Accepting cost, the next question to evaluate is how big that cost is, i.e.

The Numbers and Sizes of Offspring

Clutch size varies enormously among species. Assume that evolution should have optimized clutch size. We need to understand how it is regulated.

There are obvious possible conflicts between parent and offspring about number and size. Who controls the reproductive output? How is size balanced against number?

From the parental perspective the optimum is to produce as many as can be provisioned well enough to have a good chance to survive to reproduce. From the offspring perspective a much smaller number might be preferred.

Evidence for a genetic component in clutch size

There is wide evidence that variation in clutch sizes is an evolved genetic adaptation, as opposed to a temporary plastic response to environmental conditions. Much of the data which provides the evidence comes from studies of birds, and key among those data are Lack’s studies of starlings.

First, however, some general patterns:

There are at least 4 bird families whose members characteristically lay clutches of a single egg. All are oceanic (e.g. the albatross, frigate bird and penguins), and all depend on the same food resource (fish from the sea). A few oddballs within their families also lay single egg clutches (e.g. auks, the California condor).

Many terrestrial birds lay clutches of 2 eggs. This is the case with most condors, hummingbirds, pigeons, and most tropical passerines.

Other species mostly have only moderately larger clutches than this.

However, the opposite extreme is also observed. The partridge lays clutches of up to17 eggs, and the blue tit, though it is one of the smallest palearctic (old world arctic) birds, produces clutches of up to 19 eggs.

Are all bird clutch sizes fixed as an evolutionary optimum?

Determinate versus indeterminate clutches

Most bird species have a determinate clutch size. The clutch size is fixed by environmental and genetic conditions, but once that many eggs has been laid, laying ceases. No further eggs will be laid to replace those lost to egg predators or fallen out of the nest.

In other species, with so-called indeterminate clutch size, sensory stimuli determine for the female whether she has filled out her clutch. If an egg is removed from the nest, the female lays another to replace it.

Is clutch size an evolved response to species’ ecology?

Consider closely related species with differing ecology; is there a link between their ecology and their clutch sizes?

MacArthur studied, among many aspects of warbler biology, the clutch sizes of species that feed opportunistically on spruce budworm outbreaks, and those which feed more broadly on insects, but do not take particular advantage of outbreaks.

Both kinds of species are of similar size. The warblers feed in different parts of the foliage of coniferous trees. Among the opportunists are the bay-breasted and Cape May warblers. During years when no outbreak of spruce budworms occurs, these species lay clutches of from 4-6 eggs, with the peak and average of the distribution of clutch sizes = 5 eggs.

Between outbreak years their populations tend to decline in size.

During outbreak years both types of species produce clutches that are at least one egg larger on average, i.e. with peak and average of about 6 eggs. This is the effect of feeding opportunistically on the outbreak.

It is also the means to coexist. During outbreaks the populations of the opportunistic species increase rapidly, due to the temporary advantage they have in the competition with other warblers through their ability to take advantage of the outbreak.

‘The Lack Clutch‘ – Starlings aren’t always pests

Lack believed that birds should produce an optimum clutch whose size is determined by the parent's abilities to feed and fledge young. Selection should maximize the fitness of the species by genetically fixing the number of eggs and young in each clutch.

As brood size increases, parental effort to feed the young increases, but beyond some point (i.e. a brood size), the increment in effort cannot match the increment in brood size. The limitation may be the abundance of food available, or the time required to find and capture sufficient food to support the brood.

Each nestling in a large brood gets less food and is smaller throughout its time in the nest than those in smaller broods. Each nestling that survives (size may make a difference in the nest, but it wasn't important for the starlings), is therefore fledged at a smaller size when from a larger brood.

First, a clear effect on nestling weight:

Clutch Size Nestling Weight (gms) ----------- --------------------- 2 88.0 5 77.6 7 71.4

The period immediately following fledging is probably a period of intense stress, since the bird is thrust into the world from which it must gather sufficient food and avoid predation with no previous experience.

The success of birds fledged from broods of different sizes can only be inferred from recovery of banded birds labeled for the size of the brood in which they grew.

Lack mist netted to capture survivors 3 months after fledging, and separately attempted to find the bodies of those who died during that 3 month period.

By keeping the time interval between fledging and recapture fairly short, dispersal and winter mortality could be minimized and recovery rates maximized.

There are two ways to judge fitness:

1. Look for the brood size that leaves the largest proportion of

offspring alive to be captured in the mist nets. –or-

2. If larger broods leave more offspring (even though a smaller proportion) and there are enough large broods, then there could be a larger number from them surviving to dominate the next generation. We’ll call this “productivity”.

In Lack’s starling data, greatest % recovery and greatest productivity occur in the same clutch size, and it is also the clutch size most frequently observed in natural populations.

Here is Lack’s table…

Brood Size # Broods % recovery relative productivity 1 65 ---- ---- 2 164 1.8 3.7 3 426 2.0 6.1 4 989 2.1 8.3 5 1235 2.1 10.4 6 526 1.7 10.1 7 93 1.5 10.2 8 15 0.8 ----

Relative productivity is just the product of % recovery and egg number. Data from 7 and 8 egg clutches were combined due to the small number of birds recaptured.

Clutches of 4 or 5 eggs show equal proportional recovery, but since 5 egg clutches are larger, a larger fraction of surviving offspring arose from the larger clutches.

Among larger clutches increased clutch size is compensated by mortality during the immediate post-fledging period. Therefore, nothing is gained in fitness, and there is an obvious possibility of mortality cost to the parent. Lack also collected dead bodies two months after fledging. The rate of recovery (number of bodies) increased with clutch size.

Clutch size % recovered dead 1 – 4 0.21 5 0.35 6 – 7 0.43

Finally, note that starlings may produce two broods within a single breeding season. When they do, the same kinds of comparisons among brood sizes obtain, but the whole pattern is moved down 1 egg. Maximum percent recovery, productivity, and frequency in the population of broods occurs at size 4.

Brood Size # Broods % recovery relative productivity 1 44 2 96 2.3 5.8 3 254 4 391 2.2 8.9 5 285 1.8 8.8 6 73 1.4 8.2 7 7 ---- ----

Is clutch size genetically fixed and invariant? In the case of starlings, we already know it isn’t; the clutch size differed between early and late clutches. But…

Should there be clutch size variability?

The basic explanation is just environmental variability. If, on average, the environment provides resources which make the optimum clutch size 5 eggs, is that the case every breeding season? Obviously not. Yet is seems that clutch size is largely genetically programmed. To maintain the potential for optimized breeding in good, bad and average years genetic variation for brood size must be maintained in the population.

In theory, that minimizes the probability of extinction.

Two observations support that notion:

1) most bird species show variation in clutch size

2) many maintain a reservoir of non-breeding, but sexually mature animals. The occurrence of a non-breeding reserve is our first exposure to a reproductive strategy called bet-hedging (introduced in Stearns 1976), which will return in the discussion of life history strategies.

Geographical Variation in Clutch Size

Clutch sizes vary across geographic gradients, both within and among species. That variation is not random.

Clutch sizes inEuropean robin,Turdus turdus

1 - finches2 - tyrannidae or flycatchers3 - Icteridae or blackbirds4 - genus Oxyura in the Anatidae or ducks…5 - Thraupidae or warblers6 - Trogloditidae or wrens

house finch flycatcher

Red-winged blackbird

Oxyura vittata house wren

Lack’s observations led him to suggest that differences in brood size were due to differences in feeding behaviour. The female of most species must leave the nest to forage for her offspring. That is a time consuming process. Food availability or time may be the factor limiting clutch sizes in zones from the tropics to the poles and from mid-continental to oceanic areas.

What supports that view? Compare temperate to tropic latitudes.

Reproduction in the temperate zone is usually closely associated with the summer solstice. Why is the period surrounding the summer solstice special? Day length is the difference. Tropical day length is fairly constant, at around 12 hours. In the temperate, days are longer during the summer period, reaching 18- 20 hours.

Day length at the summer solstice increases with latitude, reaching 24 hours at the arctic circle. Thus, species at higher latitudes have a longer day during which the female can forage, and she can obtain a larger total amount of food, if the resource is sufficiently abundant.

Is food abundant?

The longer days mean that plant growth rates are at their peak near the solstice, and plant growth (and annual cycles) will generally be compressed into the summer period. Flowering is more compressed as latitude increases. Therefore, so is insect activity, which is associated with the plant food resource. A female can raise a larger clutch at higher latitude, all else being equal.

Other explanations:

In house sparrows a maximum proportion of 3 egg clutches survived, but that the greatest number of survivors per brood came from clutches of 4 eggs. Is there a reason for decreasing proportional survivorship with increasing clutch size. Foraging behaviour reduces the success of broods larger than 3 eggs severely. As clutch size increases up to 3 eggs, females increase the number of foraging flights and feeding visits proportionally. There are no further increases when clutch size increases above 3. Thus, 4 egg clutches are provisioned with no more food than those with 3 eggs.

Explanation:

Birds who have nested in areas productive enough to support a 4 egg clutch can't afford to advertise that to others in the colony by making extra foraging flights.

Sometimes it isn’t competition that limits foraging flights. Birds who make large numbers of foraging flights to feed large clutches expose both themselves and their clutches to increased risk of predation. Both they and the positions of their nests are too obviously advertised.

Among these proposed explanations, I’ve gotten in aspects of food availability, competition, and predation. Cody combined all 3 into a single model. Next we’ll look at his model.