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Fish Lake morning

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photo: Robin Song

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Natural Observations-About birds’ eggs

by KTNA Staff ~ August 9th, 2011

Earth and Beyond host Robin Song had some questions about birds’ eggs, and shares what she learned.  Audio runs 7 min 18 sec. Text is below.

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Eggs, by Robin Song                   Aug.4, 2011

In mid-July I paddled Lyra across one of the Number Lakes to hike through the forest on the north side and go have a look at the other lakes. On the first lake was the pair of Pacific loons from last summer. This year they did not have a chick. On the northeastern lake I watched Trumpeter swans through my binoculars. On the lake to the northwest a single common loon floated on the still water, its head tucked into back feathers as it took a nap. A mama teal with four ducklings swam along the channel between the two lakes, and a beaver swam past her.

Picking my way through the forest to have a look at the farthest west lake, I noticed Lyra nosing something in the lichens. I went over to her and was surprised to see a loon eggshell. I couldn’t tell if the egg was from the Pacific or the common loons. And its story was not clear, either. Had a predator snatched the egg from the nest and carried it away into the forest, or had the chick hatched and possibly a raven had carried the empty shell away? Reading about loons, I learned that they don’t take a mate until their fourth or fifth year, and then usually the first two nests aren’t successful. Predators take either the eggs or the chicks. It takes the pair two or three tries to finally get a pair of chicks past the critical stage of vulnerability from predators. If the chicks are lost, the adults spend the rest of their summer cruising on the lake, keeping each other company.

In late July I paddled across another lake to search for blueberries. After securing the canoe, I followed Lyra into the forest. We came out on the shore of another lake and the pair of common loons floating there called out in alarm. Seeing that we were not a threat, the loons stopped calling, turned in the water and swam across the lake to have a look at us. They seemed particularly interested in Lyra, who was busy lapping water and sniffing the marsh grass. Just a few yards out, the loons swam back and forth, watching us intently. The evening sun shone on their black and white feathers, their necks flashing iridescent as they turned. They lost interest in us after a few minutes, and were soon paddling farther out into the lake, there to dive for food and preen. They, too, were without young, and were spending the summer together without the stress of raising offspring.

While walking along a moose trail, I was looking for blueberry bushes and suddenly spotted something in amongst the lichens. It was part of an egg. Crème-colored and unmarked, it was fairly good-sized. Too small for a duck, too large for a songbird, I pondered what it could be as I carefully tucked it into my camera bag. Back at the cabin, I searched through my bird books. On that lake there were nesting Bonapart’s gulls and Arctic terns. When I looked up those birds, I learned that both species have eggs light brown in color, with heavily mottled patterns in brown or black.

I read that birds that have nests on the ground or on the crown of trees (such as Bonapart’s gulls) produce eggs with camouflage coloring. Birds that nest in cavities, such as chickadees, nuthatches and woodpeckers, have solid-colored eggs, usually crème, white or buff. From the size of this eggshell, I guessed that it could be a Hairy woodpecker’s. Most birds will carry the egg shells away from their nests after the chicks hatch, dropping it somewhere away from their nest tree so predators won’t be attracted to the nest by the smell of the decomposing egg shell.

Of course, the coloring is not a hard and fast rule. One thing about Nature, she can be inconsistent. While paddling other lakes in past summers, I have discovered red-necked grebe nests and found their eggs to be solid white. Why would their eggs, laid on a nest mound of mud, grass and branches, within the sparse shielding of marsh grass, not be mottled and camouflaged? When I discovered my first junco nest, back in 2004, I was amazed to see that the eggs were pale blue, with magenta-colored spots. Why weren’t the eggs green or brown, to blend in better with the nest made of grass and moose hair, tucked in the lichens and moss on the forest floor? Many duck and goose species lay pure white or crème-colored eggs, as do some grouse. Why don’t they have camouflage patterns?

I found answers in my Audubon Nature Yearbook, circa 1987. One of the articles therein is about the color of eggs and is written by Bernd Heinrich, author of one of my favorite books; The Mind of the Raven. Bernd has studied nests and eggs since he found his first one at the tender age of 10. In this article, aptly entitled “Why Is A Robin’s Egg Blue?” he elaborates on how eggs get their colors and the speculations from Naturalists including Charles Darwin and Herbert Friedmann. Many people have conducted experiments, down through the years, substituting different colored eggs in nests of various species to see how the birds react. Egg colors and patterns are under genetic control. Evolution has produced the fine-tuning. The article is long and informative, and I learned a lot. In a future program I shall go into more detail about eggs. Bernd concludes that birds, like robins, who lay small clutches and sit on them to begin incubation right away, don’t leave their nests as they have a mate to bring them food. There is a bird on the nest to cover the eggs, therefore camouflage is not needed. Ducks lay large clutches and do not begin incubating until the last egg is laid. Most duck eggs are solid white to brown, without markings. But when a duck leaves her nest, she pulls grass and down over the eggs, covering them well. Again, camouflage coloring is not needed.

There are parasitic birds that lay their eggs in other birds’ nests, leaving that bird to raise their offspring along with their own. Some parasitic hatchlings will even kill the other chicks as they hatch. The parasitic eggs closely match the host’s eggs in size, color and markings. The host bird will not reject the parasitic egg. But birds with solid-colored eggs, especially the immaculate blue eggs of robins and cowbirds, will reject a parasitic egg, as it contrasts distinctly from theirs. Experiments also proved that birds learn the color, size and shape of their own eggs by imprinting on the first one they lay.

There is still not complete agreement on just why there is so much variation in color and patterns amongst bird’s eggs. Perhaps we will never know, for sure. For example- the eggs of most hawks are radically different within the same nest, with some eggs darkly mottled and others quite pale. The reason for this is still unknown.

One could spend many years studying the eggs and nests of the vast array of bird species. A pleasant occupation, I would think. For me, the mystique of egg coloring adds to the overall beauty of the avian world, and is yet another reason why I love them so.

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