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Transcript
SOCIAL BEHAVIOR
• Birds exhibit a wide variety of
social behaviors either to live in
groups, live cooperatively, or to
compete for resources
• Main topics = territoriality,
dominance, and flocking
TERRITORIAL BEHAVIOR
• What is a territory?
• A territory is a fixed area defended continuously in either
the breeding or non-breeding season
• Acts of defense or display to discourage rivals that would
otherwise enter the territorial space
• Primary = limited to the defending individual and perhaps
its mate and progeny
TERRITORY
• The simplest territories are those with only 1 type of
resource involved
– Example = Hummingbirds defending specific flowers
• In contrast, all-purpose territories of many land birds are
used for male display, courtship, the nest site, and feeding
• These territories space individuals and thus reduce
predation, conserve essential resources, and reduce sexual
interference
TERRITORY SIZE - FOOD
• Territory size varies with body size,
energy requirement, and food habits of
the species
• Territory size has also clearly been
shown to vary with habitat quality
• Examples =
– Pomerine Jaegers, an arctic
species, will defend a territory of 19
hectares when lemming
populations are high but 45
hectares when the lemming
population is low
– Hummingbirds and Sunbirds
decrease territory size as flower
density (nectar) increases
TERRITORY – Density Dependence
TERRITORY – Density Dependence
• Territory size is also dependent on
population density and the number
of competitors
• In 1934, Huxley proposed that bird
territories could be likened to 'rubber
discs.'
• Example =
– When Tree Sparrow density is
low they only use 15-18% of their
territories and concentrate
activity in a core zone but defend
a large buffer zone
• As population density increases,
territories (like rubber discs) are
compressed (reduced in size) but only
to a certain point
– When population is high, denser
packing eliminates the buffer
zones between territories thereby
reducing size
• Therefore, there is a limit to the
number of territories in an area which
effectively limits the number of birds
present
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TERRITORY – Density Dependence
• Several studies with a variety of species have revealed that males (and
females) removed from their territories are usually replaced by conspecifics
(and, often, replaced very quickly).
• This indicates that there are 'floaters' (birds without territories) in at least
some populations and suggests, that territorial behavior may limit population
density.
TERRITORY – Density Dependence
• Winter territories of Hermit Thrushes (Brown et al.
2000)
• Used radio-telemetry to measure Hermit Thrush
movements
• dominant individuals maintained stable territories
throughout the winter
• subordinate birds, floaters, forced into a non-territorial
strategy in search of resources
• floaters (14%) moved among occupied territories, but most
were faithful to a larger neighborhood, apparently awaiting
a territory vacancy.
• The behavior of Hermit Thrushes confirmed that
competition for spatially mediated resources on the
wintering grounds, such as food or cover, contribute to
limiting populations of many species of migrant passerines
Territorial Behavior & Population Size
Territorial Behavior & Population Size
How territorial behavior might limit population size: Brown (1969)
reasoned that, in any given area, habitats vary in quality, ranging
from very high quality to low quality habitats:
At low population densities, all birds occupy high quality habitats
At higher population densities, some birds are excluded from the high
quality habitats and must occupy lower quality (but still acceptable)
habitats
At still higher population densities, high quality and lower quality
habitats are 'saturated' with territory-holders & some birds must
then occupy unsuitable habitats. Birds in these unsuitable habitats
become 'floaters.'
Territorial Behavior & Population Size
Example: Song Sparrows, British Columbia
Territorial w/ submissive floaters
Fretwell (1972) proposed that, in any
given area, habitats vary in quality or
suitability, ranging from good to poor.
Proportion of floaters increases as the number of
territorial males increases
In addition, the 'suitability' (in terms of
the fitness a bird could attain) of
habitats decreases with increasing
population densities (more birds means
fewer resources per bird).
So, when densities are high in the good
habitats, a bird may do better by
occupying the lower quality habitat (as
long as densities are still low there)
Territorial Behavior & Population Size
As density increases other population parameters
decline:
fecundity
juvenile survival
Ideal Free Distribution
Likely due to food limitation and increased
competition
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Ecological Traps
High Quality Habitat
Ecological Traps
The correlation between
population density and
habitat quality can break
down if organisms make
errors in appraising habitat
quality
This can occur if the cues
that illicit settlement become
decoupled from the
underlying resources
Low Quality Habitat
Gilroy and Sutherland 2007
Gilroy and Sutherland 2007
Ecological Traps
COSTS AND BENEFITS
• What is the central requirement for
the cost of maintaining a territory to
outweigh the benefits?
• The resources must be
economically defensible
• Defensibility is influenced by
temporal and spatial variation in the
distribution of the resource
HUNTER’S SUNBIRD
COSTS AND BENEFITS
• Resources that change rapidly in time
are used opportunistically and
difficult to defend
• Example =
– Aerial insects whose locations and
densities shift frequently are not
defendable resources
– Rarely observe swallow (aerial
insectivores) exhibit aggressive
behavior between individuals
while foraging
BENEFITS MUST OUTWEIGH COSTS
Defend territory
Cost or Benefit
Costs
• Individuals that may be territorial
over one type of resource may not
defend other resources
• Example =
– Sunbirds are highly territorial but
will sit on the same branch with
other Sunbirds when they are
foraging on passing insects
Benefits
Optimum
Size
Territory Size
DOUBLE-COLLARED SUNBIRD
3
Economics of Territory Defense
• A territory is economically defendable if the energy expended in
defense is less than the energy available
• Gill and Wolf (1975) tested this hypothesis on Golden-winged Sunbirds
– Estimated energy expended and energy gained by:
• Recording time spent by sunbirds sitting, foraging, & defending
• Using lab experiments to calculate energetic costs (calories per
hour) of each of these activities and established relationships
between nectar availability and foraging time
Economics of Territory Defense
Found that:
1 microliter of nectar, sunbirds needed 8 hours of foraging
2 microliters of nectar, sunbirds needed 4 hours of foraging
3 microliters of nectar, sunbirds needed 2.7 hours of foraging
Defense of a territory:
ensured that no other sunbirds took nectar from available flowers
nectar availability in each flower increased from 2 to 3 microliters
In sum, territorial sunbirds:
saved 1.3 hours of foraging time per day, or about 780 calories per day (= benefit)
expended about 730 calories per day in defending territories (= cost).
made a small energy 'profit' (benefits > costs) so territories were 'economically defendable.'
COSTS AND BENEFITS - NO DEFENSE
• Sites that attract hordes of potential
competitors are usually
undefended
• Example = no gull would or could
defend a dump site
• Sanderlings only defend moderate
density prey areas
– do not defend dense
concentrations of isopods
because there are too many
conspecifics
– Nor do they defend low prey
densities because it is not
worth the cost
TERRITORY—INTERSPECIFIC DEFENSE
• Territorial defense is usually against conspecifics but is also commonly
interspecific
• Examples:
– Hummingbirds will defend territories against other nectar-feeding
species
– Mocking birds will defend berry-rich feeding areas from all species
– Woodpeckers will eject other species from cavities
MOBILE RESOURCE DEFENSE
• Territorial birds can defend a
mobile resource, most
commonly a mate = mate
guarding
– Constant defense of a female
and her immediate space
can help ensure paternity
• Sanderlings will defend Willets
from other Sanderlings when the
Willets are feeding on large
crabs
– Bits of crab will fall and be
eaten by the defending
Sanderling
DOMINANCE BEHAVIOR
• Dominance and reinforcement of
its status is very common among
birds and is the driving behavior
behind territoriality
• As social ranks are established,
losers seldom challenge the
dominant individual and when
they do they almost always loose
• Dominants then have better
access to space, food, mates, etc.
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DOMINANCE BEHAVIOR
DOMINANCE BEHAVIOR
• Birds assert themselves more
effectively when on familiar
ground
• Three hypotheses for why owners win:
• 1) They are better fighters
• Therefore, territory owners win
encounters more often than
intruders
• 2) They have more information about resources and fight harder
– Predicts original owner will win
– Predicts gradual reversal of dominance
• 3) Arbitrary asymmetry of ownership – possession is 9/10 of law
• Territory owners know the area
and have a known investment to
protect – more likely to fight
harder
– Predicts present owner wins
• Krebs (1982) tested these hypothesis with great tits in England
– Removed owners and kept them in cages
– Newcomers took over almost immediately
– Data showed Hypothesis 2 was most plausible with newcomers more likely to
win once they had time to learn territory
• Example = Acorn Woodpeckers
vigorously defend their granaries
from squirrels, jays, and other
woodpeckers
DOMINANCE ADVANTAGES
• Dominance confers selective advantages
• Examples =
Dominant juncos and field sparrows
lived longer than subordinates
FLOCKING BEHAVIOR
• Birds commonly form flocks and
there are only a few species that
lead solitary life styles outside the
breeding season
Subordinate wood pigeons obtained less
food per hour than dominants
– Woodpeckers and both diurnal
and nocturnal raptors are mostly
solitary
Low-rank individuals were also the first
to leave the area
• For most other species they come
together in intra- and interspecific flocks at some point
during the annual cycle
During winter, dominant whitecrowned sparrows fed closer to the
protective cover than did subordinates,
which thus had a higher predation risk
NEW ZEALAND WOOD PIGEON
FLOCKING BEHAVIOR
• Benefits of flocking include:
– Reduced predation risk
– Increased time foraging
– Cooperative foraging strategies
– Behavioral thermoregulation
– Information exchange
• Form when benefits outweigh the cost
of being surrounded by competitors
• Most common in non-breeding season
• Food whose location is unpredictable
in space and time favors intraspecific
flocking
FEEDING IN FLOCKS - COSTS AND BENEFITS
• Insectivores often form mixed
species flocks
– Contain only few individuals
of each species, reducing
competition
– Each species retains own
niche within flock
– Contain nuclear species plus
attendant species
– Increases foraging efficiency
by increasing predator
detection
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Anti-predatory Benefits
• Safety in numbers – minor benefit
• Confuse predator – minor benefit
• Shielding – benefit for some (dominants in
center), not others (subordinates periphery)
Foraging Benefits
• Increased feeding time due to reduced
predator scanning time – major benefit
• The “beater” effect
– Prey flushed by one flock member
can be eaten by another
• Detection of predators – major benefit
– Example = ground hornbills in
Africa walk a line across fields
cooperatively flushing insects
Foraging Benefits
• Cooperative hunting – major
benefit in a very few species
(Harris’ Hawk, White Pelican)
• Locating food
– Primary reason for
intraspecific flocking in
species feeding on patchy,
ephemeral foods
– Benefit in mixed flocks
because species forage in
different ways within the
same patch
End Social Behavior
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