lecture 25 - avian demography

Matt Johnson
HSU Wildlife 365
(Humboldt State Univ. WILDLIFE 365)
I. Intro to demographics [I skipped this (I) sp 01]
A. Demography is the study of populations.
B. Central to the study of avian demography is the quantification of
lifetime reproductive output. This is the combination of four
1. Age of first reproduction
2. reproductive success, or number of young fledged per
reproductive event
3. the average survival of those young
4. longevity (itself a function of adult survival)
5. The term survival refers to the probability of surviving to a
particular age, or it can be expressed as annual survival (the
probability of surviving from a point in time to one year after
that point).
6. The term fecundity refers to the number of young successfully
reared. This can be lifetime fecundity, or annual fecundity.
II. Life History Patterns
A. General patterns - there are three major consistent patterns
1. Mortality is high at young ages, then usually stabilizes for adult
birds. Also can be viewed in terms of survival.
2. Reproductive success and effort increase with age and
3. Fecundity tends to be inversely related to longevity among
species, perhaps within species as well.
4. These patterns result in some very conspicuous changes in the
life history patterns of different species.
a. A sparrow is short-lived, breeds early, has low survival, but
high fecundity
b. An albatross is long-lived, breeds only after several years,
has high survival, but low fecundity.
c. In ecology, we call species having life history patterns like
the sparrow as "r-selected", and species like the Albatross as
K-selected. The reasons for those letters will become
clearer later. The result, is that both species may in fact
replace themselves with the same number of young. The
sparrow breeds faster, but for not as long; the albatross (or
penguin) breeds slowly, but for a longer period.
B. Annual survival and mortality
1. In general, survival tends be higher in larger than small bodied
birds, in seabirds than in landbirds, and in the Tropics than in
the Temperate zone (your book says there is some debate, and
there is, much evidence suggests the pattern is widespread, and I
firmly believe that as more and more studies are completed, it
will be clearer and clearer than tropical survival rates are high)
Karr's study was among species; a better test is within species,
or among closely related species, and these suggest t high
survival in the tropics.
2. Also, a young bird's chances of survival are roughly half that of
an adult. For small songbirds, survival is especially low in two
discrete periods. Survival is only about 50% for the first 9 days
out of the nest, when the young are poor flyers and even though
their diet is still supplemented by their parents feeding them,
they are vulnerable to predation. Next, once they finally
become fully independent of their parents, survival is only 60%
over a two week period, when they are susceptible to starvation
before they fully hone their independent feeding skills.
3. Also, females in many species have slightly lower survival rates
than males, leading to a sex-biased sex ratio. Several
hypotheses exist for this. One, the incubating females are
subjected to higher rates of predation than the males. Two,
females simply invest more energetically into breeding, and
suffer higher mortality accordingly. Third, in species where
males are behaviorally dominant, females may be relegated to
poorer quality habitats in the non-breeding season, and suffer
higher mortality rates.
4. Most small birds live 2- 5 years. 20-30 years for larger
waterbirds, such as egrets, and raptors. Ducks are somewhere in
between. As noted earlier, rough correlation with body size.
C. Fecundity. Remember, lifetime reproductive output is a
combination of age of first repro, reproductive success per season,
young survival, and longevity. The reproductive success per season
is in turn a function of the number of attempts per breeding season,
and the success of those attempts.
1. Remember, as we noted when we talked first about clutch sizes,
there is a profoundly important trade-off in reproductive effort
of success per attempt and probability of future attempts
(survival). Many bird adopt strategies to temper investments in
reproductive events to maximize their chances of reproducing
again, perhaps with greater success.
2. One such trade-off strategy is in delayed breeding.
a. Some birds breed at 1 year of age (e.g., their first spring) but
others do not.
b. Why delay? If ya think about it, the age at first breeding
determines the interval between generations, its extremely
important in determining population growth. Thus,
individuals that breed their first year would quickly
outproduce those that delayed breeding, and therefore soon
replace the others -- unless the costs of early breeding are
severe enough to offset the benefits.
c. Thus, for delayed breeding to remain a persistent strategy in
birds, it must contribute to maximizing lifetime reproductive
success, which may be most evident in long-lived species.
For example, in Adelie Penguins, among all ages, mortality
is higher (39%) for breeders than for nonbreeders (22%).
For young birds this is especially true: birds breeding at 3
years of age (the average age is 4 years) suffer an incredible
75% mortality. Breeding is indeed very costly, and for longlived species, it behooves individuals to delay the
investment of breeding until their chances of success are
high, which gets better with experience.
3. A similar trade-off exists in differential effort over a birds'
lifespan. [I skipped C- 3, 4, 5 sp. 01]
a. If a birds lifetime reproductive effort is partly dependent on
future reproduction, then it should temper its efforts to
maximize survival especially when it is young, that is, when
it has many potential opportunities in its future. As a bird
ages, it should slowly increase its effort, for future
opportunities become less likely with increasing age.
b. Support for this predicted pattern lies in differential effort
for nesting California gulls. OVERHEAD
4. Nesting success per season.
a. The success of a nesting effort is affected by many things.
The most important factors, listed in order of decreasing
importance are: predation, starvation, desertion, hatching
failure, and adverse weather.
b. In general nesting success tends to be higher in high than in
low latitudes, in cavity nesters than in cup nesters, and in
larger species with hardy young than in smaller species with
more vulnerable young.
5. Number of broods attempted per season.
a. Depends on the length of the breeding season -- which is
dependent not only the length of favorable climate and food
conditions in the place of breeding, but also on how long the
birds are there to exploit that season (usually longer for
residents than migrants).
b. Short growing seasons like those in the high temperate areas
or the arctic have short breeding seasons with few broods.
c. Birds with extended parental care also usually have only 1
d. Tropical birds usually have more broods than do temperate
birds because of a longer breeding season. However,
tropical birds usually require longer intervals between
broods. Also, they often lay only every other day, have long
incubation periods, and slow nestling growth rates. All of
these patterns point to a hypothesized shortage of food for
tropical birds, but this has proven very difficult to test
Population Dynamics.
A. Intro. [I skipped A and B sp. 01] An understanding of how
populations of animals change is central to the conservation of
wildlife. We conserve species by maintaining viable
populations. For several reasons, birds have played a dominant
role in our understanding of animal population dynamics, or
1. First, birds are diurnal and easily distinguished in the field.
(in fact many actually announce their presence and species
identity during the breeding season via bird song.)
2. Second, many birds are relatively easily trapped and
marked with bands. The enables the study of their
population via effective mark-recapture studies.
3. Third, most birds raise their young in discrete nests, making
quantification of reproduction possible, whereas it is often
nearly impossible to do so in other animals.
4. Fourth, the highly developed and conspicuous social
structure of birds (pecking order) makes it possible to study
the role of dominance and territoriality in regulating
population numbers.
5. For these reasons, the population ecology of birds is better
known than for any other group of organisms -- they form a
pillar of the science of ecology.
B. Bird numbers.
1. Population sizes.
a. Population sizes range from the tiny -- especially
common in top predators on islands. The Mauritius
Kestrel and Chatham Island Black Robin populations
have probably never been above 50 in their entire
b. To the grotesquely abundant House Sparrow and
European Starling (500 millions) -- widely spread
c. To the very concentrated, Adelie penguins nesting in
colonies in Antarctica can reach density of up to 1
million per colony.
d. But these pale in comparison to the extinct Passenger
Pigeon, was once the most abundant bird on the planet.
Nesting colonies in the mid-west and east were up to a
billion strong. Audubon watched a flock fly over his
house, at times at a rate of about 300,000 per hour,
literally blocking out the sun and darkening the sky
for....for 3 days. The young pigeons, called squabs,
were easily collected in the colonies, their fat bodies
used for meat for both humans and livestock. Hunting
stopped when populations got too low for it to remain
viable. This occurred in the 1870's....when the
population still numbered in the tens of thousands. But
the bird still went extinct, the last one dying in the Cinci
Zoo in 1914, illustrating that it is not always necessary
to kill the last pair of a species to drive it to extinction.
Some species need relatively large populations just to
remain stable.
2. Counting birds.
a. It is rarely possible for a researcher to study the whole
population of a single species. Instead, we study
localized populations in smaller, defined areas. We
then extrapolate these results to the population over a
wider area, the degree of which we extrapolate with
precision is dependent on how "representative" the
study population and study site are to the species'
population and geographic range.
b. The annual cycles of most birds fall into discrete
breeding and non-breeding seasons, over which their
numbers change dramatically, even if among years the
number fairly stable. OVERHEAD
c. Some birds are easiest to count in the breeding season
(most songbirds because they are territorial and vocal),
whereas some birds are most easily counted in winter
(e.g., waterfowl in winter flocks) or migration (e.g.,
raptors). The important point is that to examine
population changes of a species over time, called
dynamics, we need to count them with consistent
methods and during consistent periods of their annual
C. Population trends. Bird populations are rarely stable over time;
instead, they fluctuate from year to year as conditions change.
Some examples of long-term trends: OVERHEAD
1. Steady increase or decrease.
2. Irregular annual fluctuations, but overall trend.
3. Irregular crashes followed by recoveries in an otherwise
(more or less) stable population.
4. A trend whose direction is influenced by frequency of
irregular occurrences.
5. Cyclic populations.
Population Growth.
A. Equations. [I skipped all equations sp 01] There are many
forms of population growth equations, called growth models.
The three major forms are the exponential growth model, the
geometric growth model, and the logistic growth model.
1. The exponential/geometric equation models the growth of a
population in the absence of any limiting factors -resources (food, cover, mates, etc.) are unlimited.
a. Exponential - used for continuously increasing
populations. Nt = No ert, where N equals population size
(# individuals at beginning of study, 0, ands some late
time t), e equals base of natural logarithm (2.78), and r
equals the per-capita rate of population growth. That is,
the per-capita birth rate - per capita death rate. (per
capita meaning the average per individual in the
population....so that if 100 birds produce 1000 chicks in
a year, the per capita birth rate is 10 chicks per bird).
b. Geometric. The same model, essentially, except that is
applies to populations that breed in discrete intervals. Nt
= No t , where  = the geometric per capita growth rate.
It is the same as er. It is calculated as a ratio of
population sizes between years (e.g., N1/N0). BOARD
c. If r = 0 or if  = 1, the population is stable. If r<0 or
<1, the population declines, if r>0 or >1, the
population increases. These make perfect sense from
the equations above.
2. Logistic growth rate. Obviously, many populations cannot
increase forever; resources become limited and the rate of
population growth slows.
a. Equation. dN/dt = rN (1-N/K) or Nt+1 = Nter(1-Nt/K),
where K equals the "carrying capacity" of the
population in a particular area. K is determined by the
availability of resources (food, mates, cover, etc.).
Thus, as the population N get larger, it gets closer an
closer to K. So, the value of N/K gets closer and closer
to 1 as the N increases and approaches K. Thus, the
value 1-N/K gets closer and closer to 0 as N increases.
Thus, the population growth (dN/dt) approaches 0 as N
increases and approaches K. BOARD GRAPHS
b. Thus, the logistic growth at first resembles that of the
exponential/geometric models.....but as N gets larger,
the population growth begins to slow, and eventually,
when N reaches K, population growth stops altogether,
and the populations remains stable at K.
c. Overall then, species or population that are very good at
competing for resources are likely to eventually reach
their K, although it may take them a long time. In
contrast, species that are not very good competitors may
increase at first very rapidly with a fast reproduction
rate, but they will then be limited by better competing
3. Now we see the reason for the notations r and K selected.
OVERHEAD, r-selected species are good reproducers, Kselected species are good competitors. Remember this is a
Population Limitation/Regulation. – [resume here sp. 01]
A. Populations do not increase without end....they eventually are
limited by one or more factors. There are 6 major ones
1. Food-supply.
a. Lots of circumstantial evidence that food limited many
species of birds: their pops are largest where food is
most abundant, pops are largest when food is most
abundant, or natural changes in food-supply are
followed by corresponding changes in pop size.
OVERHEAD (Newton 162)
b. Some experimental evidence -- food supplementation
has shown that for some species, they are held at low
population sizes due to limited food-supply. Give them
more, and their population increases. OVERHEAD
(Newton 185)
2. Nest sites. Very good experimental evidence here suggests
that some species, especially secondary cavity nesters, are
limited by the available of suitable nest sites. OVERHEAD
(Newton 201).
3. Predators.
a. Food and other resources like nest sites can put a ceiling
on bird numbers. However, some birds may be held
below this ceiling by predators or parasites.
b. In some cases, where predators have been removed,
populations increase indicating the pop is limited by
c. These predators may act on adults and/or nests. Much
evidence suggests that as humans fragment forests,
predators such as raccoons, jays, house cats, etc. have
an easier time accessing and depredating songbirds'
nests. Thus, habitat fragmentation increases rates of
nest predation, which can limit songbird population
d. An interesting aspect of predation on adults, is the
distinction between additive and compensatory
i. In the absence of predation, many species are
probably limited by food-supply, and many
individuals would starve to death every year. In the
presence of predation, some individuals are killed
before they remove much food from the
environments, which effectively diminishes the
intensity of competition among the survivors. Thus,
predation in many cases takes a portion of the
population that might otherwise die anyway, and in
such cases predation has little overall effect on the
population. This form of predation is called
"compensatory." That is, predation compensates
mortality that would occur as starvation in its
ii. But in other cases, predation removes individuals
from the population that would have otherwise been
able to survive, and thus it adds to the factors
limiting the population. This form of predation is
called "additive."
e. In our macaroni experiment, predation was at least
partially compensatory. In the absence of predation
(green and yellow) many foragers starved...but in the
presence of predation (blue and red), fewer starved.
Falcons were killing foragers, some f which surely
would have died of starvation anyway. This fact, that
predation is compensatory, permits hunting species
without detrimental effects on their populations.
f. Also, because predation removes individuals from the
population that would otherwise remove resources if
they were alive, population can be stabilized by
predation if food is finite and not replenished (as is the
case in winter). If all individuals remain alive and
compete for food by scrambling to gather what they
can, then eventually food will be so scarce that most
individuals will die, and the population will crash.
However, by removing some foragers from the
population, predation can ensure that enough food is
available in the environment to maintain at least some
individuals. This was the case in our macaroni
experiment. In the absence of predation (yellow and
green), the foragers scrambled for macaroni until it was
really scarce, at which point their numbers crashed due
to starvation. In the presence of predation (blue and
red) some individuals were getting picked off early,
ensuring enough resources later for more foragers to
4. Parasites.
a. Parasites can hold bird population below ceilings set by
food supply n the same way as predators. Most
damaging to many birds is the parasitism impose by
brood parasites -- such as the brown-headed cowbird.
b. Also like predation, high rates of parasitism are
exacerbated by human forest fragmentation. Cowbirds
are birds that food in fields, but look for host bird nests
in forests. So they need forest "edges" between forest
and field. As we chop up our forests into smaller and
smaller pieces, we effectively create more forest edge,
even if we don't reduce forest areas all that much.
c. Thus, songbird reproduction is severely limited by
increased rates of nest predation and brood parasitism
in forest edges -- and forest fragmentation is leading to
a rapid increase in the spread of forest edge at the cost
of a loss of forest "interior" habitat. OVERHEAD
5. Weather
a. Weather can occasionally directly limit bird
populations, such as when ice storms or hurricanes kill
b. But more often, weather limits birds indirectly, by
operating through some other factor. Often, weather
influences the availability of food. In fact, a three-way
interaction that limits bird numbers is probably
commonplace in winter: weather influences food
supply such that harsh winter make less food available.
In turn, the less food available, the more time birds
spend foraging for it, and the more risks they take in
finding it, which increases the effects of predation.
Thus, weather, food supply, and predation all work in
concert to limit the birds' wintering numbers.
6. Human impacts: we'll talk more about these when we
discuss avian conservation, but briefly, here are the
pathways by which human activities can limit bird
a. Habitat destruction is clearly the primary way by which
humans negatively affect bird populations. We convert
natural forest and other habitat types to agriculture,
urban lands, flood them for reservoirs, etc....this greatly
reduces the total area which supports bird. In many
cases, bird species are dependent on certain, and
naturally rare or sensitive, habitats. It is these with
which we must be especially careful -- e.g., old growth
forests, mangrove swamps, arctic tundra.
b. Habitat degradation -- fragmentation is an example.
Not much actual area of habitat is lost, but it is reduced
in quality such that it no longer becomes suitable for
certain species.
c. Overhunting
d. Incidental killing - wires, plastics at sea, pesticides, etc.
B. Overall, although few population remains perfectly stable,
MOST remain within observed upper and lower bounds, or
limits. This pattern is called population regulation.
1. In essence, as a regulated population declines below its
long-term average, its population growth rate increases,
causing it to rise back toward the average. Conversely, as a
regulated population rises above its long-term average, its
population growth rate decreases, causing it to come back
down toward the average.
2. This characteristic behavior suggests that a regulated
population's growth rate depends on its current population
size, or density. Thus, we call this pattern densitydependence. In essence, population regulation means that
there is density-dependence somewhere in the factors
affecting its numbers.
3. Competition for food or nest sites is usually densitydependent -- it gets more severe as density rises. Predation
can also be density dependent if a greater proportion of the
population may be taken as the population rises.
4. One of the most influential forms of density dependence
regulating bird numbers is that of intra-specific competition
for habitat.
a. If habitat is limited in supply (it usually is, and is
becoming increasingly so as human alter the
environment), then there is intense competition within
species for the best habitat.
b. In birds, we see dramatic dominance hierarchies. The
dominant individuals get the best habitats, and relegate
poorer quality habitats to more subordinate birds.
c. Therefore, when a populations is low, most individuals
may have access to high quality habitat. But as the
population size grows, competition intensifies for those
high quality habitats, and more and more birds are
relegated to poorer quality habitats. Consequently, the
average reproductive success of individuals in the
population is highest when the population is low. In
contrast, the average reproductive success of
individuals in the population is lowest when population
size is high. Pop size low, reproduction high; pop size
high, reproduction low -- that IS density dependence.
d. Of course, if there were more habitat overall, or if it
was all high in quality, the population size would be
e. Thus, is easy to see that populations are limited in a
density dependent manner by the interaction between
the quality and quantity of habitat.
f. In many cases, this "pre-emptive" occupation of habitats
is so extreme that some birds actually do not obtain a
habitat (i.e., a territory) at all. Instead, they simply
"float" over the rest of the population, hoping for a
vacancy to overtake, or perhaps looking for EPCs.
These birds are called "floaters."