The general answer is as far as studies have
shown the choice is not usually made by
the parent. The adult bird puts food in
the biggest and most available mouth or the chick that is the most
demanding. In passerines incubation of eggs
usually starts when the clutch is complete, so all the chicks hatch at about the
same time, so in general there is not a big difference between them. So each has
a sort of equal chance and when one has had enough it will stop begging and
another one will have its share.
In things like raptors, owls, kookaburras, etc, incubation of eggs
usually starts when the first egg is laid, so the chicks all hatch at different
times and there may be direct competition between the chicks. The parents
display no interest in ensuring that each gets adequate food. They feed the
chick that is well enough to beg and it is not that odd for the
younger chicks to be persecuted or eaten by the older ones, even while the
parent watches. That is why survival rates in the biggest or oldest chicks is
typically highest. If there is plenty of food available, then the bigger ones
will have had enough often enough to not be the first to be fed and thus the
others then get a go.
I have been watching a noisy friarbirds nest on Farrer Ridge
since 2 November. On 24th, the behaviour of the parents
changed, with the female leaving the nest more often, and being quite restless
while sitting on the nest. On one occasion I shot one of the birds
bringing an insect to the nest, and guessed they chicks must have
hatched.
Today was the first time the chicks were visible above the
edge of the nest. I saw two faecal sacs carried away in quick succession,
but when I got home and had a closer look there were three chicks, and all
looking quite robust. I wondered how hard these birds would have to work
to feed these three fast growing kids. As I was leaving, one parent
returned to the nest from at least 50 metres away, perhaps they have already
cleaned out the ground in the immediate vicinity of their tree.
How does a parent decide which of the three gaping mouths to
put the insect in? Do they know which is which in order that each gets
adequate food?
At no stage since I began watching these birds have they
uttered a single sound. No doubt to protect the nest from
detection.
Margaret Leggoe
