Living with Birds "Why do birds nest?" by Nick Baker Tweetapedia

Great question – and it’s all tied into their reptilian ancestry.

When they conquered the land some 320 million years ago, the evolutionary secret of reptiles’ scaly success was developing a hard, moisture-retaining coating for their eggs, unlike the watery jelly (think frogspawn) that had kept amphibians stuck in or around water up to that point.

After the ‘invention’ of the egg I guess they never really looked back. It meant the predecessors of modern birds were free to crack on with other important developments, like wings and learning to fly.

Imagine the issues if you had to carry around a full clutch of developing youngsters in your belly. You’d be heavy and literally earthbound. By depositing their eggs somewhere safe, modern birds can keep things light and stay airborne. The definition of what is ‘safe’ is quite a flexible one and depends on a large range of factors: some nests are incredibly intricate and complex while others seem, to us anyway, little more than a dint or scrape in the ground. 

One clever way to ‘do a nest’ is a technique possibly used by the long extinct reptile ancestors of birds. That is to pile up a huge heap of rotting vegetation. In a similar way to a compost heap, the bacterial action of decomposition gives off heat. With a little management by adding and subtracting insulation, the eggs are automatically and gently warmed. This saves the ‘sitting about waiting’ on a clutch of eggs. Crocodiles and Alligators use this technique as well as an odd family of birds called the Megapodes. The downside is that the parents still need to attend the nest to protect it and stop things getting too hot since high temperatures are more lethal to developing embryos than lower ones. Nest technology has moved on a long way since then.

In addition to allowing birds to remain lightweight and airborne, there are other advantages to laying eggs and depositing them as a clutch in a single place.

An egg a day

If you’ve ever kept chickens, you'll know about this. A bird produces a single egg a day because birds, unlike mammals, only have one ovary. This is another possible adaptation to a life in the air; it saves weight and means a bird’s ovary and egg-making process works like a production line. A bird will keep producing until a clutch is complete. 

Sometimes the female will sit and begin incubation immediately after the first egg is laid. Development starts on day one and therefore any egg that follows in the clutch will be a day later in its development, resulting in a nest full of chicks of varying stages in development. This is known as asynchronous incubation and is seen in birds of prey. It can be likened to ‘an heir and a spare’. In a good season with plentiful prey, more chicks survive. In a poor season, anything but the eldest is an insurance policy. It’s an adaptation to less than reliable food sources.

On the other hand, most songbirds feed their nestlings on relatively easy-to-obtain invertebrates and seeds, so they don’t start incubation until the clutch is complete. This means they all hatch about the same time and is known as synchronous incubation.

All eggs in one basket

Putting your eggs in one nest allows a few more survival tricks to ensure the best chance of sending your genes to the next generation.

Despite having an egg production of one at a time, large clutches can be produced and raised in a single nest. This enables the freedom of a single bird being able to produce many more offspring in a season than would be possible if she had to nurture them internally. Some females can even start a second nest and start laying before the first clutch is totally independent; I’ve seen Blackbirds in my garden deploy this very strategy. 

It is all about turning out as many fledglings as possible in a season. The extra advantage is one of insurance. If disaster occurs, the female can flee the nest; while this is a waste of all the effort up until this point in the chicks development, she hasn’t lost everything and lives to nest another day.

The nesting strategy also allows for even more genetic diversity because if you’re a female you can ‘shop around a bit’, have a fling or two (or more if you’re a Dunnock) with other males and therefore produce a large range of genetic variation. With a single clutch having multiple fathers, it means a single clutch has a higher genetic diversity.

Nest construction

A nest’s most basic purpose is to provide a receptacle for the egg or eggs, keeping them in a known spot where they are safe and can be warmed and incubated by one or more parent birds.

Nests are as variable in size, shape, and construction as the birds themselves, and the lives of each pair of breeding birds revolve around this structure to a greater or lesser extent depending on a variety of factors. 

Birds with larger eggs, smaller clutches or chicks that hatch in an advanced state (fully feathered with eyes open) tend to have more basic nest structures. These advanced chicks are known as nidifugous (from Latin: nidus means ‘nest’ and fugere ‘to flee’) and because their time in the nest is short, not so much effort and investment goes into the nest’s manufacture. Exposed eggs such as these tend to be more elaborately marked and coloured for camouflage. Think of the nest scrape and eggs of a plover. 

Birds with a smaller body size tend to produce larger clutches of smaller eggs. Because they are so small, they are less able to package their embryos with the larger quantity of resources that are needed to produce nidifugous hatchings. On hatching, the chicks of these smaller birds are naked and blind and totally dependent on their parents for not only food but for insulation and warmth. This type of chick is called nidicolous. (From Latin: nidi for ‘nest’ and colus for ‘inhabit’) and they are little more than embryonic, pink and (I have to say) quite ugly.

Such needy nidicolous hatchlings are half-cooked birds. They require not only longer in the nest, and are therefore more vulnerable to predation, but the nest needs to be almost an extension of the parent bird, keeping them warm and dry. This is why these smaller birds tend to invest more effort and time into creating complex nest structures that are safe, camouflaged and well insulated.

Way more than 1,000 trips with building materials for most small bird nests have been recorded. Think of the incredible stretchy structure that is the nest of Long-tailed tits, for example. It is engineered to blend in and also be a warm, feather-lined bag that expands as the clutch itself grows, and one that famously has been found to contain over 1,500 individual feathers! 

It has been suggested that the building of nests such as these has enabled the evolution of eggs to be smaller and therefore allow birds to become smaller and able to exploit smaller prey items. What came first? The Goldcrest or the egg? Nests are one of the reasons that there is such a diversity of birds and why they are so successful.