How Many Eggs Does a Blue Tit Lay? Big Broods Explained

How many eggs are in a typical blue tit clutch?

The number depends on where and when the bird nests. In rich oak woodland in Britain and central Europe, clutches of 8–12 are common. In gardens, towns and conifer plantations, where there are fewer caterpillars, clutches are usually smaller, often 6–8. Birds in Mediterranean evergreen forests also tend to lay fewer eggs.

Clutch size also falls through the season. Females that start early in spring usually lay more eggs than those that start late, and older, well-fed females often lay more than first-time breeders.

Very large “clutches” of 15 or more eggs sometimes turn out to be the work of two females laying in the same box. Clutches of 14 laid by a single female have been confirmed in the UK.

How do blue tits lay so many eggs?

The female lays one egg a day, usually early in the morning, so a clutch of ten takes ten days to complete. Each egg weighs about a tenth of her body weight, and the total clutch can reach about one and a half times her weight. To manage this, she needs a lot of food in the days before and during laying.

The male helps. Throughout nest building and laying, he brings her food, a habit known as courtship feeding, which can supply a large share of her daily diet. Females with attentive, well-fed partners in good habitat can afford bigger clutches.

The nest itself is a cup of moss lined with hair, wool and feathers, built inside a tree hole, wall cavity or nest box. During laying, the female often covers the eggs with nest lining when she leaves.

Why do blue tits lay so many eggs?

Large clutches make sense for a small bird with a short life. Most blue tits live only a few years, and many young die in their first winter; only about a third of juveniles in Britain survive their first year. A pair may get only one or two chances to breed, so they put a lot into each attempt.

Nesting in holes helps too. Hole-nesting birds lose fewer eggs to predators than birds with open cup nests, so they can safely raise larger families. In the 1940s the ornithologist David Lack proposed that birds lay the number of eggs that gives them the most surviving young, and long-term studies of tits in places such as Wytham Woods near Oxford have tested and refined this idea ever since.

Timed to the caterpillar peak

The secret of the big blue tit brood is caterpillars. Chicks are fed mainly on small green caterpillars, especially those of moths such as the winter moth that feed on fresh oak leaves. These caterpillars are hugely abundant for only two or three weeks in spring.

Blue tits time their laying so that the chicks need the most food just as caterpillars peak. The female usually begins incubating only when the clutch is nearly complete, so most eggs hatch within a day of each other. Both parents then feed the brood, making hundreds of visits to the nest each day at the height of the season. A single brood can eat many thousands of caterpillars before it leaves the nest.

When the timing goes wrong, the results can be serious. Cold, wet weather in May or June reduces caterpillar numbers and makes hunting hard, and in bad years many chicks starve.

Warming springs and the risk of mismatch

As springs in Europe have warmed, oak leaves and caterpillars appear earlier than they used to. Blue tits have responded by laying earlier, on average, than in the 1970s and 1980s. Scientists study whether the birds can keep pace, because a gap between hatching and the caterpillar peak, called a phenological mismatch, means less food for chicks.

So far the species is doing well. Blue tit numbers in Europe rose by about 30 percent between 1980 and 2016, and the IUCN lists the Eurasian blue tit as Least Concern. Warm springs may even help some pairs try a second brood.

From egg to fledgling

Newly hatched chicks are naked, blind and helpless. Their eyes open after about a week, and feathers grow quickly. After about 18–21 days the young leave the nest, often all on the same morning. They look like dull versions of the adults, with yellowish cheeks.

The fledglings follow their parents for one to two weeks, begging noisily for food, before becoming independent. This is a dangerous stage: young tits are taken by sparrowhawks, jays, cats and other predators. Of a brood of ten, only one or two may survive to breed the following year, which is exactly why the parents lay so many eggs.

How blue tits compare with other birds

Most small songbirds, such as robins and thrushes, lay about 4–6 eggs. Among common garden birds, only the closely related great tit comes close to the blue tit, usually laying 7–9 eggs. Larger clutches are laid mainly by ground-nesting game birds such as the gray partridge, which often lays 15 or more eggs, but its chicks can walk and feed themselves soon after hatching.

That makes the blue tit’s achievement stand out: its young are helpless and must be fed by hand in the nest, yet it still raises families of ten or more. Its ability to learn new tricks is just as impressive, as the story of why blue tits opened milk bottles shows. Blue tits are among the most familiar nest-box birds in our guide to garden birds in the UK.