Maintaining our spring theme, this week we will continue our look at bird nests, eggs and incubation (see the last Nature Smart column for part one of this series). This time we are going to look at clutches. No, not the thing that helps your car’s transmission change gears. A clutch is defined as the number of eggs produced or incubated at one time by a bird.

In general, some species of birds will lay only one egg such as the California Condor, while many birds of prey such as the Bald Eagle will usually lay two eggs. Many song birds such as cardinals and jays will have 4 to 6 eggs while some large birds such as ducks and turkeys will lay as many as 15. The species that get the award for laying the most eggs in one season is the Brown-headed Cowbird. The adult female will produce between 30 and 40 eggs before the season is through. What is even more interesting is, she doesn’t build her own nest to deposit all of these eggs, rather she searches out other bird’s nests and lays an egg in the “host” bird’s nest.

Many factors dictate the number of eggs a bird will lay. It’s a combination of factors that determine the number of eggs an individual bird will lay. How long a species lives is one determination. The shorter the birds life, such as a goldfinch or chickadee which on average live about 5 years, the more eggs the female will lay. In addition these birds will often have two or even three clutches of eggs per season. Eastern Bluebirds often have two and some even have three clutches per season. The longer a species lives such as the California Condor which lives to 70 or 80 years lays only one egg every other year. The Common Loon almost always lays two eggs. These birds can live to about 30 years.

Food is another clutch size factor. For instance, birds that eat and feed its chicks easily obtained food such as insects will lay more eggs then a bird that specialized in food that it requires skilled hunting. Hawks and owls are good examples of this. A hawk or owl may only catch one mouse or other prey item for every 5 or 6 attempts. The harder it is to catch the food the fewer the chicks.

Birds that nest closer to the poles (higher latitudes) have larger clutch sizes due to the high concentrations of insects. If you have ever been to the arctic or even northern Canada or Alaska in summer you know what I mean.

Clutch sizes also increase with the type of nest. Cavity nesters such as woodpeckers have increased clutch sizes as compared to open nesters. This speaks to the fact that nest cavities are safer places to incubate and the mother can “invest” all of the seasons’ eggs at one time. Open nesting birds often nest several times a season just in case a predator gets their eggs.

Most birds will replace their clutch if it is destroyed by a predator or severe weather. However many single-brood species will not re-lay if their clutch of eggs is lost. In this instance if the clutch is lost early in the nesting season they may be able to replace the eggs providing the female’s hormonal system to recycle into laying mode isn’t shut off for the season. Birds such as Common Loons will not relay eggs if the first clutch is lost while other birds such as the Northern Cardinal will automatically start a new clutch of eggs.

Some birds will respond to the removal of an egg. Birds classified as determinate layers will not lay another egg if one is removed whereas indeterminate layers will lay a replacement egg in the absence of one.

Younger females tend to lay fewer eggs than older females. Why this is remains a mystery to ornithologists. One idea is, younger birds nest later in the season when clutch sizes tend to decline even in older females. The second clutch of the season (later in the season) is usually smaller then the first based on a strategic adjustment by the laying female to the prospects of finding enough food to feed the growing chicks later in season when fewer food resources are available.

As the nesting season progresses we will look at more aspects of this interesting part of a bird’s life. Until next time…