Biology 463/563 Ornithology
Dr. David Swanson, Office: CL 180
** MIGRATION **
Migration = regular seasonal movements between breeding regions and wintering regions.
- In temperate-zone and arctic latitudes, carrying capacity shows marked seasonal changes with abundant resources in the summer, but limited resources and harsh climatic conditions in the winter. Birds have 3 Potential Responses:
Examples: (1) 160 species of passerines breed in Canada, about 120 species (75%) leave for the winter and the majority of individuals of 15 of the remaining 40 species also leave.
(2) Fully 50% of birds breeding in temperate/arctic North America winter in the tropics or subtropics. These species are called Neotropical Migrants.
- Migration distances: Longest Migration - Arctic Tern = nests in the far north Atlantic and Arctic oceans, winters near the opposite pole. Some sandpipers breed in the arctic and winter in South America to Argentina or Tierra del Fuego.
EVOLUTION OF MIGRATION
- Fossil evidence suggests that birds arose in tropical environments. Migration probably arose in low latitude residents that moved northward (or southward) to exploit seasonal abundance in temperate-zone regions in response to population pressures in the tropics (especially during the breeding season with high food demands). Current migratory paths have probably evolved since the last glaciation.
- Migration tends to enhance reproductive success ---> Migrants > Tropical Residents, but temperate-zone residents have higher reproductive success than migrants. However, mortality is higher for migrants than for tropical residents (migration is a costly endeavor), but migrants survive the winter better than temperate-zone residents.
OTHER TYPES OF MIGRATION
1) Partial Migration = some of the population migrates, some is resident. This may serve as a step in the evolution of a fully migratory habit, if full migrants survive and reproduce better than residents.
2) Differential Migration = different portions of the population migrate different distances. Often sex-age classes are involved.
SEE PAGE 285, GILL. (This pattern has been well-documented for the Dark-eyed Junco, but is present in numerous other species as well).
3) Irruptive Movements = some species migrate only in some years, and migration distances may vary among years. Occurs in predators and seedeaters specializing on seeds of trees (e.g., cones, etc.). Cyclic or yearly variation in prey or seed levels results in shortages in some years, and these shortages lead to irruptions. Irruptive species in South Dakota include: Snowy Owls, Red-breasted Nuthatches, White-winged Crossbills, Common Redpolls, and Evening Grosbeaks.
- Irruptive species may behave as normal migrants, except that they stop migrating once an abundant food supply is found.
ORIENTATION AND NAVIGATION
- For successful migration, a bird must be able to find its way to correct breeding and wintering grounds. In fact, many birds will return year-after-year to the same breeding or wintering areas. How is locating these precise spots accomplished?
1) Directional Orientation = orientation of migratory flights in the proper direction (e.g., south in fall, north in spring). Appears genetically determined as inexperienced birds orient properly.
2) Navigation = finding their way back to a specific locality. Because many birds do return to specific breeding and wintering sites, this suggests that they are capable of navigation. Correct navigation likely involves previous experience.
- Experimental (Perdeck 1958) - Young Starlings were displaced about 600 km from the normal breeding areas. These birds migrated in an unchanged compass direction to winter at a site far removed from their normal range. When adult Starlings were treated similarly, most migrated to their normal wintering range.
- Migration may occur at night (nocturnal migrants = most passerines, many other landbirds, waterfowl). Allows feeding to replenish fat reserves during the day; or during the day (diurnal migrants = hawks, waterfowl, hummingbirds, etc.).
1) Visual Orientation = use of geographic landmarks (rivers, coastlines, ridges, etc.). Diurnal migrants often follow such landmarks, generally of lesser importance for nocturnal migrants, although there are documented cases of nocturnal migrants following rivers or coastlines. This may be particularly important for navigation to a precise breeding or wintering locality.
2) Sun Compass = Starlings housed in circular cages where sun was visible oriented in proper migratory direction. No directional tendency was present on overcast days. Appears that many birds can use the position of the sun to provide information of proper migratory direction.
- Birds are also capable of compensating for the changing position of the sun in the sky (changes about 15o per hour). Evidence: shifting the bird's internal clock (by changing the photoperiod) causes the bird to misread the sun's position and orient incorrectly. SEE PAGE 298, GILL.
3) Celestial Navigation = nocturnal migrants use celestial (position of stars) cues for orientation.
Evidence: Indigo Buntings in circular cage with a view of simulated night sky oriented properly, but were disoriented when the "night sky" was obscured.
- Rotation of the simulated night sky caused birds to orient according to the rotation. From these and other studies, it appears that birds use the position of constellations within 35o of the north star for celestial navigation purposes. SEE PAGE 299, GILL.
4) Geomagnetism = use of the earth's magnetic field for orientation.
Evidence: European Robins exposed to no visual cues still oriented properly for migration. If birds were housed in a steel chamber (dampens magnetic field) they lost their ability to orient properly.
- Also, homing pigeons with a magnet attached to their back failed to return to their loft on overcast days. Navigation was not impaired on sunny days as sun compass was able to override magnetic information. SEE PAGE 301, GILL.
- Orientation can be predictably altered by altering the magnetic field that the bird experiences with Helmholtz coils, either around the chamber, or attached to the bird's head. SEE PAGE 302, GILL.
- CONCLUSION: It is likely that proper orientation and navigation involve a combination of these cues. Furthermore, early experience is important in determining how birds use a particular cue.
ORIENTATION OF NEOTROPICAL PASSERINE MIGRANTS IN THE NORTHERN GREAT PLAINS
- Migrants are not capable of flying all the way from wintering grounds (in Mexico, central America, and northern South America) to breeding grounds in one flight. They must stopover at sites along the migratory route to replenish fuel reserves (as stored fat).
- In the northern Great Plains, suitable stopover habitat (woodlands) was historically found almost exclusively along river corridors, as most of this area was covered by vast grasslands.
- Do passerine migrants (most are nocturnal migrants) follow river courses during migratory flights to ensure finding appropriate foraging habitat during the day, or do they migrate along a broad front and fall out at random locations at the end of their migratory flight?
- Experimental: Measured orientation of nocturnal migrants along Missouri (west-to-east) and Big Sioux (north-to-south) Rivers in southeastern South Dakota by ceilometry. (SEE HANDOUT). Recorded number of birds and their direction of orientation by clockface method. Hypothesis = Migrants should orient differently in the two corridors if they are following river corridors during nocturnal migration.
- Results: Mean direction of orientation did not differ significantly between the two corridors in either spring (Orientation Direction = northwest) or fall (Orientation Direction = southeast).
- Conclusion: Nocturnal passerine migrants do not follow river corridors during migration through the northern Great Plains, even though this is where suitable stopover habitat is located. Instead, migrants appear to migrate along a broad front in the general direction of breeding areas.
- Why don't migrants follow river corridors?
1) Nocturnal migratory habits may make following river courses at night difficult, as most passerine migrants migrate at altitudes up to 1000 meters.
2) Neotropical passerine migrants show a great deal of behavioral plasticity during migration, so that they are capable of foraging in ways that they don't normally use. This may allow them to utilize different habitats than "normal" during migration to replenish fuel supplies.
- Examples: (a) Loria and Moore (1990). Red-eyed Vireos following passage over the Gulf of Mexico in the spring were classified as "lean" or "fat" based on average condition of birds at capture. Lean birds exhibited a greater propensity for gaining mass during stopover than fat birds. Also, lean birds were less selective of foraging microhabitats, showed an expanded feeding repertoire, and moved at a higher speed while foraging than fat birds. This demonstrates that migrants are capable of changing foraging behavior based on their energetic condition.
(b) Lean birds also gained mass more rapidly than fat birds during stopover for Wood Thrushes and Veerys, suggesting an expanded set of foraging behaviors in lean birds in these species (Moore 1991).
(c) Yellow-rumped Warblers behaved differently in their response to food availability depending on whether or not they were in migratory disposition (i.e., storing fat). Birds were given a choice (2 separate feeding stations) of constant award (3 mealworm larvae) vs. variable award (0 or 6 mealworm larvae with a 0.5 probability) feeding situations. Birds in a non-migratory condition preferred the constant award, while birds in a migratory condition preferred the variable award. When migratory birds reached maximum body mass and were ready for migration, they changed behavior to prefer the constant award situation.
TIMING OF MIGRATION
- Precise arrival and departure dates are an important aspect of migration to maximize the survival and reproductive success of the migrating birds. (Example: Cliff Swallows return to San Juan Capistrano mission in southern California on about March 19).
- Internal rhythms govern the timing of migration. Zugunruhe = migratory restlessness, indicates a state of preparedness to migrate.
- Ultimate Control = long-term climate trends, tracked by photoperiod.
Proximate Control = immediate weather conditions (e.g., warm temperatures in spring or cold temperatures in fall + tail winds + Zugunruhe ---> migration; poor weather in spring or good weather in fall + head winds may inhibit migration, even if the bird is prepared to migrate.
- Migratory Fattening = migrants develop stores of fat, which is the major fuel for migration (more energy per gram than any other fuel), just prior to migration. Long-range migrants tend to store more fat (30-47% of total body weight) than short-distance migrants (13-25%). Non- migrating birds usually store about 3-5% TBW as fat (although this increases to 10-15% in winter).
PHYSIOLOGICAL ADAPTATIONS ASSOCIATED WITH MIGRATION
1) Fat Deposition
2) Increased reliance on fat as a fuel relative to other seasons. Accomplished by increased enzyme activities in fat mobilization, transport and breakdown pathways in migratory relative to non-migratory birds.
3) Mass-specific aerobic capacity (maximum ability of tissues to use oxygen to produce ATP) increases with migratory condition in some species, but not in others.
4) Flight muscle hypertrophy ---> leads to increases in total aerobic capacity and power output.
5) Spring migrants exhibit increased maximal capacity for heat production relative to summer residents or fall migrants. May be associated with increased demand for heat production via shivering as migrants move northward into cool environments in spring. Alternatively, increased heat production capacity in spring may be a by-product of adaptation for endurance flight because physiological adaptations for long-distance flight and long-duration shivering (both are forms of endurance muscular activity) are similar. Also, increased urgency in reaching breeding grounds in spring (to establish territories, etc.) may lead to enhanced capacity for endurance flight during spring relative to other seasons.
HOW FAR, HOW FAST, AND HOW HIGH DO MIGRANTS FLY?
1) Migratory Distances:
(A) Total migratory distances may be as great as 13,000 km (one-way) for Arctic Terns and many shorebirds that nest in the North American arctic and winter at the southern tip of South America.
(B) Non-stop Flights: Long-distance flights in migrants include crossing the Gulf of Mexico for North American migrants (1000+ km) or the Sahara Desert for Eurasian migrants (1600 km). Blackpoll Warblers and several shorebirds fly non-stop from the New England Coast to northern South America in the fall, a distance of over 3000 km. Bar-tailed Godwits apparently fly non-stop from the western Alaska coast to New Zealand, as distance of 11,000 km.
- Most songbirds usually fly only 100 - 200 km per night then stop to refuel, usually from 1- 3 days.
2) Migratory Speeds:
(A) Direct measurements of flight speed - Passerines = 30-40 km/hr, Ducks and Shorebirds = 50- 80 km/hr.
(B) Measurements based on time between marking (banding) and recapture - Passerines = 25-110 km/day (average speeds, maximum speeds over 200 km/day); Ducks and Shorebirds average higher speeds: 60-200 km/day (maximum speeds up to 1000 km/day). Example: Lesser Yellowlegs traveled from Massachusetts to the West Indies (3220 km) in a maximum of 6 days (= 536 km/day)
(C) An Orange-crowned Warbler that was banded in Fairbanks, Alaska on 20 August was recaptured in Vermillion, SD on 25 September. A distance of about 4100 km in a maximum of 36 days, for a flight speed of 114 km/day.
(D) Flight speeds during spring migration are usually faster than during fall migration (oftentimes spring speeds almost double fall speeds). This is probably associated with the greater urgency during spring migration to reach the breeding grounds early to establish good territories or acquire the best mates.
3) Migratory Altitudes:
(A) Typically higher for nocturnal migrants than for diurnal migrants. Diurnal migrants usually travel below about 1500 m. During the night, migrants show an increase in altitude to maximum levels during the first two hours of the nocturnal flight with steady decreases thereafter as they approach daylight and discontinue migration. Altitude may also vary with the stage of migration, with higher altitudes along the middle of the migratory pathway, and lower altitudes near the beginning and end of the migratory route.
(B) Most nocturnally migrating songbirds migrate under 1000 m, but altitudes up to 3000 m have been reported.
(C) Shorebirds and Waterfowl routinely migrate at night up to 1200-1500 m, with altitudes greater than 3000 m reported.
- SEE PAGE 280, GILL FOR REVIEW OF ALTITUDES.