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
----------2
5
7
Nestling Weight (gms)
--------------------88.0
77.6
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. –or2. 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
1
65
2
164
3
426
4
989
5
1235
6
526
7
93
8
15
% recovery
---1.8
2.0
2.1
2.1
1.7
1.5
0.8
relative productivity
---3.7
6.1
8.3
10.4
10.1
10.2
----
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
1–4
5
6–7
% recovered dead
0.21
0.35
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
1
44
2
96
3
254
4
391
5
285
6
73
7
7
% recovery
relative productivity
2.3
5.8
2.2
1.8
1.4
----
8.9
8.8
8.2
----
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 bethedging (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 in
European robin,
Turdus turdus
1 - finches
2 - tyrannidae or
flycatchers
3 - Icteridae or
blackbirds
4 - genus Oxyura
in the Anatidae
or ducks…
5 - Thraupidae or
warblers
6 - 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.