Patterns of movement in birds:

Dispersal: moving away from area of birth or previous breeding to breed in a new area
General trends:

• natal dispersal higher than breeding dispersal
• for temperate species, female dispersal higher than male dispersal
• hypotheses:
• males are larger; out-competefemales
• evidence: in species where females are larger than males, males tend to have higher dispersal
• lower cost to dispersal for females than males
• males: reproductive success depends on territory; competing for a territory in a new area is difficult
• females: behavioral observations suggest are accepted onto territories in new areas
• in species where females are larger, females are also more territorial, so this hypothesis could also explain why male dispersal is higher in such species.

Migration: seasonal movement from breeding ground to wintering ground, then back to breeding ground again.

Patterns of migration:

• atitudinal migration: breed at one latitude, winter in latitude with milder climate
• Arctic terns breed in Arctic, winter in Antarctic.  Can feed during migration.
• Many shorebirds (e.g. bristle-thighed curlew) breed in Arctic, winter in Polynesia. 10-15 thousand km over water where can not stop to rest, feed
• Northern Wheatears (a thrush relative) breed in Greenland, migrate 2-3 thousand km over water to winter in Europe
• eastern US: many birds migrate over the Atlantic & Gulf of Mexico.  Use favorable winds.
• altitudinal migration
• some finches: breed at higher elevation, migrate to lower elevation in winter
• blue grouse breed at medium elevation, migrate to higher elevation in winter
• partial migration: some individuals migrate, others do not (ex: some raptors, goldfinches)
• differential migration: all individuals migrate, but some age &/or sex classes migrate farther than others (ex: dark-eyed juncos, American kestrels)

• costs & benefits of migrating vs. not migrating.
• migration: energetically expensive but allows movement to areas with favorable food types
• not migrating - avoids energetic cost of migration, but:
• living year round in temperate regions: energetic costs to thermoregulation; low food abundance
• iving year round in tropical regions: competition for food during breeding season
• origin of migration
• hypothesis that migration evolved independently in numerous groups that originally were year-round tropical residents
• evidence: many neotropical migrants belong to families that have many more species in the tropics so they may have originally evolved in the tropics.  Not conclusive: the same pattern would be seen if migration is primitive and speciation rates in the tropics are high
• proposed association with diet:
• birds that feed on spatially and temporally variable food sources such as fruit, nectar, need to move seasonally to find food
• such birds may be most likely to evolve long distance migration because they historically move seasonally
• evidence: correlation between migration & degree of fruit in diet in many different tropical groups
• however, there are exceptions (e.g. insectivorous New World Warblers)

Studying migration:
 

• zugunruhe: migratory restlessness
• direction of restlessness suggests direction of migration
• amount of restlessness correlated with distance birds migrate

Proximate cues for migration:
 

• daylength: main cue
• if modify day length in lab, can affect the onset & direction of zugunruhe
• weather: once daylength is appropriate for migration, actual departure may depend on weather (wait for favorable winds, for example.)

Orientation and Navigation:

• landmarks & learning:
• birds follow geographic features (this can also provide thermals, feeding opportunities); young birds may learn these features by following older birds
• sun:
• navigation experiments with starlings
• train starlings to go east to get food
• in the lab, with a stationary light, they will change the direction they go to find food depending on the time of day
• if taken into lab and put on a light/dark cycle 6 hours ahead of real day, then released outside at noon (the time it becomes light in the lab), they head south (towards the sun)
• stars
• indigo buntings placed in plantetariums at night orient zugunruhe along axis of North Star
• geomagnetism
• homing pigeons flown from unfamiliar areas on cloudy days (no sun, no landmarks) still get home
• homing pigeons flown from unfamiliar areas on cloudy days with Helmholtz coils on their heads to alter their perceived direction of magnetic field do not home; will go different directions depending on direction of magnetic field in coil.

Learned & genetic factors:

• indigo buntings
• if raised without exposure to night sky, do not orient zugunruhe
• if raised in planetarium that simulates normal sky, orient zugunruhe based on North Star
• if raised in planetarium where sky rotated around axis based on Betelgeuse, orient zugunruhe on Betelgeuse
• suggests: require experience with sky to learn which direction to orient
• if there is experience with the sky, learn to orient based on rotation of sky
• blackcaps (old world warblers)
• European populations are migratory, African populations are not
• lab-raised individuals from European populations show zugunruhe, lab-raised individuals from Africa show little/ no zugunruhe
• lab raised hybrids of European and African types show intermediate levels of zugunruhe
• since zugunruhe is related to distance migrated, this suggests a genetic tendency to migrate some given distance