Saturday, 18 November 2017

Fairy Terns mating

Birds which have been evolving for many millions of years have developed a large range of courtship and mating behaviours which are critical to successful reproduction. Recently I was able to capture a sequence of photos of the mating behaviour of a pair of Fairy Terns (Sternula nereis nereis) starting with the male approaching with a fish offering, then moving on to so called fish-wiping and ending with copulation (see note 1).

Early in 2017, I published a post showing a failed mating attempt:

Many mating attempts are not successful, with some simply interrupted and others involving a female that seems receptive, but was perhaps deceptive, only putting on a show to obtain the fish being offered.

According to HANZAB (see note 2) the bonds between Fairy Terns are poorly known. Courtship may involve fish-carrying pursuit flights, fish-parading and courtship feeding (male feeds the female). The pair in this post have no doubt courted, selected a nest site and dug a nest scrape at this stage.

The following sequence of photos seems to show this mating attempt ended successfully though we cannot know if an egg has been fertilised which is the only real measure of success.

Please click on photos to enlarge.

A male Fairy Tern in breeding plumage approaches a female with a fish offering.
The male at this stage circles the female with head held high displaying the fish, while the female maintains a semi crouched posture.
At this point I thought proceedings did not look to be going well – is she about to reject him or is she telling him to get on with it?
The male continues to circle – from time to time the male’s wings are lifted.
At this point it looked like the male was starting to approach from behind when the fish-wiping might start in earnest.
The male is moving his head rhythmically from side to side and at times the fish brushed over the female’s back and nape – hence the term fish-wiping.
The fish-wiping can go on for up to five minutes and as you watch you start to wonder if he will ever get to the point of the exercise.
The head continues to move rhythmically from side to side. The females also do this but not to the same extent as the males. The male may also do the head movements without a fish.
The male is now directly behind the female and you get the feeling the ritual is heading towards a conclusion. Still he wipes the fish from side to side.
Then he is back by the female’s side again – perhaps this ritual is not close to a conclusion after all?
Then suddenly things move quickly – the male has leapt up to mount the female.
He passes the fish and she accepts.
Copulation begins while the female seems to be more interested in the fish.
As the female swallows the fish the male finally completes the mating ritual.

A second later this four-minute (see note 3) ritual ends abruptly, however this ending is also a beginning – with fertilisation, a new generation of Fairy Terns is under way.


(1) The photos in this post have been obtained while working with a government biodiversity officer at a small tern breeding colony using a long telephoto lens to ensure there was no disturbance to the breeding birds or the colony. When photographing any birds the welfare of the birds must always come first.

(2) HANZAB = Handbook of Australian New Zealand and Antarctic Birds

(3) The times recorded on the photo files allowed me to calculate that just over 4 minutes elapsed from the first to the last photo.

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