A clutch is the total number of eggs laid during a single nesting period, by one female bird. In the early days of ornithology, egg collectors from all over the world gathered large collections of bird eggs. Clutch size is therefore the only core avian life history trait for which quantitative data exist; 

David Lack at Oxford University (UK) began studies on the life histories and clutch sizes of several local bird species. Many of these studies continue today, providing contributions to know about avian life history. Closely related birds manage to have clutches of similar size. 

For example, all tube-nosed seabirds lay a single egg, all hummingbirds lay two eggs, Most songbirds lay between two and six eggs, and some tits in Eurasia lay quite large clutches of up to 17 eggs.

Clutch size

Clutch size and food availability

Food availability may be the most important factor in influencing clutch size. Among non‐passerines, birds that feed their young tend to lay fewer eggs than those young birds forage on their own. 

The availability of food for the laying female bird is often a critical limitation; since laying an egg is energetically draining for the female even in sites with abundant food resources doing so without an adequate food supply threatens the survival of the female or her subsequent reproductive efforts. Thus, even though females are physiologically capable of laying more eggs, they usually limit their egg production (clutch size) when food is scarce.

The amount of food available for chicks also restricts the clutch size of many species. 

When female birds anticipate future food conditions at the time of laying eggs, they may adjust their clutch size accordingly. Arctic nesting raptors and Owls are good examples of this flexibility, as their clutch sizes change dramatically over years with abundant versus scarce prey. For example, female Snowy Owls breeding in the Canadian Arctic lay about 7–11 eggs in rich lemming years and will be decreased by only 3–5 in low lemming years.

Clutch size, geography, and incubation

Both among and within species, 

birds that nest in the tropics lay smaller clutches than those at higher latitudes. For example, House Wrens lay an average of 7 eggs per clutch in Saskatchewan (Canada) but averaging only three and a half eggs in Costa Rica. This established pattern generated several interesting explanations, most of which involve ecological hypotheses addressing seasonality or varying resource availability across latitudes. 

However, other factors also partly explain this pattern. For instance, environmental temperature varies during the laying period and tends to be warmer in the tropics for tropical species than for temperate breeders. In the tropics, the increased ambient temperature at the nest causes embryos to start developing even if the eggs are not being actively incubated. Yet those embryos that begin development due to ambient temperatures will less probably hatch than those incubated by the female at warmer and better-regulated temperatures; thus, tropical birds may lay smaller clutches so they can sooner begin active incubation, rather than wait until many eggs are laid.

Clutch size and nest type

In general, cavity nesters tend to lay larger clutches than birds nesting in the open space. This pattern has been attributed to the lower predation rates experienced by cavity nesters. Nestlings manage to draw attention to them by begging for food, moving around, or simply standing out in their surroundings. Because the smaller number of nestlings in a nest potentially reduces the risk of attracting predators, the more vulnerable open nesters raise few young per clutch. The young of open nesters tend to develop faster, by doing so they reduce the amount of time they are exposed to predation risk in the nest. If parents provide only a particular amount of food at a particular rate, they can either use that food to raise fewer young faster or a large number of young more slowly. It logic behind this is, that cavity nesters tend to have larger clutches with a quite longer development time, and open nesters tend to have smaller clutches with short development times.

Clutch size, female age, and time of the season

Birds produce multiple broods each season, such as Dunnocks in Europe and Eastern Bluebirds in North America, tend to lay small clutches early during the season, their largest clutches seen in mid‐season, and then smaller clutches later in the season. In contrast, with single‐brooded species, clutch size tends to be largest for birds laying during the beginning of the season and drops steadily for birds laying thereafter. 

Exactly what drives this pattern is pretty unclear. In some places, it seems that food supply declines as the season progresses and it’s clear that smaller clutches are a response to scarce food for the laying female or the forthcoming chicks. However, in some cases, a female may shorten egg production in the warmer parts of the breeding season so that she can incubate her eggs by herself directly rather than have them partially develop under environmental temperatures. Another possibility is that earlier birds will have larger clutches because they simply are in better condition or of higher quality.

In many species, young females have smaller clutches than compared to older birds. Also, younger females usually begin nesting later in the season, but when younger and older females lay eggs on the same date, the younger bird often lays a smaller clutch. There are several possible reasons for this pattern.

Individual females gradually improve their reproductive efficiency or may invest more effort in reproduction at later stages in life. Alternatively, individuals themselves do not change over their lifetime: at any given time, the young ones may include many inferior individuals that will die relatively early and thus are selected out, but the older birds in the population are individuals of higher quality that have been more efficient at both surviving and producing offspring all along. 

One such study on Common Terns in northern Germany concluded that almost all perceived reproductive superiority of females as they get old resulted from their steady improvement as they grew older.

Number of broods per season

When comparing a bird’s clutch size in temperate zones versus the tropics, it is essential to note that most tropical species have quite longer breeding seasons than higher latitude species. Tropical land birds often lay eggs over four or more months occasionally all year long, whereas species from temperate zones lay within less than 2 months. Arctic species typically lay within an even shorter time, probably less than a month. Longer breeding seasons at lower latitudes allow birds living there to have several breeding attempts per season than their counterparts living at higher latitudes. The same impact appears within temperate latitudes of the northern hemisphere, with bird species from further north laying fewer clutches per season but with larger clutches, and with overall fecundity varying little with latitude.

Clutch and egg replacement

Most birds tend to replace their clutch if it is destroyed, but many single‐brooded species will not relay if they lost the hatched offspring. To lay a replacement clutch usually requires the female’s hormonal system need to cycle back to laying mode, and the rapid yoking-up phase of preparing for ovulation must be completed and this takes 5–8 days in most passerines and longer in larger birds.

However, if only one egg is removed from the clutch, birds respond in different ways. Determinate layers lay only a fixed number of eggs and will not lay a replacement egg if one is being removed from the nest during the laying period, whereas indeterminate layers will lay replacement eggs until the clutch reaches a particular size. 

Some indeterminate species are capable of prodigious feats:

for example, the frequent removal of one egg every day from the nest of a Northern Flicker induced the bird to lay 71 eggs in 73 days, whereas the regular clutch size of this species will range between six to eight eggs. Domestic chickens, which have been selected over hundreds of years for egg production, can lay up to 352 eggs in 359 days. Chicken hens, like other birds, stop laying eggs if they are allowed to sit for incubation upon a full clutch.

Some species in the laying process act as indeterminate layers only at a certain point. For example, some female gulls will lay replacement eggs only if the eggs are removed before they will sit on two or more. Variation also exists in ways in which birds determine that a “complete” clutch has been laid; 

• some species count the eggs visually, 
• whereas others seem to depend on the feel of the eggs beneath them. 

The tendency of many birds to lay replacement eggs taken from the nest during the laying period may have helped save some species from extinction. Wildlife biologists able to “double clutch” many endangered species by removing some eggs to raise in the laboratory with birds concurrently raise their replacement eggs in the wild. This practice has been an important aspect of captive‐breeding programs for many endangered bird species.

Eggs

All eggs have the primary function of protecting the embryo as it develops. Yet the eggs of various bird species differ vastly in shape and size, structure and shell coloration, and the relative proportions of yolk and albumen. From the smooth white eggs of some domestic chicken to the granulated, avocado‐like capsule eggs of the Emu, bird eggs present a spectacular array of diversity. The study of egg diversity is a specialized branch in ornithology which is termed oölogy.

Egg structure

Bird eggs are large, especially when compared to shell‐less human eggs, with the size of the period at the end of this sentence. The only mammals with eggs similar to birds are the monotremes the four species of echidnas and the Platypus which lay soft, leathery eggs instead of giving birth to young. Embryos within the large external eggs of birds, monotremes, and reptiles must be quite resource-independent during early development stages unlike all other mammals, whose mothers internally supply sustenance to the embryo. Thus, a bird egg must be packed with all of the nutrients like protein, fats, carbohydrates, and water that the embryo will be needing for development until it hatches.

Eggshell color and texture

After the eggs of most species leave the shell gland of the female, pigment glands in the oviduct walls deposit subsequent layers of color on them. The egg’s background color is usually deposited first; any streaks, spots, or other darker markings are added later. The variation in egg pattern and coloration across species is usually associated with the breeding environment. Many birds, especially those that nest in open spaces, lay cryptically colored eggs to avoid detection by visual predators. In North and Central America, for example, the speckled markings and color of Killdeer eggs, together with the wood chips or small stones used to line the nest, act as camouflage and help the nest and its contents blend into the surroundings. 

Colorful eggs do not always act as camouflage, however. Although tinamou eggs are one the most beautiful in the world (they come in many colors and are shiny), those of tinamou species do not blend in well with their surroundings. It’s also been hypothesized that such conspicuous egg coloration gradually heightens vulnerability to brood parasitism or predation to incentivize the male to incubate more attentively. Alternatively, vibrant egg color may have benefits to the developing embryo, possibly providing protection from ultraviolet radiation or antimicrobial defense.

Some birds lay white eggs without any additional markings. These species typically nest in dark holes, begin incubation with the first egg, or cover their eggs when they leave the nest. Because these eggs are hidden from visual predators, they do not need colors for camouflage. Additionally, pigments are metabolically expensive to produce or may need dietary precursors that are hard to obtain. For hole‐nesting birds, the white color also provides an added advantage of helping the eggs be more detectable to the parents in the dark.

Bird eggs vary in texture too. Most eggshells have a matte finish like that of a chicken egg.