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Pick up a bird book
containing facts and figures about breeding activity and you will discover
that common terns lay two or three eggs that they incubate for roughly
three weeks and, one month later, a youngster makes its first wobbly flight.
Such bare statistics mask the most critical period in the life of a chick.
These early days are equally hectic for the parents. Glued to the BirdCam
filming the tern colony at Belfast Lough RSPB Reserve, it has been an
eye-opening experience to follow the short - and sometimes turbulent -
history of different families. What did we see?
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Females
proved to be the rulers of tern society. Not only did they take on the
lion's share of incubating the clutch but also they excavated the shallow
depression into which the eggs are laid. The birds made this by pushing
their breast against loose cockleshells and slowly rotating, kicking shells
out from beneath them like a digging terrier. A handful of revolutions
later and, hey presto, a neat cup-shaped hollow was the result. During
the subsequent 'big sit' lasting 20 days or more, some females chose to
while away the incubation hours by inserting more and more shells under
the eggs and finished up peering down from a mound. In complete contrast,
Charlie Dimmock types indulged in fastidious gardening and surrounded
themselves inside intricately woven grass walls.
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A tell-tale sign of
imminent hatching is a tiny hole or crack in the egg's surface. It was
sometimes possible to see this with the BirdCam but we never witnessed
the preamble that took place inside the shell itself. Tern chicks are
born with a hard calcium tooth located, unusually, on the top of the bill.
Known as an 'egg tooth', its purpose is to help the chick chip its way
out of the shell. To use it properly, the baby bird lies on its back inside
the egg and taps upwards to derive maximum leverage. If all goes well,
the shell will gradually weaken and split into two, allowing the youngster
to roll free. One mother was keen to assist and nudged away the final
piece of shell. Comedy then played a hand. When freshly hatched the chick
is bedraggled and wet. The inside of the egg is much the same so that,
in her eagerness to greet the baby, mum finished up with eggshell stuck
on her head.
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 In many clutches all
the eggs hatched within hours; in others, a two-day interval separated
the arrival of each chick. For their time together as siblings, sharing
a nest peacefully was not a problem. Youngsters frequently propped up
against each other for a snooze. When an adult arrived with fish, begging
was unanimous, even though there was dinner for only one. Rivalry, tantrums
and bickering were noticeable by their absence. On average, chicks could
stand on their own two feet just three days after hatching. At this stage,
because they could walk, some parents decided to chaperone broods into
vegetation rather than keep them in the original nest. The motives for
this decision were twofold.
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Heat was one factor.
Brooding eggs and helpless chicks in the full glare of strong sunshine
caused sitting birds to pant uncomfortably.
To cool the chicks (and perhaps also themselves) adults were observed
leaving the colony and returning with soaking wet belly plumage. Once
offspring became mobile, shifting them to the shade of vegetation seems
to have been an attractive proposition. A quite different factor in moving
house appeared to be security. Late clutches attracted unwanted attention
from a growing surplus of unmated birds. Nuisance attacks took the form
of buffeting the incubating adult and attempting to brood unguarded eggs
or newly hatched chicks. The mood of parents reflected anxiety so as soon
as the chicks could stumble forwards - never mind walk - they were led
into concealment.
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By the tenth day chicks
had worked out that the unfeathered stumps poking from their sides were
wings, not arms. This discovery ushered in a daily exercise regime of
frantic jumping up and down: like a mini helicopter trying to take off
without the aid of a propeller. Shortly after, a row of transparent waxy
sheaths containing pre-packed feathers sprouted along the length of the
'arm', even though the remainder of the plumage was still entirely downy.
Because the wing feathers grow to become the longest feathers of all,
they emerge first. Other tracts of large feathers follow quickly, especially
across the rest of the wings and tail. Within a month the only reminder
of a fluffy childhood is fragments of down still clinging to the head.
Adolescence is over and the bird is a juvenile with a full set of wings.
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 Although the parental
bond will endure for a while longer, in some cases all the way to winter
quarters in the Southern Hemisphere, a juvenile is capable of driving
off other adults that attempt to feed it. It seems odd that a hungry young
tern will only accept fish from its parents and, perhaps even more remarkably,
that it can even recognise them. However, this feat of memory is nothing
compared with what it will do over the coming two years. In this period
the bird, now an immature, does not return to the place it was born. Instead,
with very few exceptions, it will remain south of the equator for its
first birthday and not migrate north again until it is nearly two years
old. At a future date we hope to see it back on Belfast Lough playing
a part in a drama that, in a few short weeks, produces energetic balls
of feathers capable of touring the planet.
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