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Matt Johnson
HSU Wildlife 365
Ornithology
ORNITHOLOGY
(Humboldt State Univ. WILDLIFE 365)
LECTURE 23 – CLUTCH SIZE
I.
Clutch size.
A. Eggs encapsulate a pair’s genes to be potentially passed on to
the next generation – they are the currency of life, and their
number and success greatly influence a bird’s fitness.
B. Yet considerable variation exists: Procellariiformes: 1 egg,
Galliformes: up to 23 eggs; Cowbirds: an egg a day for a
month. Many altricial songbirds lay 2-12 eggs, many
precocial birds lay up to 20 eggs.
C. Also variation within a species: Great Tit: 2-14, Snow Goose
3-6; Northern Flicker 3-12. But some species are more fixed:
precocial shorebirds lay 4, oceanic Pelecaniformes always
lay 1, hummingbirds and doves always lay 2.
D. The most intriguing variation of all, however, is the relatively
consistent trends in clutch size with respect to several
environmental variables…both among and within species!
See Table 20-5.
E. A question that has interested ecologists for over 50 years is
the evolutionary significance of this variation. Why does it
exist, and what is the “optimal” clutch size?
F. The first question to consider is that which sets the upper
limit on the number of eggs and young a bird can produce.
1. In general, altricial species are limited by the feeding
abilities (meaning food supply, parental quality, and costs
of time spent off nests) of the parents to bring food to
those helpless young.
2. In precocial birds, the upper limit on clutch size is set by
the nutritional requirements for egg (especially yolk)
formation.
3. But while these factors set the upper limit, they don’t
explain the variation in clutch size.
G. Several competing hypotheses exist:
1. “Belly limitation.” They lay what they can effectively
cover.
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a. This can be rejected for most altricial birds by
experimentally increasing clutch size – they can
incubate more than what they normally sit on.
b. Could be important in some precocial shorebirds.
2. “Food limitation.” Birds lay as many eggs as they can
get food to produce. Indeterminate layers clearly reject
this hypothesis.
3. “Population control.” Clutch size is gauged to mortality
rates so that a population does not overpopulate itself.
a. This explanation relies on “group selection.” It is
clear that natural selection operates on the individual
– to maximize the fitness of individuals.
b. Thus, this hypothesis does not hold because this
system is open to “cheaters.” Any bird that cheated
and had a higher clutch size than her neighbors would
pass more genes on to the next generation. And then
her offspring would do the same, and so on, until the
group was no longer limiting itself, but was instead
comprised of females all independently maximizing
their own reproductive output. Thus, all hypotheses
that rely on group selection are not “Evolutionarily
Stable Strategies”, and they are unlikely to be correct.
4. “Lack’s Hypothesis.” Clutch size is that which produces
the most surviving offspring. (there are numerous ways of
saying the same thing).
a. OVERHEAD/BOARD graphs:
 # young hatching increases with clutch size
 but wt of young (and hence probability of young
survival) decreases with increasing clutch size (cuz
less food brought per chick)
 So, # young surviving (= # hatched * survival) is
highest at intermediate clutch sizes.
b. let’s review scientific process:
 The phenomenon is that clutch size varies (Table 205).
 A potential explanation is Lack’s Hypothesis for
Clutch Size.
 IF, Lack’s H. is true, THEN we’d predict that if we
experimentally increase and decrease birds’ clutches
(experimental treatment), they wouldn’t produce as
many young as if we left them along (control
treatment). [this is a prediction of Lack’s H]
c. At first (early 70’s), this was the most widely
accepted hypothesis.
d. Experimental evidence provides some support.
Adding eggs to clutches increases the number of
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young hatched, but their average survival is reduced
compared to control clutches. See Table 20-6 for
magpies and Figure 20-7 for Great Tits.
H. However, clutch size is often a bit lower than Lack’s optimal
clutch size would predict. See Figure 20-7. Why? There are
three hypothesis for the fact that Lack Hypothesis predicts
clutch sizes a bit too large.
1. Lifetime vs. annual reproductive success – the “trade-off
hypothesis”.
a. A theoretical expectation of trade-offs between
(adult) survivorship and reproduction. The more a
pair invests in a single reproductive event (i.e., a
clutch), the higher the probability for success in that
event. BUT, that high reproduction comes at some
small cost in adult survival probability. And the
longer a bird lives, the more chances it has to
reproduce again. So, birds should not “put all their
eggs in one basket” but instead spread their risk out
over their lifetimes, to maximize the total number of
young they manage to raise to independence over
their whole lives, not just in a single season.
b. Because of this trade-off in Lifetime Reproductive
Success (LRS), the optimal clutch size in a single
season is a bit less than the number that would
produce the most young from that single clutch.
c. Evidence for this trade-off hypothesis has been
equivocal. In some species, it has been shown that
adults with large broods lose more weight during the
breeding season that do adults with small broods (but
linking this wt loss to survival has been tricky).
d. However, in a study of tree swallows, females with
experimentally enlarged clutches did not lose any
additional weight, nor did they survive less well (from
year to year). Hmmmm…..
2. The second hypothesis for Lack’s high predictions is “the
nest predation hypothesis.”
a. Idea here is that increased predation pressure could
create selection pressure (could “favor” in other
words) small clutch sizes – by three mechanisms:
 First, larger clutches take longer to lay, which will
extend the time of vulnerability. So smaller,
shorter-to-lay clutches might be favored in intense
predation-pressure environments. This logic
seems consistent with the observation that clutch
sizes are lower in the predator rich tropics.
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
Second, larger clutches are noisier (and hence
more conspicuous to predators) than small
clutches. This logic seems consistent with the
observation that birds that nest in tree cavities
(which are relatively safe from predators) have
larger clutches than do species in similar taxa and
similar environments that nest in open cups.
 Third, larger clutches entail more energetic
investment and time for parental, which might
preclude re-nesting in case of failure in some
environments. This logic seems consistent with
the observation that clutch sizes are lower in low
elevations (where the breeding seasons are
relatively long and re-nesting after failure is
possible) than in high elevations (where there is
no time to try to re-nest at all).
b. However, experimental test on cup nesters in which
young are added to existing broods has not consistently
raised the likelihood of getting nailed by a nest predator.
So again, the case remains mysterious.
3. Third, the “seasonality hypothesis” stresses that the
seasonal availability of resources greatly influences the
number of eggs and young parents can raise.
a. This one is a little complicated, so you need to
concentrate.
b. Food availability in seasonal environments follows an
wave-like pattern: low in the winter, rising in the
early spring, high in summer (spring and summer
correspond to breeding season), declining in fall, etc.
See figure 20-8.
c. For birds that do not migrate, their populations are
likely most limited in the harsh winter, when
conditions are harsh.
d. In the breeding season, the adult populations of
course remain as low as they were in the winter, but
at this time food is (by comparison) everywhere.
e. So in the breeding season there is a “surplus” of food
availability to birds whose population sizes have been
limited by a harsh winter. The bigger the difference
between summer and winter, the bigger the surplus.
f. This hypothesis postulates that it is this surplus of
food that is available in the breeding season that sets
clutch size.
g. The (non-intuitive) deduction then is that clutch sizes
will be largest not where predation is weakest or even
where total food abundance is highest in summer.
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II.
Clutch sizes will be biggest where there is the biggest
difference between summer and winter food
availability. This logic nicely explains why clutch
sizes are lower in the tropics than in the temperate
zone, and often highest of all in near the arctic (big
seasonal difs in subarctic). It also explains why
clutch sizes are higher at high (more seasonal) than
low (less seasonal) elevations.
4. Complicated and interesting stuff, eh? Clutch size has
been, and remains, one of the most intensely studied and
thought-provoking areas of ecology.
Brood reduction.
A. One way birds can cope with uncertainties about the
maximum number of young they can raise in a particular year
is to lay the number of eggs that can be successful in a good
year, and reduce that number to the correct number for that
year's conditions as those conditions arrive. This is the
“Insurance Egg” hypothesis we talked about previously.
[There is another explanation: in some species the “extra
egg” is smaller then the others, and it may be we are seeing
the clutch size evolve to a smaller number….a snapshot of
evolution in action.]
B. This "brood reduction" can come in several forms.
1. First, parents may lay an extra egg and hatch an extra
young, only to kill it in all but the most productive years;
this is called avian infanticide. It is relatively rare but
conspicuous and well-documented (Moorhens in public
park ponds in Europe often perform infanticide to the
horror of park visitors).
2. Alternatively, the eggs may hatch asynchronously,
ensuring dramatic differences in the size of the chicks.
This difference in size then can result in the dominant
older chick getting most of the food in poor years, which
results in either the younger chick starving, or in some
species the older chick actually kills its sibling. This is
called siblicide.
C. To not end on a bad note, the propensity for some species to
lay more than they can normally raise has been helpful for
some species of threatened birds. Whooping cranes for
example usually have a clutch size of two, but rarely fledge
more than 1 chick. If 1 egg is stolen and raised in captivity,
we can (potentially) double the reproductive output of the
species.