably Ireland's most recognisible avian migrant, and certainly its most impressive, traveling from South Africa in early spring to avoid the harsh winters there and returning from Ireland when the cold, autumnal weather reduces the numbers of its invertebrate food.

Swallow, Hirundo rustica

H. rustica is often referred to as the “Barn Swallow” in some countries, due to its habit of nesting in barns and farm outhouses, forming a bowel shaped nest from mud and other materials. It has been shown that in mate selection, females prefer males with long (2), symmetrically patterned tails (3). Long tailed individuals arrive earlier, giving them more choice of mates and symmetrical tails signal lowest degree of fluctuating asymmetry and hence the fittest individuals. It would therefore be expected that long tailed individuals would produce more individuals, which was the case in studies in Denmark and Canada in the 1980's (4). However, genetic analysis of offspring produced showed that long tailed males only fathered 59% of the offspring in their nests compared to 96% of the nestlings of short tailed birds. So while long tailed males mate earlier and are preferred by females, they experience a higher percentage of cuckoldry. Unfortunately for them, it is thought that the very thing that gives them an advantage in attracting a mate in the first place (their long tails) is the ting that is their undoing: long tailed males tend to have less maneuverability when feeding, meaning they have to spend longer hunting for food and consequently less time protecting their mates.

References:

1. Sterry, 2004. Collins Complete Guide to Irish Wildlife p. 74
2. Pape Møller, 1990. Animal Behaviour 39 pp. 458-465
3. Pape Møller, 1993. Behavioral Ecology and Sociobiology 32 pp. 371-376
4. Smith et al., 1990. Behavioral Ecology 2 90-98