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Matt Johnson
Lecture Notes
ORNITHOLOGY
(Humboldt State Univ. WILDLIFE 365)
LECTURE 7 - FORAGING ECOLOGY
I.
Relationships between birds and food.
A. Bird populations responses to food availability.
1. Birds, especially in the non-breeding season, are primarily
interested in getting food. Thus, bird populations and
communities show some characteristic responses to changing
food availability, which is a word that simultaneously describes
food abundance (how plentiful food is), how easily food is
obtained (how easy it is to capture/gather), and how quickly it
returns (food productivity, or turnover rate).
2. These responses include:
a. tracking ephemeral food resources (e.g., seabird populations
following fish and crustacean populations in marine
environments)
b. responding to regular annual variations in food supply (e.g.,
grebes and others congregating at Mono Lake when brine
shrimp numbers are near their peak)
c. concentrating in local habitats that support high food
abundance (e.g., shorebirds congregate ephemerally at
exposed mud flats)
d. switching food types in response to availability (e.g., nonmigratory hummingbirds in urban environment changing
from flowers to feeders for the winter)
3. These responses can be characterized by two types:
a. Functional response. This describes the change in an
individual bird behavior in response to variation in food
availability.
b. Numerical response. This describes the change in
population size of a species in response to variation in food
availability.
B. Influence of food on spacing patterns and social systems.
1. Food is rarely distributed randomly. Birds distributions also
often "match" (loosely) the distribution of their major foods.
(e.g., redstarts in Jamaica, shorebirds at Mad River Estuary).
2. Territory size is often related to food availability, becoming
smaller where food is plentiful. (e.g., Ovenbirds and ants in
II.
III.
IV.
Jamaica OVERHEAD, hummingbirds in mt. meadows and the
number of Indian paintbrush).
Finding food.
A. Search tactics.
1. Research suggest that many birds have "search images" when
looking for food. That is, they have a mental picture, or
template, of what they're looking for, and when what they see
what matches the search image, they strike quickly. Small
deviations from the image may cause the food item to be
ignored. This is probably a mechanisms to increase strike speed
and minimize search time.
2. Sometimes, birds will make dramatic shifts in preferred food
types with little change in their availabilities, suggesting a
"threshold" in availability which, once crossed, favors a
specialized diet on only 1 to several prey type. DRAW
GRAPH.
B. Neophobia. Russ Greenberg documented that some birds are
reluctant to try new situations (a condition called neophobia) when
foraging; whereas others are not reluctant (neophilia). These
extremes represent two different foraging strategies.
a. Neophobia is useful less risky than neophilia (curiosity killed
the bird), but also prohibits the bird from exploiting
unpredictable resources.
b. Neophilia is the reverse; flexible, but risky. Neophobia seen
most in stable, predictable environments.
Capturing/handling food.
A. Basics. Some foods are large and yield high calories, but are
difficult to eat (hard nuts, spiny insects, fiesty snakes, etc.). Thus, a
bird must balance handling time and energetic content when
foraging. Again, different species take different strategies here.
Tropical Puffbirds will spend 15 minutes with a bug, but it eats BIG
BUGS. Small Flycathers eat, you guessed it, small flies.
B. Tool use. Some bird lower the handling time of difficult to capture
prey by using tools to aid in their capture. Classic example is the
Woodpecker Finch of the Galapagos that pries insects from
decaying wood with twigs.
C. Culture (learned behaviors). Other species have evolved learned
behaviors, passed on from generation to generation, to reduce
handling time. Eurasian Oystercatchers take up to a year to learn
how to effectively capture and open bivalve mollusks. Also, the US
reintroduction of thick-billed parrots was unsuccessful in part
because the juveniles that were released were unprepared to harvest
pine nuts efficiently - their primary winter food source. The
learning process was difficult to foster as they were bred in
captivity. Re-intro also struggling due to predation on young.
Eating food (including re-finding it).
A. Role of nutrition. Not much is known, but it appears that migrating
species of songbirds are especially well adapted to use abundant
berries in the fall to build up fat reserves. Berries may be more
important than insects not because they are more common, but
because they provide a better balance of nutrients.
B. Changing food requirements, hyperphagia.
1. Foraging rates (feeding speed and/or or time devoted to feeding
each day can increase) often double or more as birds prepare for
migration, probably the single largest physiological demand on
vertebrates.
2. Small birds double their body weight as little as 10 days;
forming a layer of fat that is stored on their belly that comprises
a solid sheet from chin to vent.
C. Relocating cached food.
1. Classic example is that of Acorn Woodpeckers which store
thousands of acorns in communally used granaries - often an
extended family group shares a granary.
2. Much work done on the capacity for species of corvids (jays) to
store and relocate cached pinyon or pine nuts.
a. Clear progression of "memory", "nut specialization", "nut
dependence" and multualism.
Species
Clark's
Nutcr.
Pinyon
Jay
Stellar's
Jay
Scrub
Jay
V.
Bill
shape
Chisel
like
Chisel
like
Generalized
Generalized
Subligual
pouch
Yes
Expandable
esophagus
No
# seeds
carried
90
# miles
traveled
14
Habitat
of cache
Mts
No
Yes
55
6
Pine For.
communal
communal
No
Yes
18
2
Upslope
No
No
5
0.5
Pinyonjuniper
Seed
testing
100%
rattle
98%
click
76%
visual
none
b. The relationship is mutualistic between the pine and
nutcracker in that the cones have evolved to be "most
harvestable" by the nutcracker's bill and its behavior. So the
nutcracker gets good timing of reliable food rich in lipids
and proteins; the pine gets good dispersal (bird's don't find
them all) and free "planting."
c. Food caching is a way to survive highly seasonal
environments punctuated with periods of markedly high and
low food availability. It’s a functional response to food.
d. Other approaches are to migrate away for more favorable
conditions….that would be a functional response (in that
each bird migrates)….and also a numerical response locally
because the populations size diminishes as food declines.
Optimal foraging theory. All of these observations led behavioral
ecologists, especially ornithologists, to develop a rather sophisticated
body of theory in the 70's and 80's called "optimal foraging theory."
% of nuts
in diet
>50%
Degree of
dependence
Great
10%
Moderate
<5%
Little
<5%
Very little
The literature for this is VAST, we'll just go over the most basic and/or
interesting aspects of it, but you'll probably see it come up several times
in your stay here at HSU, if you haven't seen it already.
A. Basics. It's not called optimal for nothing.
1. The idea is that foraging is a balancing act between the energy
gained from eating, the energy spent while foraging, and the
time spent foraging. This can be seen as this formula:
Profit = (energy gain - energy cost) / foraging time
Where:
energy gain = caloric value of a food item
energy cost = energy used to find, capture, handle, and eat food
item
foraging time - time spent finding, capturing, handling, and
eating food item
2. The BIG assumption is that natural selection has been so
efficient, and birds so capable of behaving as natural selection
would have them, that they always behave in ways that
maximize the balance of this equation, maximize their profit.
This isn't a valid assumption; how often do you behave
optimally? We're animals partially shaped by natural selection
too, just like birds. Nonetheless, it is a useful tool to help us at
least understand the principles behind foraging behaviors; a
framework from which we can identify and examine deviations.
B. Lab findings.
1. Lab findings have shown that, in general, birds behave as we'd
expect them to from this model. For example, Great Tits that
were served mealworms of various sizes on little conveyor belts
ate large and small worms when were both were scarce, but
selected only large high-profit worms when they were so
common that the investment of time in small worms lowered
their foraging profit.
2. However, sometimes birds don't adhere to these rules. This
usually happens when the birds aren't maximizing profit. They
may still be behaving "optimally," but they may be optimizing
something else. For example, some make foraging decisions
such that they minimize foraging time, to be safe from
predators.
C. Practical utility? Thus, optimal foraging theory is just that - a
theory. Reality often deviates from it, but we couldn't understand,
or even recognize, those important deviations if ornithologists
hadn't devised the theory in the first place. It has greatly increased
our understanding of food.
D. IN general, most lab work has been done on temperate spp. with
highly variable diets (corvids, parids). These birds’ environments
are very flexible, and they often rely on switching prey and making
decisions to survive. Birds in more stable environments, such as
specialists in the tropics, tend to be more stereotyped.