Tuesday, September 20, 2011

Fall migration

Nocturnal bird migration as seen on radar

That's right. Fall migration. Happens every year at this time.  Millions of birds heading to the tropics for the winter. Some of them for the summer. The summer you ask? Is that right? Yes it is! Some of our little warblers migrate far enough south that their wintering grounds are in effect a second summering ground. In other words, the birds experience not only boreal summer, but austral summer as well.

Every night during Sept and Oct, these birds lift off from wherever they spent the day refueling and fly, typically through the night, and land in a new location, further south in the early hours of the morning.
These nightly flights are in such numbers that they can actually be seen on radar.

Last night was no exception. In fact, last night was one of the strongest flights yet this fall. It showed on the radar as well.  It was almost solid green from Duluth to Brownsville.

So how does one tell birds from weather? It's pretty easy. Weather moves in straight lines. Always.
Weather also produces much higher intensity (reds, blues, orange, etc) than birds usually do.  Birds always produce a "donut" or circle around a radar transceiver.

Here's the way the radar works: radar sends out a radio signal that travels at the speed of light. This radio signal bounces off whatever objects are in its way and returns directly to the receiver at the tower. The time difference between when the signal is sent and the signal is received, determines how fast and what direction the object is traveling.  The difference in frequency determines the cross-section, or how large the object is. This also determines the intensity of the signal that is displayed on the weather website.  Dark green is a weak intensity, light green is more intense, yellow is higher intensity, etc.

On nights with heavy flights of birds, the sheer number of birds in the sky typically produces light greens to yellows on the radar display.  Nights like this usually have hundreds of thousands of birds migrating all at once.  The coolest thing, is that all passerines call during nocturnal migration. Everything from chips to "tseeps" to "tsips", peeps and "sips" can be heard from birds passing over at night.

Cloudy nights are typically better for hearing migrants because the clouds keep the birds flying close to the ground. On clear nights, they fly high up and are more difficult to hear over all the crickets.

A CD of flight calls can be purchased from oldbird.org that contains all of the flight calls of most passerine migrants. If you memorize these, you'll be able to identify the birds that pass over your house at night.

Good luck out there! Happy nocturnal birding!

Friday, September 16, 2011

The Bald Eagle: The majestic symbol of freedom

adult Bald Eagle

If you haven't heard already, I've been working for Mississippi Explorer Cruises as an Interpretive Naturalist this summer.  It's been a fun job. I get to talk to lots of people and I get to tell them all about the natural and cultural history of the ports we go out of.   I also spent a lot of time scanning the banks looking for wildlife. So far, I've seen just about everything you can see on the river. A few weeks ago, we saw a lone Beaver. A week before that, we saw a pair of River Otters playing on the bank. Deer are plentiful on the islands and are often seen swimming between them.  We even occasionally see fish, either swimming just below the surface or jumping clear of the water.

At the start of our cruises, people often ask what we have a chance of seeing and a lot of people ask about Beaver or Otter, but the number one thing people ask if we're going to see is Eagles.

Now, if you know anything about birding on the Mississippi River, you know that the answer is always yes. We almost always see eagles during our cruises.

In fact, Eagles have rebounded quite well since the pesticide DDT was banned in the 1970s.
In the 1960s, their population hit an all time low and the species was placed on the Federal Endangered Species list. Since then, through a number of acts passed by congress, their population has steadily increased again.  In 2007, their population had increased sufficiently that they were removed from the Endangered Species list.  This year's population estimate for Pool 9 on the Upper Mississippi River at Genoa was 100 pairs.

The Mississippi River has long been one of the strongholds for this species, with its plentiful supply of fish. Although, at their low point, the species almost disappeared from the river valley.

Fortunately, Alaska, the primary stronghold for these birds, due to its remoteness was never very highly assaulted by DDT and they remained there in decent numbers throughout the 1900s.  Today, over half of the world's 9,700 breeding pairs of Bald Eagles lives in Alaska.

Here on the Mississippi River, every winter, we enjoy Bald Eagles in their hundreds that gather around the open water below the dams and at warm water vents.  In many places, you can stand in one spot and count over 200 eagles just by turning in a circle.

Today, Bald Eagles enjoy federal protection in many forms.  They are protected under the federal Migratory Bird Treaty Act of the 1918, the Bald and Golden Eagle protection act of 1940. They are also protected as our national bird and part of the symbol on our national seal.

Bald Eagles are known as builders of some of the largest nests in the bird world.
One of the largest nests on record measured over 9 ft across and weighed in around two tons. Most nests, however, a much smaller, measuring around 4-6ft across and weighing around half a ton.

They begin the nesting season in January, adding to and "remodeling" the same nest they used in previous years, every year making it a little bigger.  Here in WI, the female lays 2-4 eggs sometime between late Feb and mid March.  The young hatch roughly a month later, and fledge around the 4th of July.  Once they leave the nest, the young are tended by the parents until they are able to fend for themselves, at which point they begin the long process of learning to fish. It's quite amusing to watch a young eagle attempting to fish. They are quite clumsy and sometimes quite pathetic at it.  They learn quickly however and soon become master hunters.  Bald Eagles reach maturity at around 3-5 years old, at which time they gain the diagnostic white head and tail.

Interestingly,  at the time they leave the nest, the grown young are often bigger than their parents. The reason is because of the extra baby fat they still retain and the down feathers that they haven't lost yet.
A recently fledged young Bald Eagle can have as many as 8 or 9000 feathers.  A typical adult has around 7000 feathers.

In 1782, the Bald Eagle was selected as our National Emblem because of it's long life (nearly 30 years), great strength and majestic looks.  Their range covers the entire North America continent from Alaska to Florida, from Canada to Mexico. They are perhaps the single most well known bird in the world, and the one that everyone loves to see.

They are truly a majestic bird, they sit at the top of the food chain and are a symbol of strength and fearlessness.  Hopefully they will be around for many centuries to come.

"May the wind under your wings bear you where the moon walks and the sun sails"
--J.R.R Tolkien