Saturday, 25 February 2017

Caspian Terns – unusual behaviour

On two occasions this week I have observed Caspian Terns on the Gippsland Lakes and captured some photos of unusual behaviour, or if not unusual, then behaviour I have not seen before.

The Caspian Tern (Hydroprogne (Sterna in some guides) caspia) is our largest tern. They are usually found alone or in pairs, sometimes in small parties - rarely in large flocks – and they range from coastal and near coastal waters to far inland rivers and lakes.

Caspian Terns, including dependent young, turn up on the Gippsland Lakes at this time of year following their dispersal east from breeding rookeries on islands in the Port Albert – Corner Inlet area. Several of the birds in this post have been banded and flagged by the Australian Wader Study Group on islands such as Clonmel Island south of Port Albert.

The first interesting behaviour observed was a juvenile Caspian Tern with a parent on a sand bar near Crescent Island. When I first sighted the young bird lying prostrate on the sand I thought it was dead.

Please click on photos to enlarge.

The Juvenile Caspian Tern (with a parent), looked to be dead?
The parent bird has a silver band on its right leg – there is no flag. Also note the streaked black cap - this adult bird has moulted back to non-breeding condition.

As the boat moved I noticed the juvenile bird move and as we came side-on I saw the bird was resting and possibly begging for food from the prone position.

From side-on the juvenile bird’s head and bill are slightly lifted – it certainly was not dead.

I have not seen young terns lying in this position before.

Later in the week I visited the sand islands in Jones Bay, a favourite birding location, and found about a dozen Caspian Terns actively flying about chasing food and loafing on one of the sand islands. I spent an hour or so observing both adults and juveniles and parents coming in with fish to feed their young.

This juvenile has just been given a fish which it is swallowing. 

The parent bird had a quick wash in the nearby water, a preen and then was off again to hunt for more fish. As soon as the parent had left, the young bird moved into the water. I thought it was going to have a wash, having just been fed, but no, it picked up and played with a stick.

Juvenile Caspian Tern playing with a stick.
  
It soon dropped the first stick and then found another which is rotated about in its bill.

Here the young bird has another stick.

Here it tosses and catches the stick. Perhaps this is practice for handling fish which must be positioned so they go down head first?

Once again some new behaviour – I have not seen Caspian Terns playing with sticks before.

There were two juveniles at first and then after sitting patiently for 30 minutes or so two more flew in, making four young ones. Two of the juveniles were flagged, V6 and U8. One of the juveniles stayed close to two adults which I assume were its parents.

The two adults and this juvenile belong together.

The young one often approached one of the adults and begged for food as it is doing in this photo. Note that both parents have extensive black caps indicating breeding condition though in some photos white streaking of the cap is evidence that they are moulting back to non-breeding condition.

The young bird stuck close by the parents even when one went in for a bath.

The juvenile has followed one of its parents into the water but did not bathe itself.
The parent’s bath is somewhat restricted by the young one standing close by.
As the parent flies back to shore to preen the young bird gives a rather pathetic call to be fed. 

On the same sand islands about 70 Red-necked Stints rested at first and then later moved around the shoreline feeding in company with about 14 Red-capped Plovers and one Curlew Sandpiper. They provided some interesting activity and a few photo opportunities when the Caspian Terns were quiet.

Three Red-necked Stints bathing.
Curlew Sandpiper feeding with Red-capped Plovers and Red-necked Stint.
Curlew Sandpiper feeding beside a smaller Red-necked Stint – a good size comparison between these small waders.

Silver and Pacific Gulls hung around the Caspian Terns – ever the opportunists! I wondered if they were hoping to pick up a dropped fish.

Adult Pacific Gull.
This juvenile Pacific Gull sat quietly - unmoved as the Caspian Terns were active around it.

The most interesting behaviour I observed however was an adult Caspian Tern with a fish. It made repeated shows of delivering the fish to various juveniles and one immature bird but in the end, did not give over the fish. This adult announced its arrival with a loud rasping call just like the other adult Caspian Terns coming in with a fish. It landed near and approached one of the juveniles with the fish and the juvenile responded as if to take the fish.

Adult with fish approaches a juvenile.

With most fish deliveries, the hand over and swallowing of the fish happens so quickly it is a struggle to capture the action with photos. However, with this bird the fish was proffered but then withdrawn.

The young bird calls but does not take the fish.
The young bird seems to know it is not going to get the fish and loses interest.

The adult with the fish departs.

The adult with the fish flew out over the water and circled back to the island once again making an entry with a loud rasping call. On this second entrance it approaches the same juvenile bird.

This is the second approach by the adult to the same juvenile bird as before.
Once again the fish is not handed over and the adult bird flies off.

For the third time the adult bird circles around and returns to the same juvenile.
Two Silver Gulls fly over and the pair of Terns duck but do not fly.

By this time the fish is starting to dry, so the adult bird flies to the water and gives the fish a wash – I assume to keep it wet. The juvenile follows still trying to get a feed.

The adult gives the fish a wash.
In the water once again it looks like the fish is being offered and certainly the young one is squawking to be fed.

But no, the adult is off again without passing over the fish. 

This behaviour went on for some time and included approaches to other juvenile birds. Two of the flagged juveniles, V6 and U8, were both approached by the adult with the fish. The adult was also flagged, orange 63.

Here adult 63 with the fish approaches juvenile U8.

I think this approach was to juvenile V6.
A flight shot as 63 circles the island calling with the fish. 

By now this has gone on for over 20 minutes with three of the young birds approached. Each time the adult moves close, the young one calls with open bill and even on occasion the young lunged towards the fish in an attempt to grab it, but each time the adult jumped back to prevent the fish from being taken.

By then I was completely puzzled as to what is going on here and speculating on a number of scenarios to try and understand why the fish is not being handed over.

At first I thought the adult was trying to get the juvenile to follow it because it was uncomfortable handing over the fish in my presence – though other adults had no such problem!

However when the adult approached other juveniles I started to think the adult bird was confused about which bird was its young. Or perhaps there were two offspring belonging to the adult and it was not sure which one should get the fish!

Then I thought maybe this adult has lost its young and it still had the urge to feed a young one but couldn’t bring itself to feed a bird that was not its own?

Sitting on the log watching this unusual behaviour, which I was at a loss to understand, I even speculated that the bird was simply teasing the young ones – it certainly looked like that type of behaviour from a human perspective.  

At home looking at the photos on my laptop I noticed that No.63’s black cap is in more advanced moult back to non-breeding condition than the other adults. Perhaps this bird has reached a stage where its young are on limited or no feeding and must now fend for themselves but the bird is still going through the motions of offering fish.

However why it is offering fish to three different juveniles does not make sense?

No.63 flies in yet again with the fish.
No.63 appears to offer the fish to U8 which is sitting down and by now does not bother to stand in an attempt to obtain the fish from 63.
After a half-hearted bill-open prompt U8 loses interest.
Then the immature (or more advanced juvenile) came over to try its luck with 63.
However 63 turned away with the fish.
63 departs again without giving the fish to either U8 or the other juvenile.

At this point 63 flew about 200m to another island where it landed on its own. Then I heard the rush of a hundred Silver Gull’s wings as they suddenly took to the air from their resting place close to where I was sitting. Looking up to see what had spooked them I saw three Whistling Kites fly over. Apart from the Red-necked Stints and Red-capped Plovers, the island I was sitting on was now deserted and the session observing the Caspian Terns was over.


I am left to ponder No.63’s fish offering behaviour for which I have no acceptable explanation. If anyone has seen this behaviour before or has an explanation or a suggestion, I would appreciate you letting me know.