Wednesday, 7 February 2018

Fairy Terns – from chicks to fledged and flying juveniles

Fairy Terns (Sterna nereis) are endemic to Australia, apart from two separate and small sub species populations in New Caledonia and New Zealand (North Island). They are mostly found along the southern Australian coast including Tasmania though they do extend up the WA coast to about Exmouth Gulf-Ningaloo Reef and the east coast to Sydney.

The Australian population of the very similar Little Tern (Sterna albifrons) occupies the northern coasts of Australia and is also widespread in Asia, Africa, Europe and the Americas.

The Australian distribution of the Little and Fairy Terns can be seen in the following BirdLife Australia Birdata maps. Asian Little Terns, especially Japanese birds, visit Australia in non-breeding plumage over our summer.

Please click on photos to enlarge.

Sighting records for the Fairy Tern
Sighting records for the Little Tern

In Victoria, the two populations overlap and on the Gippsland Lakes in East Gippsland, Little and Fairy Terns, both largely colonial breeders, often breed together though the Fairy Terns start first and are followed a few weeks later by the Little Terns. Single pairs of both Little and Fairy Terns have been found breeding alone however this is not common.

The background story on conservation efforts involving renourishment of sand islands in the Gippsland Lakes to improve breeding opportunities and success for Little and Fairy Terns can be found here in BirdLife East Gippsland’s newsletter The Chat, see page 16:

The breeding season in SE Australia is from September to March, though recent breeding events on the Gippsland Lakes have occurred between October and January.

Over the past two breeding seasons (2016/17 and 2017/18) I have been privileged to work with Faye Bedford, DELWP Biodiversity Officer, to monitor the small tern breeding colonies on the Gippsland Lakes and to assist this season with photographs for a chick to juvenile age classing project.

As the title suggests this post will present photos showing the development of Fairy Terns from recently hatched downy chicks through fledging to fully fledged and flying young.

A pair of Fairy Terns mating, including fish wiping, can be seen in an earlier post here:

The nest is a simple excavated sand scrape which may include the placement of small stones and or shells around the rim. The nesting sites we observed were on islands with extensive flattish open sand formations topped with an abundant shell surface which was high enough to avoid inundation from the surrounding lake waters and low enough to achieve a suitable moisture level to ensure the humidity around the eggs was satisfactory. Given the small terns used the sites immediately after their formation by dredging it was obvious they found the conditions satisfactory.

Photo showing the breeding area. Surveying/counting small terns in breeding colony.
Fairy Tern on nest scrape.

Incubation commences after the last egg is laid so that sibling chicks hatch at about the same time, and takes from 19 to 21 days (Little Terns are similar) with both parents sharing incubation. I observed on several occasions fish presentation at change over. One or two eggs is common and occasionally three for Little Terns. We observed two nests with 3 downy chicks however two chicks per brood was the norm.

The precocial(1) young hatch with a covering of down and are mobile and leave the nest scrape within 2 to 3 days. Terns will construct their nest scrapes beside woody debris if available in which case their chicks will often stay by this cover and excavate shelter hollows by and under the woody debris. Two pairs of chicks which we observed stayed by the woody debris where they hatched for the full fledging period of about 3 weeks. Where woody debris was not available chicks headed for the nearest plant cover for protection.

On our first visit to the breeding colony on 2/11/17 there were many birds on nests and mating was observed but no chicks were seen. On the second visit on 14/11/17 no mating was seen, many birds, both Fairy and Little, were on nests and there were good numbers of chicks seen. The oldest chicks were estimated to be between 3 and 5 days old at most, giving an estimated date for the start of incubation of the first Fairy Tern eggs to hatch of 21/10/17, give or take a day or two.

It should be noted that the breeding colony builds up over time with more birds joining the colony and commencing breeding over a period of several weeks. This results in a large range of ages and stages for the juvenile birds with some birds still sitting on eggs, downy young not long hatched and a range of young birds through to fully fledged fliers.

The following sequence of juvenile Fairy Tern photos taken on successive visits to the colony give the age of the oldest juveniles present for each visit based on the estimated age of the most advanced chicks found on our second visit on 14/11/17 being 3-5 days old. By sheer luck we were fortunate to be able to follow two pairs of chicks which hatched from nests by woody debris close to where our small boat pulled in and where we set up each visit to monitor the colony.

The downy young come in a variety of colours and variable amount of blotching from almost none to extensive. The chicks come in two basic colour types, either grey and what I like to describe as ginger or golden, however some chicks are a light blond colour. This variation has been described as dark and light morphs respectively. No doubt evolution has favoured birds with feather patterns that help them blend into their background as protection from predators. So, the down that chicks hatch with and the first set of feathers grown by fledging juveniles have rich dappled patterns to provide camouflage.

The young are very vulnerable before they can fly and are totally dependent on their parents for protection and food. One parent stays with the young at all times while the other goes off to hunt fish – this duty is shared by the parents. Later when the young have fledged and are capable of short flights they crèche up along the shoreline where they wait for parents to come in with food. Even at this well-advanced stage there are always some adults present.

From first hatching, the young are fed small whole fish which they swallow head first. If the fish are too large they are rejected in which case, the parent eats the fish.

Three to five day old chicks (photos taken on 14/11/17)

This Fairy Tern pair nested between the two sticks and hatched two young, one grey and one a ginger colour with both well blotched. I called this pair the “parallel stick chicks”. The grey chick is about to be fed a small fish.
A typical 1-2 day old golden or ginger hued chick with a good amount of blotching.
This pair of chicks are 3-5 days old and are siblings which I called the “yoked stick pair”. One is blotched and other has very little blotching. Both are the golden morph and the one on the right is nearly blond. They stayed together and by or near the yoked stick for most of their fledging time.

This is the blond “yoked stick” chick. At 3-5 days old note the primary flight pin feathers are the first feathers to develop in the fledging process. 

When a parent signals danger the chicks lie flat and very still on the sand and shell grit until they are given the all clear signal from a parent. 
A parent brings a fish to the “yoked stick” pair however they showed no interest, indicating they were well fed at that point, so the parent departed with the fish – I assume it ate the fish itself.
This pair of chicks are no more than three days old – they were a little wobbly on their feet and there are no pin feathers showing on the wings.

At 3-5 days old the chicks are quite active. They never seemed to stay in one place very long. They took brief rests and short sleeps usually when digesting recently swallowed fish and at other times made short excursions away from the yoked stick. At times these excursions would result in trouble such as the time they tried to nestle in under an adult Fairy Tern – not their parent - sitting on a nearby nest. 
At first they seemed to be accepted, but soon the short-lived foster parent rejected them with a few pecks and they scurried back to their yoked stick refuge.

Another incident involved a nearby nesting Red-capped Plover. The blond “yoked stick” chick ventured over and into the Red-cap’s nest scrape. The Red-cap of course was not happy with the situation and tried to evict the Fairy Tern chick. However this aggression towards the chick resulted in one of its parents attacking the Red-cap. While there may have been some advantage for the Red-cap to nest in a small tern colony, in the end it was unsuccessful probably because of the endless disturbances and too much time off the nest. Similarly, a pair of Hooded Plovers attempted to nest within the colony. They too did not appear to be successful.

Ten to twelve day old chicks (photos taken on 21/11/17)

Our third visit to the colony was seven days later on 21/11/17.

The “yoked stick” pair had a scrape at both ends of the stick where they spent their rest times, either together or singly, in the shelter afforded by the stick.
The “yoked stick” pair at 10-12 days old. Note the feathers developing on the mantle (back), wings and tail. There is no sign of feathers on the body and head which are still down covered at this stage.
This is the dark or grey morph “parallel sticks” chick which is about 10 days old.
This is the light or ginger morph “parallel sticks” sibling to the chick in the above photo. It has just been fed. Note feather development is similar to the “yoked stick’ pair – they are the same age give or take a day.

The blond “yoked stick” chick taking shelter at the yoke end of the stick.

Each time the chicks entered the depression they vigorously raked some sand out with their feet. The scrape enlarged as the chicks grew. There was another scrape at the other end of the stick. Woody debris such as this provides highly valuable protection for developing chicks.

The “yoked stick” pair showing feather development at 10-12 days old. Their heads and bodies are still downy.
Another photo showing feather development on the wings at 10-12 days.
While the oldest chicks in the colony were 10-12 days old this one is only 3 or 4 days old. This chick is one of a pair and both rejected the fish which I think may be too big for them to swallow.
Parents across the colony were kept busy bringing in fish for hungry and rapidly growing chicks – there were still birds on nests including both Fairy and Little Terns. This is the “yoked stick” pair again and the chick on the right got the fish this time.

Seventeen to nineteen day old chicks (photos taken on 28/11/17)

On 28/11/17 we made our fourth visit to the colony when the oldest birds were between 17 and 19 days old. They were not fully fledged at this stage and could not make flights away from the colony when the adults spooked and took to the air.

My first 17-19 day old chick photo taken on our fourth visit.

Note the mantle, wing and tail feather development and now feathers are replacing the down on the head. The body still appears to be only down covered at this stage though white body feathers are developing below the down.

At 17-19 days fledgling pairs are still sticking together and sheltering in scrapes by woody debris.

Seen on their own the chicks still looked small however when they stood beside a parent their size became apparent.
Both the “yoked stick” pair were still at the yoked stick where they hatched some 18 days earlier again showing that if woody debris is available, the chicks will use it for shelter. The nearby “parallel sticks” pair were also still in the same location where they hatched.
This is the “parallel sticks” pair at 17-19 days old.

The 17-19 day old birds were making exercise flaps of the wings – the build-up to flying which was still some days away yet. 
Same bird as above – this shot shows feather development on the wing. 
At 17-19 days old the young birds are spending a lot of time preening the new feathers as they displace the down and waiting to have their now insatiable appetites satisfied.

A moment after the above photo was taken and the parent is coming in with a fish and the young bird gets excited. Note the wing feather development – still a way to go to achieve sustained flight. 

Twenty-five to twenty-seven day old chicks (photos taken on 6/12/17)

On our fifth visit to the colony the oldest young were 25-27 days old and fully fledged. They were capable of flight and readily joined the adults when they took fright and flight in response to a disturbance or threat such as a raptor flying over. However, the colony still contained a large age range with many younger juveniles and many still fledging and not capable of flight.

Most of the oldest juveniles at this stage are what I call fliers and they spend their time at or just above the shore in a loose crèche where they wait for incoming parents with food and are still protected by the presence of adults. They are not, as far as I could see, going out to fish for themselves at this age. When they do finally leave the breeding colony it will be with their parents who they will depend on for some time before they are cable of catching fish for themselves.

This is a 25-27 day old juvenile Fairy Tern. It is fully fledged and can make short flights.

                                   Same bird as above showing wing feather development.

Even at 25-27 days old and fully fledged these birds are still waiting to be fed and will settle in a scrape/depression to rest and wait just as they did when they were young chicks. They are still very dependent on their parents.

This juvenile Fairy Tern is a little younger than the above 25-27 day old bird – note the down around the head and flight feathers are a little shorter. Many of the advanced juveniles entered the water to bath. This one is still a little wet and needs some preening to tidy up. 

This bird is about the same age as the one in the last photo above. The mantle and wing feather patterns and colours were highly variable between birds. 

Many of the young birds exercised their wings. Birds of this age are capable of short flights.
Right from downy young through to fully fledged juveniles, the down and later the mantle and wing feather patterns and colours was highly variable between birds.

Nine days later, on 15/12/2017, the seventh visit found the breeding colony completely deserted with no Fairy or Little Terns of any age present and visible remains of dead birds. An investigation of the breeding area found no evidence for the abandonment of the colony. At this stage, many young would have been incapable of leaving the colony as many of the juveniles could not fly.

In the absence of any human or dog foot prints it can only be assumed that a flying predator has attacked the colony, forcing the adults and any young that could fly to leave. The remaining juvenile young would have been cleaned up by a range of predators including gulls, ravens and raptors once the adults were not there to protect them. The 2016/17 breeding season ended the same way. However many juvenile birds did survive and leave the colony with their parents.

Another visit to the site 14 days later on 29/12/2017 found a small number of Fairy and Little Terns adults plus fledged juveniles.

Since the breeding colony was abandoned it is difficult to estimate the age of the juveniles. Also as the juveniles reach the fully fledged stage, the birds at this age tend to all look the same with further development with age showing longer flight feathers and bill length increase.

This juvenile Fairy Tern could be as old as 48-50 days.
This juvenile Fairy Tern is developing some yellow in the bill compared to the bird in the above photo.

A search of many possible/suitable roosting locations across the Gippsland Lakes for the adult Fairy and Little Terns with juveniles that had left the breeding colony failed to find the birds. A small number were found along with a small number of juveniles, however the bulk of the birds appear to have left the Lakes?

Tracking the developing juvenile Fairy Terns once they leave the breeding colony is very difficult. Even if they could be found, getting close enough for photos would be hard. However the “post juvenile” development involves a complete or almost complete moult of the juvenile feathers starting for some in February-March with some birds attaining their first immature non-breeding appearance by as early as April.

At a distance, in their first immature feathers, they will be difficult to separate from the adult birds which by then will have moulted out of breeding plumage and back to non-breeding plumage. It should be noted that the development from post juveniles to first adult breeding plumage and the associated moults and their timing is poorly understood - per HANZAB.

Post breeding on the Gippsland Lakes, it is assumed that the Fairy Terns and their young head west along the Gippsland coast and the Littles head north up the east coast of Australia. The reality is we do not have a strong handle on post breeding movements. All we can do now is hope they return next spring for another breeding session.

(1)  Precocial is a term used to describe downy young with open eyes that are mobile and leave the nest within 2 to 3 days – many species of birds produce precocial young, for example gulls, plovers and mound nest builders such as the Malleefowl.  

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