Early Birds Catch Worms, Then Lay Eggs
Most birds lay their eggs at sunrise, but NOT robins! They lay their eggs at mid-morning. That's several hours later than most birds lay eggs. For robins, this makes good sense. Robins eat a lot of earthworms during the breeding season, and they use those early dark hours to hunt for worms because worms are most available before the sun gets too high. Robins lay their eggs mid-morning after feasting on worms. A robin can then fly over to her nest and lay her eggs easily, but most other birds seem to need a long period of quiet before they can lay eggs. Those other species can get a big breakfast even if they eat late because they don't want worms anyway!
An Egg a Day is Work
If you think laying an egg is easy, think again! Robins lay only one egg per day for good reasons. Female birds have one working ovary, unlike mammals, which have two. Ovaries are the organs where eggs are produced. A bird's ovary looks like a tiny bunch of different-sized grapes. These "grapes" are the ova, or actually the yolks. The one ovum about to be released looks huge. One or two are about half this size, a few more are a bit smaller, and the rest of the ova are tiny. About once a day, the largest yolk is ovulated. That means it pops off the ovary and starts traveling down a tube to the outside of the robin's body. This tube is called the oviduct.
If a female robin has mated with a male, the yolk will become fertilized. If the robin hasn't mated, the yolk still goes down the oviduct and will be laid like a normal robin egg, but it won't develop into a robin. As the yolk travels through the oviduct, the tube's walls slowly secrete (drip out) watery proteins called albumen to surround the yolk. Near the end of the trip down the tube, the oviduct secretes calcium compounds. The calcium compounds will become the eggshell, but the egg will remain a bit soft until it is laid. You can imagine why the formation of an egg is a tremendous drain on a mother robin's body!
Stopping At Four
Robins usually lay four eggs and then stop. Like most birds, they lay one egg a day until their clutch is complete. If you remove one egg each day, some kinds of birds will keep laying for a long time, as if they can stop laying only when the clutch of eggs feels right underneath them. Robins normally lay four eggs.
On The Nest
Until they've laid a full clutch, robins allow all the eggs to stay cool so the babies don't start to develop. That's pretty smart! It means all the babies hatch close to the same time. Mother robins may start incubating their eggs during the evening after the second egg is laid, or after all the eggs are laid. They sit on the eggs for 12 to 14 days. The female usually does all the incubating. Even in good weather, she rarely leaves her eggs for more than 5 to 10 minutes at a time.
It's mom's job to maintain the proper incubation temperature, keeping the eggs warm during cold weather and shaded during really hot weather. She also must turn or rotate the eggs several times daily. She hops on the rim of the nest and gently rolls the eggs with her bill. Turning the eggs helps keep them all at the same temperature and prevents the babies from sticking to the insides of the eggshells. Males only occasionally sit on the eggs, but they hang out in the territory throughout the daylight hours and respond immediately if the female gives a call of alarm. A male may even bring food to feed his mate, but usually she leaves the nest to feed herself.
Some birds, like hawks and owls, lay their eggs when weather is still very cold, and start to incubate as soon as the first egg is laid. The egg they laid on the first day hatches out a day before the egg they laid on the second day, which hatches a day before the third day's egg. Therefore, the oldest baby may be a lot bigger than the smallest baby. If hunting is very bad and the babies are very hungry, the biggest may sometimes eat the smallest. The oldest baby leaves the nest before the later babies, too.
Sharing Her Body Heat
The eggs must be kept warm to develop. A robin's body is 104 degrees F. or even warmer. Feathers insulate by keeping the bird's body heat inside, and the outer feathers can still feel cool to the touch. That's why female robins need a special way to keep their eggs warm. They have an incubation patch, or brood patch, which is a place on their bellies where their feathers fall out. A mother robin shares her body warmth by parting her outer feathers and then pressing her hot bare tummy against her eggs or her young nestlings. Outer feathers cover the bare area so the brood patch is hidden. (It's a little like keeping the oven door closed so the heat stays inside.) Scientists who hold a female robin for banding will often blow on the tummy feathers to see if a brood patch is hiding underneath.
Many birds apparently sense the egg temperature with receptors in the brood patches. This helps the birds determine how much time to spend on eggs, and they can change their incubation behavior accordingly. For example, they may sit more or less tightly on the eggs, or leave the eggs exposed while going to feed or drink.
The End of the Egg: Hatching Out
Fighting its way out of the egg isn't easy for a chick. First it breaks a hole in the shell with its egg tooth, a hard hook on its beak. Then it must struggle with all its might, between periods of rest, to get out. No wonder hatching may take a whole day. The eggs usually hatch a day apart in the order they were laid. Naked, reddish, wet, and blind, the babies require A LOT of food. Now it becomes a full time job for both parents to protect the nest, find food, and feed the clamoring babies during the 9-16 days they spend in the nest.