Eggs - care required when handling! - December 30th

As it is a wet day here on Rangatira, one of my jobs this morning was to sort out the eggs that we have collected and pack them safely away.

Black robin eggs are small and cream coloured with light brown speckles on them. The female will lay up to three eggs at once and each one will weigh about 3 grams. The females do gain weight before they lay the eggs, but with an average female weighing only about 20 grams, the eggs are a huge percentage of her body weight!

What a collection of eggs!



The egg shell has a thin membrane between it and the chick developing inside, which you have probably seen if you’ve ever peeled a hard boiled hen’s egg. The egg shell and membrane are permeable, so they let gasses and water in and out of the egg through tiny pores in the eggshell.

The developing embryo in the egg gets its food from the egg white, which is full of protein and liquid and the egg yolk which is full of nourishing fats. By the end of incubation the egg will have lost about 10-15% of its weight due to loosing water across the shell. After developing in the egg for about 18 days the embryo will start pipping. First it pips internally, which means that it breaks through the membrane between it and the shell. The parents can tell when this is happening and stay closely on the eggs during this time. Once the embryo has made its way though the membrane, it uses its egg tooth, a small tooth like projection on its beak, to break open the shell. This is a huge effort for such a tiny creature and the chicks hatch out quite exhausted. They do still have a bit of the yolk left inside their bodies in an internal yolk sack and this gives them food and energy for a couple of days after hatching.

The black robin chicks are then nestlings for approximately another 21 days before fledging. Sometimes the female will have one nestling and still be sitting on an egg that failed to hatch. When this happens we remove the dud egg from the nest. We very carefully break the egg and write down why it didn’t survive. We also dry the shells so that Melanie can take them back to the lab. She then puts the shells under a scanning electron microscope and looks at the location and numbers of pores that are on the egg shells.

A nicely dried egg

Dissecting eggs that have been incubated for over 20 days, can be really smelly work and we make sure to do it outside not to cause too much of a stink in the hut. One egg that we dissected last week was so rotten that it exploded as soon as Melanie broke it, and the smell was spectacularly bad! Thankfully the one I did today just looked like it had been scrambled inside the shell, and only smelt a little bit. So I carefully washed it out and put the eggshells out to dry, then went inside to a hot cup of tea and some wet weather board games.