Fluctuating Asymmetry in Common Tern (Sterna Hirundo) Chicks Varies with Hatching Order and Clutch Size

Fluctuating asymmetry (FA), small random deviation from bilateral symmetry, often increases with stress during development. Common Terns (Sterna hirundo) typically lay two to three eggs that hatch asynchronously. I predicted that C-chicks (last of three) should have greater FA than A- and B-chicks at hatching and that FA should be higher in chicks from smaller clutches, because of differences in parental quality. Tarsus length of newly hatched chicks was measured across three years, and middle toe length was measured in one year. Sample sizes exceeded 100 chicks in two of three years. Variation in tarsus FA with hatching order and clutch size was statistically significant in one year (P < 0.01) and nearly so in another (P < 0.10). No significant differences were present for toe FA. A-chicks from three-egg clutches appeared to have the lowest tarsus FA among categories of chicks in both years, and in one year they were significantly more symmetrical than B- and C-chicks from three-egg clutches. As predicted, A-chicks from three-egg clutches were also more symmetrical than A-chicks from two-egg clutches and singletons. However, C-chicks did not differ significantly from B-chicks in tarsus FA. Fluctuating asymmetry also varied with hatching date, but no clear pattern emerged. Fluctuating asymmetry was not associated with trait size or body mass, although there was significant variation in body mass and toe size among groups, C-chicks being relatively small and A-chicks and singletons relatively large.