Lumen 22-6

Bird Navigation, or Should We Make a Left Turn Here?

 

Posted – July 25, 2022

 

Irrelevant tid bit – They knew many things but had no idea why.  And strangely this made them more, rather than less, certain they were right.

                Neal Stephenson        “Anathem”

 

 

I can find the airport, with help, but birds migrate thousands of miles all on their own.

 

During migration, usually to and from breeding and non-breeding sites, birds have to deal with dehydration, rapid metabolism to get energy (convert fat to energy), oxygen supply (RBC and lung size increase), and lack of sleep.  All serious issues, but this Lumen focuses on navigation.  How do the birds know where to go?  Frequently young birds migrate separately from the adults, so no guidance from birds who have made the trip before.

 

Everywhere I looked for information on migration, I found the story of the Bar-tailed Godwit (Limosa lapponica), a long legged shorebird with a long, slightly upturned bill.  Traveling as

Bar-tailed Godwit

individuals or in flocks, and some flocks are entirely first-year birds, they fly 7,200 miles nonstop from their breeding site in western Alaska to their non-breeding site in New Zealand in September – a trip of 8.1-11 days across the open Pacific Ocean.  Before leaving on migration, they double their weight in 2 weeks: a 1.5 lb. bird has more the 10 oz of fat under their skin and in their body cavity.  Digestive organs such as the intestines and gizzard shrink and atrophy, and pectoral (flight) muscles and heart muscles double in mass and lungs increase capacity.  After arriving at the non-breeding grounds in New Zealand and Australia, they regrow the digestive system.  With the return of spring in the north they repeat the weight gain and digestive system atrophy to fly 6,000 uninterrupted miles to China and Korea in 8-9 days.  There they regrow the digestive system to enable them to eat and gain weight for the flight of 4,000 uninterrupted miles back to their Alaska breeding grounds in 5 days.  What an amazing feat every year.

 

I will also mention another remarkable migration by the Blackpoll Warbler (Setophaga striata), that weighs less than an ounce.  Some of these birds after nesting near Nome, Alaska

Male Blackpoll Warbler

fly across North America to New England where they jump off over the Atlantic Ocean for a 2,000 mile nonstop flight to their wintering area in South America, a 3 day trip.  This tiny bird makes this flight alone, not as part of a flock, meaning the young-of-the-year have no help finding the way.

 

To navigate birds need multiscale and multisensory cue integration in the brain (what a great sentence, wish I could remember where I stole it from).  Basically, it means that birds need more than one tool to not get lost.

 

How do we go about learning the secrets of bird migration?

 

            RESEARCH TOOLS

1.  Bird banding – One of the oldest tools, putting a numbered band on the bird’s leg.  To learn where the birds go and how long it takes them to get there, the birds have to be recaptured.  With song birds the recovery rates are very low, but for waterfowl, who are intensively hunted, much was learned by hunters reporting bands on harvested birds.

 

2.  Geolocators – These devices are attached to birds and records where they go and when they were there.  However, they have to be recaptured to access the data.  This works reasonably well when working with a breeding population where many birds return to their previous nesting location, making recapture easier.

 

3.  Radio transmitters – Small transmitters are attached to birds, which are relocated using antennas tuned to the right frequency.  This technique can track many birds in a population, but the limited range of the transmitters requires long hours in the field using directional antennas to locate birds.  This works well for studies of populations in a relatively small area.

 

4.  Satellite transmitters – These operate on the same principle as radio transmitters, except that the transmitters are strong enough to broadcast to satellites.  This is expensive but very good for tracking the long distance migrations of birds big enough to support these larger transmitter units.

 

5.  Weather radar – Radar (including Doppler radar) used to predict weather can also locate, track, and count migrating birds.  Because of the large number of weather radars in the U.S., this is a useful tool for learning about regional migration flight corridors, stopover sites, timing, and numbers.

 

6. Motus - The Motus Wildlife Tracking System is an international collaborative network of more than 750 coordinated receiving stations (towers) primarily across North America that pick up signals from radio tagged birds.  A small transmitter on a small bird could pass close to many towers during migration.  Your tagged bird could be recorded on any of the towers, and these data are shared among the cooperators.  This can record details of routes, stopover habitats, and the length of stops.  A relatively new and exciting tool not fully developed yet.

7.  Laboratory studies that mimic natural circumstance to see how the birds respond to changes in weather, sun light, star positions, and other natural cues.

 

 

BIRD NAVIGATION TOOLS

What have we learned from all these research tools?  I repeat, “To navigate birds need multiscale and multisensory cue integration in the brain.”  Birds likely use one tool for local navigation, where they are familiar with landmarks, and another set of tools for crossing continents or oceans.

 

1.  Smell -  Volatile chemicals can remain remarkably stable and repeatable across hundreds of miles.

 

2.  Landmarks/landscape – Mountains, rivers, coast lines, etc.

 

3.  Sun – Movement of sun and concurrent shifts in bands of polarized light (invisible to us but birds can see them).

 

4.  Stars - Not the star’s position in the sky but its apparent lack of rotation around Polaris (the North Star).

 

5.  Quantum Avian Compass – We have long known that birds have iron crystals in their beak, which were thought to interact with the Earth’s magnetic field in some way to create a compass.  We now know that these crystals are not magnetite, but macrophages, a type of white blood cell active in supporting the immune system that has nothing to do with magnetic sensing.  But birds can detect the Earth’s magnetic field, and the search is on to identify the mechanism.  The leading hypothesis centers on radical pairs – magnetically sensitive chemical intermediates formed by photoexcitation of cytochrome proteins in the retina (wow, another sentence for the record book.)

 

Birds’ compass does not work through polarity (finding the direction to the north pole), but instead they detect the inclination of the geomagnetic field lines.  Inclination is the angle between the magnetic fields lines and the horizontal plane.  At the poles the angle is 90 degrees and 0 degrees at the equator.

 

Light striking cytochromes in the eyes causes electrons to form radical pairs.  Radical pairs have a property called spin (they do not spin, that is just the name of this property), in singlet and triplet states.  The inclination of the Earth’s magnetic field influences the ratio of these two states, creating chemical products that create a detectable signal that the birds use to recognize the way to travel.

 

If you want a clearer or more detailed explanation of this process, you will have to ask someone else.

 

In closing I want to tell one more interesting migration story – despite a long annual migration,  Only a few sites can be very important to a bird.  The Whimbrel (Numenius

Whimbrel

phaeopus) (another long legged wading bird but with a down turned bill) is a long distance migrant that generally nests in the Arctic and winters in the tropics.  One bird that was followed for 9 years always stopped in mid migration at Boxtree Creek in coastal Virginia; nested in same part of the Mackenzie River delta in Northwest Territories, Canada; and wintered at Great Pond, St. Croix, U.S. Virgin Islands.  Together these three sites – a nesting site, a wintering site, and a migration stopover site – included only about 500 acres along an annual migration route of about 18,000 miles.  Over this long trip, only a few small spots are important to the Whimbrel. 

 

 

Bird migration is amazing, fun to watch, and not nearly understood.  Get out and see it this fall or next spring.