Bird Island’s Sooty Terns – the story continues

Downy chicks standing in the colony (Chris Feare)
Downy chicks standing with a parent in the colony (Chris Feare)
Parental care is still a full-time occupation (Chris Feare)
Parental care is still a full-time occupation (Chris Feare)

Eggs are now hatching over most parts of the Bird Island colony and the parents are highly attentive and protective. Chicks younger than three days are largely confined to their nests, only a few seeking shelter beneath some of the herbaceous vegetation close to their nests. Older chicks, but still covered in down and without true feathers, are beginning to run away from their nests when disturbed and at this stage we stop visiting parts of the colony where such chicks are present. It is a dangerous time for them. Following a disturbance the chicks try to return to their nest sites. They are capable of doing this, possibly being attracted to their parents’ call, or perhaps already having a good sense of colony geography around the nest. But when they pass though neighbours’ small nest territories they can be pecked viciously by the owners, sometimes resulting in injury or death.

A half-grown chick, partially feathered but still with down on its face and belly (Chris Feare)
A half-grown chick, partially feathered but still with down on its face and belly (Chris Feare)

The chicks grow slowly, taking about two months to reach flying stage. During this time their plumage changes gradually from the protective down with which they hatch, through development stages involving the replacement of down with feathers, and the subsequent growth of these feathers until the young birds resemble adults in shape, but with shorter bills and tails.

However, at the flying stage they differ greatly from adults in their colour. While adults are black above and white below, young birds on leaving the colony are largely dull black, with white or beige spots on their backs, grey underwings and a pale grey area at the rear of their bellies. For an unknown time after fledging they continue to be looked after by a parent and occasionally we have seen a youngster being fed by a parent in the air over the beach. The young Sooty Tern flies beneath the adult, which regurgitates food into the chick’s bill – a delicate operation akin to mid-air refuelling in aircraft!

Fully grown chick ready to leave on its first ocean venture: Sooty Tern shape but very different colour (Chris Feare)
Fully grown chick ready to leave on its first ocean venture: Sooty Tern shape but very different colour (Chris Feare)

One of the big unknowns about Bird Island’s juvenile Sooty Terns is where they go during their five or six years of immaturity. We have had only two recoveries of ringed juveniles, one in Sri Lanka and the other inland in Northern Australia. The latter was found after a storm and was possibly blown inland, but nevertheless it must have been in the region at the time. It had left Bird Island only four months previously and so must not have dithered on its 8000 km journey across the Indian Ocean!

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s