The last word–

QUACKERSI was watching a duck and her eight chicks walking in a line across the grass. All of a sudden a couple of other chicks entered the group. The mother duck immediately weeded out the stranger chicks and sent them on their way. To us they looked identical, so just how did the mother duck achieve her feat? Is it just that animals are exquisitely sensitive to visual differences between members of their own species? Or was the mother duck relying on non-visual information as well, and if so, what?

● Chasing away non-descendant

young is called “kin discrimination”

and is often considered less efficient

in birds than in other animals.

However, eider ducks have been

reported to discriminate against ducks

that are not part of their family unit.

Coots have also been seen to do the

same thing, but neither species seems

to use appearance as the way to

recognise their young.

Many birds use acoustic

recognition and can identify each

other’s voices. Swallows, finches,

budgies, gulls, flamingos, terns,

penguins and other birds that live in

larger flocks do this. Odours can also

play a role in determining how some

birds recognise each other.

In ducks, sound seems to be the

principal method of recognition: they

have been fooled into returning to

the wrong nest, only to be greeted

by a portable cassette player rather

than their ducklings.

The ability to recognise their own

young saves colony-living birds from

expending energy raising someone

else’s offspring. It also stops ducklings

running the risk of aggression from

adults if they beg food from the wrong

ones. Natural selection favours

individuals who know who they

are talking to.

Waterfowl have long been thought

to be unable to keep track of their

own young. They have been seen to

lose their own ducklings to another

parent, or to mistakenly accept and

care for non-descendant ducklings.

This has been put down to the fact

that birds do not generally have a

central family unit.

Ducks do behave in a different way

towards their own ducklings, though.

Parents sometimes favour their own

offspring over non-descendant

young, as with the duck in this

question, or they may tolerate or

encourage the ducklings to mix.

Consequently, some provide what

is called alloparental care, a form of

adoption. This is seen when a duck

is able to increase the chances of

survival of her own offspring by

accepting non-descendant ducklings

into her entourage. Her own ducklings

might be better off because the risk

of any individual being eaten by a

predator is lower if it is part of a bigger

group. To improve the advantage even

more, the non-descendant ducklings

may be positioned at the edge of the

brood, further away from parents. This

has been seen in Canada geese;

the adopted goslings were noted to

generally potter further away from

their adoptive parents than the

biological offspring, and therefore

not as many survived.

Jo Burgess

Department of Biological Sciences

Rhodes University

Grahamstown, South Africa

FOOLED IN BLACKPOOLFrom the top of Blackpool Tower (approximately 150 metres) on the UK’s west coast, can you see the curvature of Earth along the Irish Sea horizon? I thought I could, but my friend disagreed. If I’m wrong, how high would we have needed to be?(Continued)

● Seeing the curvature of the Earth

can mean either seeing the surface

of the Earth in front of you fall away

towards the horizon, in the same

way that you see the ground fall away

when standing on a rounded mountain

top, or seeing the horizon as a curved

rather than horizontal line.

It is actually possible to see the

curvature of the Earth, in the second

sense outlined above, at any height:

for example, sitting on the beach,

standing on the deck of a ship or

looking out of a plane window. This

is to be expected, because a view

from any point on a sphere such as

the Earth will give the horizon as a

disc. The height of the viewpoint

will simply determine its size.

The visual cues employed to see

the curvature of the Earth are many,

but judging the line of the horizon

relative to the horizontal is generally

not one of them. Instead, two more

obvious cues are noting that the

distance of the horizon is the same

in any direction, and seeing that the

texture gradient – the way a view

changes in appearance and perspective

with distance – of the sea or land is

constant within that distance.

I agree that increased viewpoint

height will yield a richer set of cues,

especially those associated with

seeing the horizon in the second

sense, delivering a more obvious

curvature. Nonetheless, this curvature

can still be noted at sea level.

John Campion

Psychologist and vision scientist

Liphook, Hampshire, UK

THIS WEEK’S QUESTIONCavorting cavitiesI have often noticed that my fillings

feel a bit strange when I’m bouncing

on a trampoline. Several other people

I know also report this. Oddly, it

doesn’t happen on the landing, when

I am experiencing greatest deceleration,

but on the apex of a bounce, at

the point when I am momentarily

weightless or just beginning to fall.

What causes this? Does anybody

know if jumping astronauts notice

any filling-based irritation while

horsing around in zero gravity?

Jasper Fforde

Hay-on-Wye, Powys, UK

“Waterfowl have often been seen to lose their own ducklings to another parent, and to mistakenly accept and care for non-descendant ducklings”

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“It is possible to see the curve of the Earth at any height, even from a beach. This is because a view from any point on a sphere gives the horizon as a disc”