Spring and early summer are fantastic times for birds and bird lovers. We get the opportunity to see birds in all their glory, building nests, carving out a territory and tending to their young. Somewhere in all their busyness, the female takes centre stage with egg production.
Bird eggs are virtually self-contained life-support systems. All they require for the embryo to develop properly are warmth and oxygen. Oxygen diffuses into the egg through microscopic holes formed by the imperfect packing of the calcium carbonate crystals that compose the eggshell.
Birds have evolved their eggs into a wide variety of sizes, shapes and colours making the eggshell a truly amazing piece of evolution. It’s a device that keeps the egg from drying out whilst allowing reproduction away from water.
Although most bird eggs are “egg-shaped,” some, such as those of owls, are nearly spherical. Fast-flying, highly streamlined birds such as swifts tend to lay long, elliptical eggs, while those of guillemots and shorebirds are more pointed at the narrow end. Such eggs help adults to efficiently cover them during incubation and they are especially advantageous for birds that nest on bare ground or cliffs because, when disturbed, pointy eggs tend to roll in circles, rather than away from the nest – and possibly over the cliff.
Small birds tend to lay proportionately large eggs. For example, the egg of a wren weighs about 13 percent of the wren’s weight, while an ostrich egg weighs less than 2 percent of an adult’s weight. Egg weights still vary considerably ofcourse – a small hummingbird’s egg could be one hundredth of an ounce whilst the ostrich, a whopping three and a half pounds!
For anyone who has been watching ‘Springwatch’ recently, Chris Packham was totally taken by the splendour and variety of birds eggs when he came across them as a youngster and we tend to agree.
Picture – common tern eggs