Tail feather elongation in Great Skuas Stercorarius skua: a sexual ornament signalling individual quality?

In breeding Great Skuas Stercorarius skua on Skúvoy, Faroe Islands, central tail feather elongation was longer in males than females and was related to head size, laying date, egg volume, diet composition and, only in males, the tendency to incubate the clutch.

Sexual ornaments, such as elongated tail feathers, allow birds to attract conspecifics of the opposite sex. In skuas, the smaller species have clearly elongated central tail feathers, while in the larger species, the elongation is marginal. We evaluate here if the modest elongation of the central tail feathers in Great Skuas is associated with ecological factors and thus could still function as a social signal.

We caught 47 breeding Great Skuas on their nests on Skúvoy, Faroe Islands in 2013, measured their central tail feathers, other biometrics and reproductive success.

Central tail feathers were more elongated in males (mean 15.2 mm, n = 20) than females (11.3 mm, n = 27). This sexual dimorphism persisted when corrected for body size, as males were smaller than females. In both sexes, longer tail feather elongation was associated with a shorter head, an earlier laying date, and a higher bird-based proportion in the pair’s diet, while no spatial patterns were found. Moreover, during our catching attempts, males with a longer tail feather elongation were quicker back at the nest to incubate the clutch. Egg volume, but not chick body condition, increased with parent tail feather elongation. In a model including laying date, head size, and diet, only laying date persisted as a significant predictor of the tail feather elongation, as earlier laying birds had shorter heads and ate more birds.

We argue that the modest tail feather elongation in Great Skuas may signal individual quality for both sexes, and in males specifically also the nest attentiveness, even though this trait may seem ecologically irrelevant at first glance.