Food stress, nestling growth, and fluctuating asymmetry

It has been suggested that fluctuating asymmetry (FA) is an indicator of pe rturbed development. Our study tests the validity of a fundamental assumpti on of asymmetry theory: that environmental stress imposed during developmen t increases FA. We deprived tree swallow (Tachycineta bicolor) nestlings of food for 3 h every morning from days 3 to 15 of brood age. We obtained 2 r epeated measurements of the right and left ninth primaries, outer rectrices , and tarsi, and one measure of body mass on days 8, 12, and 16. Food-depri ved nestlings had significantly smaller character sizes (all characters for all days), significantly lower wing and tail feather growth rates, and sig nificantly higher tarsal and mass growth rates than control nestlings. Howe ver, food deprivation did not cause experimental nestlings to exhibit signi ficantly greater FA than control nestlings. Nestling mass and FA were not a ssociated within treatments or across broods, and brood size had no signifi cant effect upon the character size, growth rate, or FA level in nestlings. Measurement errors were small and size scaling, antisymmetry, and directio nal asymmetry did not confound FA estimates. Our results suggest that the n inth primaries and outer rectrices of tree swallows are unreliable indicato rs of FA-stress relations because of strong stabilizing selection for symme try in this aerially insectivorous species.