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Matt Johnson
Lecture Notes
ORNITHOLOGY
(Humboldt State Univ. WILDLIFE 365)
LECTURE 13 - MIGRATION
I.
II.
Intro
 Ahh, bird migration. One of the most profoundly beautiful
phenomena in all of Nature. Birds' abilities to span the earth,
flying high over mountain desert and sea, the arduous journeys
they undertake, and their rich rewards upon reaching their
destinations are truly inspiring.
 Every fall, an estimated 5 billion land birds of over 200 hundred
species leave North America for central and South America. Right
now....great pivotal points of the year. Similar numbers depart
Europe and Asia for Africa. Their numbers are...in
short...staggering. Many of these migrants' populations are
declining; we'll touch briefly on the reasons for those declines
here, but we'll focus on them more explicitly later in the semester
when we cover bird conservation.
 This lecture we'll focus on the what's, where's, and why's of avian
migration. Next lecture we'll discuss the how - how bird manage
to navigate such vast distances.
Patterns of migration. - Migration routes and patterns vary
considerably among species, but some general patterns do emerge:
A. In North America, most migration routes are oriented North-South,
for 2 reasons:
1. First, climatic conditions vary more consistently N-S than they
do E-W; migrants fly South for warmer winter weather, and
North for the abrupt spring bloom of resources.
2. Second, major topographical features of North America are
also similarly oriented along N-S lines: mountain ranges
(Rockies, Sierra, Appalachians, coastlines, and deserts.
Exceptions -- altitudinal migrants, a few Rocky Mt-coast
migrants
B. In the Old World, birds initially migrate E-W in accordance with
major topographical barriers (Alps, Med Sea, African Deserts, then
fly N-S to reach their latitudinal destinations)
C. Birds in the southern hemisphere migrate to the North for their
winter (our summer). Great examples include many of the
seabirds we see off west coast in summer and fall. Many of them
nest on islands in South Pacific (NW Zealand, Australia, some off
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III.
of S. America), then they winter (our summer) of Calif. Coast
before returning.
D. Migration routes sometimes reflect recent range expansions, most
easily seen by Old and New World species dispersing across the
Bering Sea to colonize N. Alaska and N. Siberia respectively.
1. Arctic Warblers (an old world species widespread in Siberia)
recently breeding in Denali Nat'l Park in Alaska fly west across
the Bering Sea to join their comrades in N. Siberia before
migrating south to SE Asia for the winter.
2. Similarly, Pectoral Sandpipers (a new world species) have
recently expanded into N. Siberia. These Siberian Pectorals
first migrate east in the fall to join Alaskan Pectoral Sandpipers
before flying South to winter in South America.
3. Sometimes we don’t even know where they winter -Spectacled Eider extravaganza.
Evolution of migration. So, why do birds migrate?
A. Where to look for answers.
1. Some of the most interesting findings come from comparing
similar species that do and don't migrate, or migratory and nonmigratory populations within a species.
2. For example, Common ringed Plovers show long distance
migration (Scandinavia to S. Africa), short distance migration
(N. Europe to N. Africa), and no migrations (resident on
British Isles). What environmental characteristics have
resulted in the evolution of this pattern? OVERHEAD Leapfrog migration
B. What is migration anyway?
1. Migration is the predictable seasonal movements of individuals
in response to seasonal variation in climate and resources.
2. Not to be confused with nomadic wandering, which is often
tied to unpredictable fluctuations in climate and resources. Red
Crossbills moving en masse into the N. US in response to
severe winter weather and an abundance of pine seeds farther
south is not migration.
3. Migration does vary in spatial extent:
a. Some species migrate very short distances, just a matter of
a few miles, up and downslope with seasons -- altitudinal
migrants -- this is especially well developed in tropical
species exploiting seasonal resources such as fruit and/or
nectar that vary with respect to wet and dry seasons that
travel slowly up and down mountain slopes.
b. Many species along CA coast are short-distance migrants;
breeding in Pacific NW and wintering in S. Calif. And N.
Mexico (kinglet, gnatcatchers, several warblers)
c. Other species are true long-distance migrants, breeding and
wintering ranges widely separated -- Arctic terns are the
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champions, flying 12,000 km one way from Arctic
breeding colonies to Antarctic wintering waters. Buff
breasted Sandpipers breed on the N. Slope of Alaska and
winter on the pampas grasslands of Argentina. Swainson's
thrushes breed here in our own back yards and winter in
Central and S. America.
C. Cost-benefit analysis. A useful way to approach the evolution of
migration is to examine its costs and benefits. For it to evolve and
persist, costs < benefits.
1. Costs.
a. The benefits better be big cuz the costs are high. Migrants
run the gauntlet. More than half of all Northern
Hemisphere migrant land birds are thought to not return.
Of 100 million waterfowl that migrate to southern
wintering grounds, only 40 million return.
b. Not only is the flight itself energetically demanding and
risky, it makes migrants susceptible to periodic chance
weather events such as hurricanes and sandstorms.
c. The survival costs associated with migration are high; but
there's more than that. Migrants, because it takes so long to
fly from wintering areas to breeding to grounds and back,
have less time they can devote to reproduction. Migrants
have smaller clutches and (especially) fewer breeding
attempts than residents breeding at similar latitudes.
2. Benefits
a. Migrants aggressively exploit favorable seasonal
opportunities and resources.
b. Birds migrate south for the winter to travel to relatively
benign climates supporting higher food availability than N.
winters. Then they fly north to take advantage of the spring
bloom of abundant resources.
c. Some experts assert that many landbird migrant species
evolved from tropical species that move small distances
following changing resource levels. Thus, the warblers and
tanagers and vireos etc. evolved to fly North to take
advantage of less competition, less predation, and abundant
food that characterize the temperate spring and summer.
d. I believe it is best to think of this as a push-pull process.
They don't fly S and then "return" to breed, nor do they fly
N and return to winter.
D. Inverse relationship between survival and reproduction.
1. Birds that breed in the temperate zone that do not migrate
(temperate residents) take full advantage of the spring bloom they reproduce like crazy from the start of the bloom in early
spring to its end in mid-late summer. Their reproductive rates
are very high. But then they get smacked in the winter - their
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winter survival rates are very low. Thus, their populations are
held below what can be supported in the summer by high
winter mortality rates.
2. Birds that breed in the tropics that do not migrate (tropical
residents) have the opposite pattern. They live year-round in
relatively benign conditions -- it easy for them to find enough
food to survive, but when it comes time to reproduce, there are
so many other species with whom they must compete for food,
and whom will search for their nests to depredate for food, that
they have a hard time reproducing. Their survival rates are
very high, but clutch sizes are low, nest predation rates are
high, and thus their reproductive rates are very low.
3. Migrants travel between these two regions, the tropics and the
temperate zone, trying to get the best of both worlds. They try
to be in the temperate zone for the big spring bloom and
associated high rates of reproduction, and they try to be in the
tropics for the relatively benign tropical winter months and
associated high rates of survival. But they pay a price - and
that is migration. Migration is both risky (high mortality) and
time consuming. High mortality in migrations prohibits
migrants from boasting the high survival rates of tropical
residents, and the time it takes to migrate prohibits migrants
from fully exploiting the productive temperate spring and
summer - they produce fewer broods and therefore boast lower
reproductive rates that temperate species. They wind up
somewhere in the middle.
E. Differences between ages and sexes in migration patterns.
1. These differences in cost-benefits of migrants, temperate
residents and tropical residents can also be applied to different
migrations strategies within a species.
2. Sex differences. Often, males and females winter at different
latitudes. Usually, males winter farther north than females, but
in raptors the reverse is true.
3. Age differences. Usually, immatures winter farther N. in some
species (shorebirds) but farther south in other (warblers).
Confusing, eh?
4. Dark-eyed Juncos. Very good case study by Nolan and
Ketterson.
a. Adult females winter farthest south; immature females and
adult males in the middle, and immature males farther
north.
b. Nolan and Ketterson hypothesize three factors operating:
OVERHEAD
i.
greater migrational mortality favors young to fly
shorter distances (wintering farther N)
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ii.
iii.
IV.
V.
males reproductive success is dependent on time of
return to breeding grounds, so they winter farther N
to get back earlier
survival is easier farther S, so adult females that are
unaffected by the previous two factors winter the
farthest S.
Migratory Feats
A. Some examples:
1. Arctic terns
2. Blackpoll warblers Slide and
3. Bar-tailed Godwit and Bristle-thighed Curlew; OVERHEAD
4. Fallouts - Slides
B. Fueling migration
1. Hyperphagia - fat stores more energy (but requires more water)
than either carbo or protein. #’s on board they can double
their weight in just a few days
2. Fat deposition - birds need to put it on before migration, and
replenish it en route.
1. Staging areas. Especially important for shorebirds. In fact,
conservation of shorebirds hinges on preservation of their
staging areas. Copper River Delta in Alaska, Bay of Fundy
in Canada, Delaware Bay.....Humboldt Bay??? A little bit.
2. Stopover ecology. Much work as focused on how quickly
birds replenish their fat reserves, which foods they
concentrate on, how the risk of predation varies, etc. Some
outstanding opportunities exist for stopover ecology work
on Lanphere Dunes.
C. Flight ranges. Various authors have tried to derive formulas for
the "gas mileage" based on the depletion of weight, timing of
arrival, and distance between points in migration. They vary
considerably, but generally conclude that small migrants can cover
about 2500 km in 100 hours if 40% of their weight is in fat. That
will get them across any of the world's major geographical barriers
to migration.
Timing of migration
A. Internal rhythms influence migration timing.
1. Photoperiod and circannual cycles clearly important because
even in captivity under constant temp and food, migratory birds
exhibit zegunruhe - migratory restlessness. They get fidgety
during the night and bump into their cages to the south or north
depending on the season. Much more about this nest lecture.
2. External factors also play a role, especially at fine-tuning
migration. Many migrants wait at staging areas for favorable
weather conditions to initiate their flights - strong tail winds,
clear nights, etc.
B. Age and sex differences.
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1. Males usually depart N for breeding grounds before females, as
they need to get back sooner to establish territories.
2. Young in songbirds depart before adults on fall migration
(showing that their route is innate/exploratory and not learned).
Young in other species go with their parents or after.
C. Differences in taxa.
1. Time of day.
a. Soaring birds fly during the day to take advantage of rising
columns of hot air -- thermal soaring. OVERHEAD.
b. Aerial insectivores (swifts and swallows) also migrate by
day so they can feed along the way.
c. Most other passerines fly at night - when temps are cooler,
winds are slower, and air is more humid.
2. Speeds.
a. Most birds migrate at speeds very close to the "speed of
maximum range" as predicted by power curves -remember? E.g., 6 hours at 15 mph covers 90 miles, but 5
hours at 20 mph covers 100. Speed of maximum range is
always slightly higher than speed of minimum power
b. These speeds vary from 20-40 mph; higher for species with
high wing loadings (waterfowl and some shorebirds) lower
for species with lower wing loading (passerines).
3. Altitudes.
a. Raptors and passerines fly relatively low (800-3000 m)
b. Shorebirds and waterfowl tend to fly much higher, where
air is thin and flying efficient, but very cold and low on O2.
Their large bodies and arctic adapted morphologies can
handle the cold better - the record is for swans - 29,000 feet
or about 9000 m).
c. Generally, birds climb as they get lighter (as fat is burned)
just like pilots do because optimum flying altitude increases
as fuel is burned up and weight declines.