Black tern plunge feeding at Pennington Flash

During most of their stay at Pennington Flash, two juvenile black terns were observed feeding for prolonged periods using three different methods. The commonest method was dipping-to-surface, rhythmically rising and falling to snatch prey from the water with virtually no hovering. A second method which was deployed on a few occasions involved the birds rising high into the sky with the hirundines to feed by chasing flying insects in the air.

However, on one occasion a third method of feeding was observed. One of the terns flew along a feeding route between two of the green buoys that are at the side of the spit. It flew very low over the water, at times hovering as can be seen in the photo above, but more often just flying slowly, almost bat like, before plunging into the water when it would almost completely disappear below the surface for a second before re-emerging and flying up again. The tern continued feeding in this way for a distance of about 100m between the buoys before flying up high and going back to the start of the route. It repeated the same feeding route several times before returning to the dipping-to-surface method of feeding.

Fortunately I managed to capture this method of plunge feeding in a short video below. Watching the video in isolation and it would be easy to think that the bird was either injured or cleaning itself in someway, but in the field it was obvious that it was feeding because it kept returning to the same route. It seemed to me that there was obviously something in the water and close to the surface that it was feeding on, perhaps a shoal of small fish.

By taking a few screen grabs from the video you can see more clearly what it is doing and it becomes obvious that it is feeding.

It flies low over the water looking for prey.

It then plunges into the water bill first going for it's prey.

On this occasion it only goes partway under.

Same again, low over the water targeting prey.

Plunging in bill first.

This time it goes further under water with only it's wings visible above the surface.

Before re-emerging.

It continues along it's feeding route, again low over the water.

Once again almost completely submerged.

It's back!

Finally it turns and heads back to the start. It repeats this several times before returning to the dipping-to-surface method.

The book "Birds of the Western Palearctic" says, "Far less common [method of feeding], surface-plunging; sometimes immerses head and bill, occasionally submerging up to wingtips". 

I'm not sure if the behaviour I observed is true surface-plunging as described by BWP, or if it's something else. I would imagine that what BWP means by surface-plunging involves the bird sitting on the water and snatching prey just below the surface and what I observed is just a variation, but I may be wrong. It's very difficult to find any information on this type of feeding apart from those few lines in BWP which is one of the reasons why I wanted to share this post.