Monday, 30 January 2017

Whiskered Tern Breeding colony - Lake Omeo, Benambra

This Whiskered Tern breeding colony was discovered by members of the Bairnsdale and District Field Naturalists Club in early December 2016, at Lake Omeo located near Benambra in East Gippsland’s high country, when members visited the lake to check on some rare plants.

For more information about Whiskered Terns please refer to two previous posts:

Lake Omeo is about 700 metres above sea level. At the current knee-deep water depth, the lake is about 5km long and nearly 2km wide at its widest point. The lake has no outlet as the original drainage to Morass Creek was cut off by faulting and block tilting many thousands of years ago. The Lake is normally dry, however after good rains it can have sufficient water depth to support many water birds and breeding events such as the Whiskered Terns now breeding there. In the past, the lake sometimes even had enough water depth to support water sports such as water skiing and sailing, but it only fills to that extent on very rare occasions these days.

Please click on photos to enlarge.

Looking towards the north-eastern end of Lake Omeo – Benambra township is on the right-hand side of the photo. The peaks in the background are The Brothers.

Whiskered Terns are colonial breeders and usually breed in shallow marshes and flooded grassland where aquatic insects are numerous. They are nomadic in Australia and are quick to exploit flooded areas along inland rivers. Lake Omeo lies just north of the Dividing Range at the head of the Murray River catchment.

It is possible the Whiskered Terns travel between the Gippsland Lakes, where they are often found in large numbers during inland droughts, and the Murray Darling Basin (MDB) by flying up the Tambo River, crossing the Divide at a low point near Benambra, and then flying on down the Mitta Mitta River, across Lake Dartmouth and beyond. However, in doing so they seem to have noticed Lake Omeo and taken advantage of the rare water and breeding opportunity there even though the MDB currently has lots of water following good inland rains in 2016.

Looking south from out on the Lake – the peaks in the middle background are The Sisters which are on the Great Divide – water falling on the other side of the Sisters runs into the Tambo River which discharges to the Gippsland Lakes and then Bass Strait.

In early December 2016, 60 to 70 breeding pairs were reported to be on nests with eggs in the middle of the lake and more birds were building nests. When I made my first visit to the Lake on the 4th of January 2017 there were no nests in the middle of the lake. This breeding attempt had clearly failed. However, the birds had made a fresh start in more substantial aquatic vegetation near the NE end of the lake.

Looking south across Lake Omeo and the aquatic vegetation where the Whiskered Tern breeding colony is located.

It is hard to estimate the number of Whiskered Terns present and the number of nests. However my conservative guestimate on the 4th of January was 250 adult birds with 100 breeding pairs on about 100 nests. Of about 30 nests inspected, only two had one just- hatched chick. Given an incubation period of 18 to 20 days, egg laying for the new breeding event must have commenced on about the 12th of December. Hatching is synchronous, that is, incubation starts after the last egg is laid and the chicks all hatch at about the same time.

While Whiskered Terns are colonial breeders each pair defends a small territory around their nest, so nests are spread across a large area. The nests were located in a species of rush and the same plant was used to construct the nests.

Whiskered Tern nest - this one is more substantial and more protected than many of the other nests.

Side view of another nest – some nests are anchored in the surrounding vegetation while others float while being constrained laterally by the rushes.

One of the 30 nests checked on the 4th of January. The egg on the nest has a small hole – the start of another chick working its way out of the egg.

This was the other just hatched chick (4/01/17).

Hoary-headed Grebes apparently often nest in association with Whiskered Tern colonies and this was the case at Lake Omeo.

Hoary-headed Grebe nest – with egg covering vegetation removed for the photo and then carefully replaced.
The Hoary-headed Grebes were very secretive around their nests – I never saw one either on a nest or leaving a nest.

Both Whiskered Tern parents share nest building, incubation and care of the chicks. A hide was used to photograph adult birds on the nest because even though they leave the nest on approach, they soon return once you are concealed in the hide.

Pair on a nest.
Adult on a nest.

This bird has just flow in to the nest and is about to settle on the eggs.

Partner birds would stand on the nest beside the incubating bird or rest nearby.

At first I thought this bird was collecting nest material. It tried to lift two reeds.

Then it tried to lift one reed.

In the end it just perched on the floating reeds – I suspect it was the partner to one of the birds on a nest I was photographing.

Adult Whiskered Tern in breeding plumage.

Adult Whiskered Tern in breeding plumage.

Non-breeding Whiskered Tern – most of the Whiskered Terns at Lake Omeo were in breeding plumage.

Flock of Whiskered Terns – I could not work out why they were massing above this point on the Lake – they did not look to be feeding?

There are 49 birds in the photo above and there were probably another 15 birds outside of the frame making 64 birds altogether.

A slightly cropped version of the photo above – the WT’s are all looking down – but what were they so focused on?
Another visit to Lake Omeo was made on the 22nd of January to see how the breeding colony was progressing. What a difference 18 days makes! There were still a few birds on nests with eggs, however most of the eggs seen on the 4th of January had hatched and there were many young birds ranging from recently hatched unfledged young to fully fledged flying young. This was good to see as it meant, unlike the early December breeding attempt that had completely failed, this attempt had successfully produced many young.

On one count, there were at least 50 birds flying this way and that across the SW half of the lake searching for aquatic food. However Whiskered Terns also hunt over dry land for insects and other small prey such as skinks. One bird was spotted with a small skink in its bill flying around the breeding colony looking for its young. Another bird was seen with a fish.

Whiskered Tern with a skink – probably taken in the dryland grazing paddocks beyond the lake.

The colony is a very noisy place as birds come and go with food for young. When they approach with food there is a lot of calling from both parents and young to locate each other for food delivery. The young birds appear to be very mobile both pre-flight swimmers and fledged flyers. Once chicks hatch they are quick to move away from the nest and into the shelter of the rushes – they would be easy standout prey on the nests.

Nesting birds and their young are very vulnerable to predation and nesting in colonies is one way to counter predation. Only one raptor was seen while at the colony, a juvenile Swamp Harrier. Swamp Harriers are very effective hunters in wetland habitats and young water birds are often easy prey. To counter this threat, the Whiskered Terns band together to aggressively mob threatening intruders such as Harriers and birdwatchers.

This juvenile Swamp Harrier was hanging around the Whiskered Tern breeding colony and was no doubt preying on young Terns.

At one point the young Harrier flew across the breeding area and was immediately attacked by 15 or so Whiskered Terns – on this occasion the Harrier was driven out of the area.

The young were very wary. The non- flying birds could swim well and took shelter in the reeds, leaving their nest almost immediately as nests are very visible to predators. The fledged flyers were also wary and took flight early, not allowing close approach for photos. I am sure the parents warn the young birds to hide or fly at our approach.

Chick hiding in the reeds.

Another chick hiding in the reeds.

An older chick with developing wing feathers held up out of the water while swimming strongly.

These three juveniles were probably siblings and all could fly. From hatching these young ones have fledged to the point of being able to fly in about 18 days. They are still showing some down around the head and face.

One juvenile has flown, the second has just taken off and the third bird was not far behind its siblings.

The period from the start of incubation to a fledged flying bird, can be as short as 36 days. For birds that respond opportunistically to flooding events, a short time to raise young is an evolutionary advantage.

It was very interesting to observe a Whiskered Tern breeding colony in East Gippsland as these events are uncommon in our area and certainly are rarely recorded.

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