Sunday, September 26, 2010

Migrating Monarch Butterfly Spectacle -- not just in Mexico!

A monarch butterfly, Danaus plexippus.

I was in Cape May Point, New Jersey, on Saturday, September 18th, expecting to see migrating hawks. Many birds follow the Atlantic shoreline south during autumn migration. As New Jersey gradually narrows into a tip at its southern end, migrants end up concentrated at the “point” with the Delaware Bay in front of them -- across their path. They sometimes wait a day or two for a north wind to help them across the water. The place can fill up with eagles, hawks, and songbirds. It’s a great place for autumn bird watching.

But this year butterflies stole the show. Monarch butterflies also get funneled to New Jersey’s point. On Saturday there may have been a half a million big orange and black butterflies in the tiny town of Cape May. Butterflies filled the sky. They hung from the flowers and trees. It looked like the town had been transformed into a Walt Disney movie. 

Gathering in a roost for the night. 

On Sunday morning, the wind was just right. The butterflies headed out in orange clouds across the bay. They passed by for hours. It was spectacular.

They are going to Mexico. As unlikely as it sounds, each tiny individual from the summer’s last generation flies the whole distance or dies trying. For some that is a distance of over 3000 miles. They can fly at about 12 miles per hour, for about 50 miles per day.

Once they get to Mexico, they spend the winter in a dormant state in chilly mountain refuges. The migrating generation lives about seven months, which is much longer than the few weeks typical for monarchs born in spring or summer. The migratory generation doesn’t finish developing sexually until the spring following migration. Then they rouse, mate, and start coming back north.

But they don’t have to fly all the way back. The return flight is generational. Individuals fly part of the distance, lay eggs on milkweed plants, and then die. The eggs hatch into caterpillars that eventually become adults that fly further north to lay eggs of their own. After a few generations, each traveling further north, the continent is repopulated all the way to southern Canada. Before you know it, a migratory generation is born and it’s time to migrate again.

Populations from eastern North America fly to Mexico. The western ones tend to end up in southern California.  In cities all over the continent we can glance out of our office windows, even on high floors, and see the big orange butterflies passing by. The last of them will go by around Halloween; they have to beat the first frost. 

There is more information about urban insects in my book,  Field Guide to Urban Wildlife, which will be published by Stackpole Books in spring 2011. 

Sunday, September 19, 2010

Sparkling Birds

I took this photo of the World Trade Center Light Memorial this year on 9/11. The normally still columns were filled with thousands of shifting silver dots of light that glittered as they moved. At the time I didn’t know what they were.

They turned out to be migrating birds!

Hundreds of thousands of birds fly over New York City each autumn on their way to their southern winter homes. Light from the sun, moon, and stars helps them navigate. But artificial lights can interfere with their natural abilities to find their way. Unlike most autumn migrants, the birds that flew down Broadway on the night of 9/11 did not pass overhead unseen; instead they flew into the memorial lights. They became disoriented and stayed there, circling, illuminated like silver sparks. 

The lights were turned off five times during the night for 20-minute periods to allow the birds to fly away.

But migrating birds are still in danger. Brightly lit buildings confuse them too; many birds die every year when they fly into office building windows. The phenomenon is gaining recognition and famous sites like the Empire State Building and the Chrysler Building dim their lights from midnight until dawn during fall migration. It saves a lot of birds. Let us hope that it catches on.

You can read more about it here:  'Lights out' help migratory birds

There is more information about urban birds in my book,  Field Guide to Urban Wildlife, which will be published by Stackpole Books in spring 2011. 

Sunday, September 12, 2010

The Buckeyes are Leaving

The buckeye butterfly, Junonia coenia

Buckeye butterflies are passing through New York, heading south. I keep seeing them in the open sunny places that they like -- lawns and parks, roadsides, empty lots, and fields of weeds.

They are heading to the warmer places where they live year round like the southern United States, Mexico, and the Caribbean. Every spring some of them return to repopulate the northern United States and parts of Canada. Their southbound autumn flight is particularly noticeable along the coasts where at times they seem to be everywhere.

Adult buckeyes are about two and a half inches across. They are brown and tan, with a pair of orange bars on each forewing, and a series of large “eyespots” that may scare off predators when the wings open suddenly to reveal them.  (Oh no! Says the attacking bird -- some giant thing is looking right at me!) The caterpillars vary from dark greenish to gray or black with orange and yellow lines and rows of fancy spikes.

The buckeye caterpillar -- slim and stylish. 
 The butterflies sip nectar from asters, chicory and peppermint flowers, and some others. The caterpillars are especially fond of the foliage of plantains, snapdragons and toadflax.

Most of us are familiar with sexually dimorphic species, like lions or cardinals, where males and females look different. But buckeye butterflies appear in different forms in different seasons. The buckeye’s wings are tan underneath in spring, but red underneath in autumn; the change is probably linked to lower temperatures and shorter days. The difference is so pronounced that scientists call the forms by different names -- linea for the tan one and rosa for the other. 

Saturday, September 4, 2010

Don't touch the parsley worms!

Black swallowtail caterpillars are called parsley worms because they are commonly found on plants in the parsley family, Apiaceae, which includes carrots, fennel, dill, hemlock, Queen Anne’s lace, parsnips, anise, and coriander. I found this caterpillar in the Shakespeare Garden in New York City’s Central Park where it was sharing a fennel plant with  two other parsley worms and a host of wasps and bees.

The black swallowtail caterpillar, Papilio polyxenes.

It is easy to see some features of caterpillar anatomy on a fat three-inch long specimen like this. It has three pairs of tiny true legs near its head, on the right. The four pairs of feet in the middle and the pair at the rear end are called prolegs; they have rows of hooks called crochets on the tips that help them hang on. But one of its most interesting features is usually hidden.

It has an osmeterium, also called a “stink gland,” which it can use to startle and repel would-be attackers. (Other kinds of swallowtail caterpillars have them too.) When I gently tapped this caterpillar, it instantly reared up on its proglegs, stuck out his bright yellow osmeterium, turned to touch me with it, and just as quickly pulled it back inside its slot above the face.

The caterpillar's forked yellow osmeterium is usually hidden. 

The bad smell this left on my finger was impressive. The smell is usually described as that of rancid butter, but it is an angry insect version of rancid butter that seems much worse to me. So I learned a lesson; don’t touch the parsley worms. And if I were a caterpillar predator I would definitely look for a better-smelling meal. 

There is more information about urban insects in my book,  Field Guide to Urban Wildlife, which will be published by Stackpole Books in spring 2011.