Sunday, 27 May 2018

Holding Court

Rockabill is alive with the sound of courting terns: shrill calls to mates, the helicopter-like warnings not to encroach on a particular tern's territory, females demanding to be fed... It's pretty loud!

Courtship in terns occurs in three main stages, with both sexes taking an active role in mate choice. The first stage takes place in the air, in circling displays above the colony. The aerial nature of this display may allow potential mates to size each other up, perhaps literally: there is some indication that the length of tail streamers in roseate terns is linked to reproductive quality, and may help terns decide on a suitable mate1.

With wings spread wide and sunlight shining on them, terns can see how colourful each others flight feathers are. This may seem like an odd thing to say about a white/grey/black looking bird, but terns can see in the ultraviolet spectrum, and their newest flight feathers are covered in barbules which reflect this type of light. Terns are unusual in that they replace their flight feathers two and even three times a year. This happens slowly, and in overlapping stages, so that the second and third wave of feather growth can be seen at the same time. An abundance of new, “colourful” feathers may indicate good condition, and make that tern more attractive2.

 Roseate terns strike a pose. Taken under NPWS license by Luíse Ní Dhonnabháin.

Terns pair off at the peak of this spiral, and after much displaying of the male's prized fish, he leads the female back to his nesting territory. Here after much synchronised posturing, the male hands over the fish to the female, after which they copulate for the first time (in common terns up to 50 copulations have been recorded for a single pair)3.

Roseate tern pair mating. Taken under NPWS license by Luíse Ní Dhonnabháin.

In the second stage the pair head out to sea to feed, occasionally visiting the nesting territory.

In the final stage the female stays at the nest site while the male forages, bringing back food to the female while she lays the eggs. Some terns prefer to rob fish from their peers rather than head out fishing themselves. The time saved means that these terns can feed their mate more often, resulting in females with higher body masses and better outcomes for their fledglings. In spite of this, these kleptoparasites seem to make up only a small proportion of the colony's population4.
Roseate female wishes male would just hand it over. Taken under NPWS licence by Luíse Ní Dhonnabháin.

The arrival of the first eggs on Rockabill this week marks the end of the courtship period for the early birds, but there are still many more wooing away!

We hope you've been enjoying National Biodiversity Week, and made it to some of the programme's events. Until the next time!

Luíse Ní Dhonnabháin
& The Rockabill Team.

1 Palestis, B.G. et. al. (2012) 'Tail length and sexual selection in a monogamous, monomorphic species, the Roseate Tern Sterna dougallii' Journal of Ornithology , 153 (4).
2 Cabot D. and Nisbet I. (2013) Terns. HarperCollins:London.
3González-Solı́saf J., Sokolov E., Beckerd P.F. (2001) 'Courtship feedings, copulations and paternity in common terns, Sterna hirundo' Animal Behaviour, 61(6).
4 García, G.O., Becker P.H, Favero M. (2012) Kleptoparasitism during courtship in Sterna hirundo and its relationship with female reproductive performance.' Journal of Ornithology, 152(1).

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