There’s a good reason the birds all look the same age

ELISABETH M. BROWN

Among many familiar animals, the young are small copies of the

adults: all furry mammals (humans included), lizards, snakes, frogs

and toads, fish, snails, crabs, lobsters and others. Animals with

hard outer shells, like crabs and lobsters, have to shed their shells

to grow; with each molt the animal is larger.

But some animals have a different system.

Visitors to the open space are familiar with the intensely black

stink beetles, Eleodes, often seen walking park trails. When

challenged, they stand on their heads for a threat display. If you

persist, they can give off a bad odor.

Sometimes there are several in a small area, but they are always

about the same size.

Why don’t we see any small ones? The answer is the beetle you see

has always been this size. When it was younger, it was a grub (a sort

of naked caterpillar), living in the soil and feeding on underground

plant parts. These animals undergo a major body reorganization when

they grow up; they go from grub to adult beetle. Other animals that

do this are moths and butterflies. The caterpillar rolls up into a

pupa and emerges as a full-sized adult.

Among vertebrates (animals with backbones), the rule is that the

young are small. Bear cubs, cougar kittens, speckled baby deer, lambs

and colts cavort with their parents and gradually grow to adult size

and proportions.

But when did you last see a small robin on the lawn with its

mother? A small gull at the beach? All the house sparrows at Cafe

Zinc are the same size.

Birds fly for their living, and there’s the difference.

You can’t arbitrarily change the dimensions on an airplane and

expect it to fly the same way (or at all). So it is for birds. Expert

birders can identify a bird by its characteristic flying pattern. How

it flies is a careful balance between shape and size that varies

between species and types of birds. Every species of bird has arrived

at a particular engineering solution. Change the size and it all

falls apart.

Birds that must fly to feed themselves do not leave the nest until

they are fully grown. Their feathers are often colored differently

from their parents (as in the case of gulls and hawks), but when the

young fledge and leave the nest, they must be adult size and weight.

The exceptions are the ground birds -- quails, partridges,

pheasants, shorebirds, chickens, roadrunners -- and those that can

paddle on the water to their food, like ducks and geese. These birds

can grow up somewhat like young furry animals do. That’s why we see

ducklings or small chickens growing up as miniature copies of their

parents. But they don’t fly until they’re full sized and their adult

flight feathers have grown in.

Finally, what about penguins? Interestingly enough, since they

must “fly” underwater to catch fish, they follow the flying birds’

rule. Their parents feed them until the young are adult size, at

which time they must walk to the coast and jump into the Great

Southern Ocean.

It could be a metaphor for life.

* ELISABETH M. BROWN is a biologist and the president of Laguna

Greenbelt Inc.