Migration Why do birds migrate?



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Migration




  • Why do birds migrate?

  • If birds already live on the metabolic edge, why add to the annual energetic burden of molt and reproduction

  • Why fly thousands of miles to another continent?

  • Migration is the regular seasonal movement of animals between disjunct areas of their home range

  • Birds are not the only animals that migrate

  • Seasonal migration of African mammals over the Serengeti, one of the most spectacular sights in nature

  • Squid, salamanders, bats and butterflies all make equally perilous migratory journeys

  • Annual migration of Monarch Butterfly is as much a feat as global treks of many birds

  • Migration remains one of ornithology's deep mysteries

  • We can track its course, note species patterns, worry about our destruction of breeding and wintering habitats…

  • But we still don't understand why birds do it

  • Many hypotheses have been proposed to explain migration

  • Birds obviously seek a more favorable climate when they migrate

  • May be trying to maximize the available food supply, tracking local abundance of resources over a large area (Serengeti)

  • Pennant-winged Nightjars migrate to follow the rainy season

  • From breeding grounds in South Africa to winter quarters (rainy season) in Sudan

  • Following peaks of termite populations, their favorite prey

  • Migration to the North in Spring enables birds to take advantage of the annual northern seasonal peak in food supply

  • Fuels annual cycle of reproduction and molt




  • Migratory birds are already showing signs of change

  • 20 species now arrive ~21 days earlier

  • 7 species of warblers have shown range shifts averaging 65 miles farther north in the past 24 years, none have shifted farther south




  • Hummingbirds are migrating 12-18 days earlier in response to climate change (Johnson 2013)

  • Food availability for nestlings may be a problem




  • Our common northern birds are really southern birds who merely visit us for a few months each year to take advantage of a free lunch

  • Migratory birds aren’t fleeing the cold so much as exploiting a seasonally rich food supply




  • By migrating to Northern Temperate areas in Spring and Summer birds are taking advantage of the longer day length

  • Increases the amount of time available for foraging

  • May be fewer predators in the Northern Temperate Zone

  • More seasonal climate slows the ability of the resident predators to build up very large populations

  • Another benefit of migration is that it might allow birds more competitive elbow room

  • Territorial behavior more practical when available resources are not already taxed by large populations of resident species

  • Migration also beneficial from the standpoint of evolution

  • Natural selection acts on migratory populations at three levels

  • Breeding grounds

  • Wintering grounds

  • Hazardous process of migration itself

  • Migration also promotes the geographic dispersal of birds

  • Might lead to geographic isolation and increased rates of speciation

  • Seen through the lens of life history strategies, migration is a trade off

  • Life history strategies balance survival and reproduction

  • Life history strategies represent the pattern of age-specific birth and mortality of a population or species

  • Every individual organism has a finite amount of time and energy

  • Allocating any part of that energy to any structure or function means there is less energy for other things

  • Every choice the organism makes on how to allocate that limited energy involves a trade-off, affecting all other choices

  • Reproduction, for example, carries a huge energetic cost, especially for the female

  • All of these various life history strategies represent a compromise between conflicting demands

  • Represent a choice about how to allocate limited time and resources, how to balance reproduction and survival

  • Migratory birds achieve a balance between the strategies of resident birds

  • Higher reproductive productivity than tropical birds, but lower survival

  • Lower productivity than temperate resident birds, higher overall survival rate (escaping the worst of the winter weather)

  • Tropical birds achieve a different balance between survival and reproduction

  • Smaller clutch sizes, higher rates of nest failure, higher juvenile mortality

  • But adults live longer, can attempt to nest several times during the year

  • None of these hypotheses has been decisively proven…

  • Most likely that migrants are taking advantage of the superabundant seasonal food supplies in Spring and Summer in the northern latitudes

  • Perhaps all of these hypotheses are involved to some degree…

  • Given the high cost and danger of migration, ecological payoffs must be substantial for such a behavior to have evolved in the first place

  • Over 200 North American species, about 5 billion birds, leave each year for that ultimate road trip to the southern sunshine

  • The same number leave Eurasia each year to follow the summer sun to Africa

  • Not all birds migrate…

  • Even among migratory species there are non-migratory local populations/races

  • Chinese House Sparrows migrate, but American House Sparrows stay put

  • European Starlings are year-round residents in Great Britain, but Central European populations are migratory

  • North American migrants follow north/south migration routes, along four major paths or flyways:

  • Atlantic Flyway

  • Mississippi Flyway

  • Central Flyway

  • Pacific Flyway

  • Routes are an accident of geography

  • Follow the north/south alignment of:

  • Major river systems (Mississippi)

  • Major mountain ranges (Appalachians, Rockies, Sierra Nevada)

  • In Eurasia major mountains and rivers follow a more east/west axis (Alps, Mediterranean Sea, North African deserts),

  • Many species migrate east to west, although they still end up much farther south than when they started

  • Their migration axis is also skewed because their destination, the continent of Africa, is relatively displaced to the west of the Eurasia landmass

  • This influence of geography on migration is especially interesting in light of Jared Diamond’s Guns, Germs and Steel: the Fates of Human Societies (Norton 1979)

  • Ties major differences in human cultural development in the New World versus the Old World to same geographic influences

  • Traditionally, the best way to study the migratory habits of birds has been to band them with aluminum leg bands, and try to recover banded birds

  • Recovery rates are extremely low, usually less than one percent

  • For species that migrate by day, and whose routes are funneled through a narrow area, observers can simply count the passing birds

  • Like the hawks that pass over Hawk Mountain, Pennsylvania every year

  • Since World War II, radar has been used as a tracking tool for migrating flocks

  • Now have miniature radio transmitters small enough to attach to migrating birds, can then track from the ground or by plane

  • Migration patterns are complex, may vary within species, with different races or even different sexes migrating to different areas

  • The Fox Sparrow has six distinct races along the Pacific coast

  • These races would compete with one another if they were forced to share the same wintering grounds

  • They leapfrog over one another in migration, farthest north flies farthest south

  • In many species of passerines, males arrive at the breeding grounds before the females

  • Male Red-winged Blackbirds arrive up to five weeks before the females

  • Male Purple Martins also arrive earlier, seeking out the best sites for their communal nests

  • In many species, males, females, and juveniles may migrate to different latitudes

  • Males generally migrate farther north, presumably because their larger body size makes them more tolerant to cold

  • In species where the female is larger, it is the females who migrate farther north

  • Environmental conditions, such as weather and food supply seem to act as a proximate cue for migration

  • But the ultimate cue for migratory readiness probably comes from photoperiod or day length

  • Experiments with White-crowned Sparrows suggest that adrenocortical hormones and the pituitary hormone prolactin are involved in the physiological control of migration

  • Our basic knowledge of migratory control, however, is sadly lacking

  • Migratory birds show a characteristic pre-migratory nocturnal restlessness, called Zugunruhe

  • Caged birds will sleep briefly after sunset, then wake up, hop and flutter around with increasing energy, until about midnight when they start to settle down

  • Some birds, like White-crowned Sparrows also show compass orientation

  • During Zugunruhe, sparrows kept in circular cages oriented toward the South during Fall migration, and toward the North during Spring migration

  • Bramblings kept in cages under artificially shortened days never develop Zugunruhe

  • Cross-fostering experiments (Harris 1970) determined there are both learned and innate components to migratory behavior

  • Harris switched 900 young between non-migratory Herring Gulls (Larus argentatus) and migratory Lesser Black-backed Gulls (L. fuscus)

  • Band recoveries showed many of the non-migratory argentatus now followed their new parents to France and Spain, suggesting learned behavior

  • The migratory fuscus gulls, on the other hand, showed an innate tendency to migrate, and left their foster parents behind

  • North American birds winter in the Caribbean, central and South America

  • Some birds stop along the Gulf Coast in Louisiana, Texas and Florida for the winter

  • Birds typically fly hundreds of miles a day on their epic migratory journeys

  • Many of our local birds, like the Yellow-crowned Night Heron, leave Louisiana for the Caribbean and northern South America

  • The Arctic Tern flies from its breeding grounds in the Arctic as far south as the Antarctic, a journey of up to 10-15,500 miles

  • The tiny little Blackpoll Warbler flies nonstop every autumn from New England to the Lesser Antilles and Venezuela

  • Other species, like Clark's Nutcracker, may migrate only a few miles to lower elevations in their native mountain ranges

  • Most migrants fly at night, especially smaller passerine species, resting and foraging on land during the day

  • Many species fly non-stop, especially birds like swifts and swallows that can fly by day and feed on the wing

  • Others take comfort where they can find it...

  • Of the North American passerines that set out, only half ever return

  • Among the ducks and other waterfowl, the toll reaching 60% or more

  • For those who survive nature’s gauntlet, humans add further perils…

  • Good weather with favorable winds can start birds moving

  • Bad weather can quickly exhaust migrating birds, and blow them thousands of miles off course

  • When birds crossing the Gulf of Mexico meet an advancing cold front from the North, a fallout can occur

  • Tens of thousands of migrants settle on the first coastal landfall to wait for better conditions to finish their voyage

  • Flying such great distances day after day uses incredible amounts of energy

  • Williams and Williams (1978) concluded that the equivalent physical demand for a human would be running a four-minute mile for 80 hours!

  • For five species migrating across the Sahara Desert, Ash (1969) found an average weight loss of 26 - 44%

  • To pump themselves up for such an energy draining flight, birds build up extensive reserves of fat just before departure

  • Bobolinks add 50% of their own body weight in stored fat prior to migration

  • Laboratory tests on the Ruby-throated Hummingbird showed that a 4.5g bird, having 2g of body fat, could fly nonstop for 26 hours at an average of 40 km/hr

  • Gives it an effective range of about 1,050 km, enough to cross the Gulf of Mexico (Lasiewski 1962)

  • Black-necked grebes (= eared grebe - Podiceps nigricollis) reinvent their bodies in preparation for migration

  • Balance of digestive tissue versus flight tissue shifts back and forth, in response to seasonal demands (Templeton 2005)

  • To help fatten up on brine shrimp in Mono Lake, the grebes:

  • Double their weight prior to migration

  • Double the size of their digestive organs

  • Shrink pectoral muscles by half (already flightless from molt of primaries)

  • 2 -3 weeks prior to migration the grebes:

  • Shrink digestive organs to 1/3d former mass

  • Reduce leg muscle volume

  • Increase heart size

  • Double size of the pectoralis flight muscle (returns to its original size)

  • Migrants returning across the Gulf of Mexico often bypass southern Louisiana, presumably to replenish their fat reserves at richer foraging grounds farther to the North

  • This local migratory phenomenon, a subject of some controversy, is called the coastal hiatus



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