Sunday, 10 December 2017

Small Terns - defence tactics

The Australian Fairy Tern (Sternula nereis nereis) is a colonial breeder though occasionally there are solitary breeding pairs. The Fairy and the closely related Little Tern (Sternula albifrons sinensis) usually breed together each spring on some Gippsland Lakes islands where suitable habitat occurs.

Nesting pairs seem to be reasonably tolerant of one another with nests spaced at least 2 metres apart or more. Nesting pairs with eggs or young seem to allow other Fairy or Little Terns within a metre or two of their nest scrapes or the mobile young already out of the nest. However minor squabbles do break out from time to time among the adults when invisible boundaries are crossed. Competition among birds of the same species is referred to as agonistic behaviour.

Please click on photos to enlarge.

Two Fairy Terns in dispute – altercations are usually short and there is rarely any actual contact – aerial chases are common.
Another Tern has strayed too close to a Fairy Tern on a nest and its partner is warning the bird off with calls, open bill and up-raised wings.

The major advantage of course with any colonial breeding species is the strength-in-numbers factor with many eyes to spot approaching predators and many birds to undertake attacks to drive predators from the breeding site. Also, larger numbers mean the inevitable losses to predators are shared, so breeding pairs have a better chance of at least some of their off-spring surviving.

Often when alarmed, the whole colony of adults will take to the air and be very vocal, however they usually circle around at height and soon return to their nests and young. Often when this occurs it is caused by a raptor passing by or over the colony. Any raptor from small Goshawks, Whistling Kites and Swamp Harriers to White-bellied Sea-Eagles elicit the alarm response. Most raptors are attacked and seen off by several Fairy and or Little Terns.

A Whistling Kite has flown over the breeding colony and several Little Terns have attacked the Kite. I have not seen any actual contact – continued dive-bombing close to the head seems to be enough to drive predators off.

The Whistling Kite is seen off by two Little Terns.

By observing the response of the Fairy Terns to various species it can easily be seen which ones are regarded as a threat. In addition to raptors I have seen Ravens, Silver and Pacific Gulls, Pied Oystercatchers, Masked Lapwings and Bar-tailed Godwits attacked. Ravens and Gulls are no doubt a serious threat to eggs and young in the breeding colony. I am not sure how much of a threat these other listed species are, however I guess Oystercatchers and Lapwings may take eggs and small chicks?

Two Juvenile Pacific Gulls landed on a small sand island near a breeding colony of Little and Fairy Terns.

One of the juvenile Pacific Gulls.
An attack was soon launched by several Little Terns and kept up until the Gulls moved away.
It was hard to catch the Terns as their dives were at high speed – even the Gull is still looking towards where the attack came from after the Tern has passed.

In another location, a Silver Gull tried to enter the breeding colony of Fairy and Little Terns.

In this instance Fairy Terns launched an attack.
At the last moment the streamlined Fairy Tern projectile opens its wings to bank strongly away from the determined Silver Gull – both birds screamed their displeasure.

The Silver Gull was not inclined to go, however after persisting with their repeated dive-bombing the Gull eventually departed.

It is heartening to watch the Little Tern, our smallest tern, and the only slightly larger Fairy Tern bravely defend their breeding colonies against much larger predatory birds.

No comments:

Post a Comment