Migration, Orientation, Movements

Migration: An annual two-way movement, usually based on seasonality What is migration? Migration: An annual two-way movement, usually based on seasonality Wintering Area Breeding Area

We have learned about migration from many efforts An example of a “watched” migration: the Broad-winged Hawk A medium-sized hawk from Eastern No. America

Tracking Migrating Songbirds One of first successful attempts to track the migration of a songbird (Swainson’s Thrush) With a radio transmitter From Cochran et al. 1967. Living Bird

Radar Also Provides Much Information Use of radar Radar detection of birds arriving near Houston, TX (images from S.A. Gauthreaux, 1999) Trans Gulf migrants arriving Apr 25, 0031 GMT Trans Gulf migrants leaving Apr 25, 0159 GMT

Basic Types of Migration Obligate annual migration Latitudinal migration (neotrops) Elevational 2. Irruptive migration (food highly variable) Rough-legged Hawks Snowy Owls Crossbills Redpolls 3. Partial migration Some individuals of a species migrate while others do not. Adapts to local conditions

Differential Migration Sex and age classes may have different migration characteristics Example: Dark-eyed Junco (Slate-colored race) Migrate south to eastern US to Gulf Coast, but migration distances of individual juncos shows different average wintering distributions of males, females, and young. 1. Adult females migrate farthest south 2 young males stay farthest north in Indiana & Ohio. 3. Adult females, young females settle at intermediate latitudes.

Fat as fuel for migration Necessary for long-distance migration Fat yields 2x as much energy & water as carbohydrates & protein Migrants fatten rapidly before migration and can add it quickly while feeding at resting points along the way (called stop-over sites) Stop-over sites are very important for birds crossing vast areas of inhospitable areas (e.g. ocean, large lakes, deserts)

Timing of migration Photoperiod Local conditions (food, vegtation changes) Weather patterns (cold, warm fronts)

Weather That Prompts Migration In fall, largest flights occur in the southward flow of dry polar air immediately following the passage of a cold front. In spring, major movements occur in warm southerly air flow on the west side of high pressure centers. Once airborne, passerines usually fly downwind regardless of wind direction.

From Able, K. (ed.) 1999. Gatherings of Angels

When migrating over land, songbirds progress in a series of relatively short flights, up to 200 miles or so, interrupted by 3-5 days of rest. Length of stop-over depends on local weather, how much fat the bird has, and refueling conditions In one October flight, 500,000 ducks flew from eastern Saskatchewan & Manitoba to Lousiana in a little over 1 day! (=1512 miles; mean ground speed 41-50 mph) A flock of Snow Geese flew from James Bay Canada to Louisiana in 60 hours!

How Fast Do Migrants Fly? Most songbirds migrate at about 20-30 mph in still air Waterfowl & shorebirds: 30-50 mph Tail winds or head winds can drastically impact bird speed and (potentially) survival.

Movements are Remarkable Homing behavior Homing pigeons that return to their lofts from Distances of 100’s of miles Other documented displacements Migration Latitudinal Altitudinal

Migratory feats A. Arctic shorebirds White-rumped Sandpiper Baird’s Sandpiper

Migratory Feats Arctic shorebirds: Stop-over locations

Migratory Feats Blackpoll Warbler

Long-distance migration in Catharus thrushes From Outlaw et al. 2003. Auk 120: 299-310.

Migratory Feats

Migratory Feats Short-tailed Shearwater

Importance of Stopover Sites

To Migrate (and move about beyond the range of local cues), Birds: Use a two step process Consult a map to determine the compass course to their goal Use a compass to navigate to the goal (Wiltschko and Wiltschko 2009)

Use of Map and Compass Compass mechanisms are relatively well known Magnetic Sun Stars Combinations Maps less so Geophysical gradients Landmarks Learned and innate components Recent Review in Wiltschko and Wiltschko (2009)

How Do Birds Navigate? Geomagnetism Solar storms that disrupt the earth’s magnetic field, also disrupted the orientation of Ring-billed Gulls. A bar magnet interferes with a pigeon’s ability to return to its loft on overcast days.

Birds “see” the inclination of the magnetic field, not its polarity Direction of arrow in He (natural) or H (experimental) does not matter, it is the angle e relative to p that is sensed Birds “see” this by photopigments in the eye. “Radical-pair” model suggests photon absorption leads to formation of a pair of radicals (singlet and triplet) with unpaired electrons. Light triggers this and the ratio of single to triplet states depends on magnetic field. Birds compare singlets to triplets using pigments in eye, which form patterns on the retina.

moves across sky—requires a clock Sun-compass orientation -Not constant moves across sky—requires a clock -Not always available Sun rises and sets rapidly, but is slow to change position around noon --birds use this information --because rate of change varies with location on earth, birds also learn to use this mechanism

Polarized Light At sunrise and sunset polarization forms a band at 90° relative to the sun. So, position of sun (and E or W) is noticeable even with clouds.

Star compass The “Emlen cage” for testing orientation in birds.

Integration of Cues Magnetic field provides a reference system for linking sun position and internal clock with geographic direction Young birds rely on sun compass more than magnetic, older birds adjust misinformation from sun compass (in experiments) by consulting magnetic field

More Integration Night migrants set compass with polarized light before star compass is visible Magnetic compass may also be used at this time The magnetic, sun, and star compasses are integrated components of a single complex system that provides directional information

Landmarks are a Start to the Map Navigating around the home range, can be expanded to navigate back to a familiar area (homing) Can be done by route reversal, or better yet by keeping track of direction to home and using compasses to navigate in that direction, then local cues Seems to only be done by very young birds Different from migration to a new location (first time migrants) Older birds use a real map, coordinate based, system

What about the Map? Gradients used are not known Geomagnetic field Olfaction Gravity Sound (ocean roar)

Compass and Map Innate (direction to migrate) and learned components are involved

Why do birds migrate ? Temperate Origin: birds escape from the inhospitable climates in the north which negatively impact a bird’s life history. Tropical Origin: migrants aggressively exploit favorable opportunities. Neotropical migrants are essentially tropical species that temporarily exploit the long days & abundant insects of high latitude summers. (rather than as temperate birds that tolerate the tropics to escape the northern winter)