Would you raise the bird that murdered your children? - Steve Rothstein
This is a little bee-eater’s nest.
But this is not a little bee-eater chick.
It’s a newly hatched honeyguide—
and it’s much more lethal.
When its mother placed it here, she
punctured all the other eggs in the nest.
However, one little bee-eater chick
survived the attack and is now hatching.
Although the honeyguide nestling
is still completely blind,
it instinctively stabs
the little bee-eater chick
with its sharp, hooked beak.
And over the following weeks,
the host parents devotedly care
for the hatchling
that murdered their offspring.
This is but one example
of brood parasitism,
an evolutionary strategy in which one
animal tricks another
into rearing its young.
It’s especially well-known among birds.
By depositing their eggs
into a stranger’s nest,
brood parasites are able to shift
the major costs of parenting onto others.
Brood parasite chicks usually
hatch early,
then monopolize their host parents’
attention.
Some stab their fellow nestlings to death,
while others shove the remaining
occupants out of the nest.
Meanwhile, others are less
harmful to their hosts.
Not all brood parasites kill
all of their host’s offspring outright.
Brown-headed Cowbirds
usually outcompete them
by begging for food louder,
more frequently, and with a wider mouth.
Among the most benign,
black-headed ducks lay their eggs
in other nests to be incubated.
However, a few hours after hatching,
they simply saunter off.
But in the most egregious cases
of brood parasitism,
why don’t host parents take a stand?
In fact, hosts will often drive adult
brood parasites away from their nests,
and many take their defenses further.
But whether a hosts can recognize
and reject parasitic eggs and nestlings
seems to depend on a few factors.
Eastern phoebes will accept
a Brown-headed Cowbird’s
speckled egg into their nest,
though theirs are pure white.
The gray catbird, on the other hand,
is an expert at rooting out
the very same parasites.
It memorizes what the first egg
in its nest looks like,
which is usually its own,
and tosses any aberrations.
This retaliatory adaptation can fuel
an evolutionary arms race
where brood parasites evolve eggs
that closely mimic their host’s.
Interestingly, birds that do reject
parasitic eggs
are usually clueless when it comes
to parasitic chicks.
Reed warblers are good
at ejecting poorly matching cuckoo eggs.
But if one hatches in their nest,
they’ll care for it even after
it’s grown six times their size.
Though chick rejection is
a rarer phenomenon,
there are some noteworthy examples.
While incubating its eggs, the Australian
superb fairy-wren sings to them,
imparting a unique note that its chicks
use as a kind of password.
When a cuckoo is in the wren’s nest, it
hatches first and pushes the others out.
But, perhaps because it hatched sooner,
the cuckoo chick wasn't able
to learn the password,
and so it doesn’t croon
the right begging call.
At this point, the adults usually abandon
their nest and start another.
Altogether, host species show
a remarkable variety of responses.
This seems to partially be
a result of how long
brood parasitism has been
in their environment,
and thus how much time they've had
to evolve suitable adaptations.
In fact, studies have shown that those
hosts that reject parasitic eggs
less frequently can visually distinguish
between their eggs and a brood parasite’s.
They simply lack a response
to the visual information.
This is probably because,
before brood parasitism appeared,
responding would have likely
had no adaptive value.
And even when hosts do
recognise a parasite,
getting rid of it may not
be the best option.
The host, especially if it’s small,
might not be able to kill the parasite—
or could risk breaking
its own eggs in the process.
Unless the brood parasite
kills all of the host’s young,
it may be best to simply
foster the imposter.
Brood parasitism tends to evoke
horror and disdain.
But why should it be thought of as any
more objectionable
than predator-prey relationships?
And is it ever productive to impose
human morals onto other animals?
Or does it end up saying more about us
than it does them?
Whichever way you swing it,
brood parasitism is yet another example of
the fascinating turns evolution has taken.
