NWA 9/20/13

A Northwoods Almanac for 9/20 – 10/3/2013

First Significant Migration of Broad-winged Hawks

If the autumn winds are right, Hawk Ridge Nature Reserve in Duluth is THE site in the Midwest to observe migrating raptors that originate from far northern summer breeding areas and which winter as far south as South America. Why Duluth? Most raptors are reluctant to cross large bodies of water, so when they encounter the largest freshwater lake in the world, Lake Superior, the birds naturally veer southwest along the lakeshore. Funneled along the western tip of Lake Superior, they concentrate in sometimes extraordinary numbers on the bluffs overlooking East Duluth and can be easily seen from the overlook at Hawk Ridge.

The raptor count that took place on 9/11/13 at Hawk Ridge in Duluth totaled1838 raptors including 1,185 broad-wings, 41 kestrels, and 510 sharp-shins. The 9/12 count was even better with 7,228 broad-wings among the 7,833 total raptors! But while the 9/11 raptor count was a good number of birds, the bigger news was the 6892 migrating non-raptors that flew over the ridge, including among others 271 Canada geese, 79 common nighthawks, 1,349 blue jays, 1,496 cedar waxwings, 107 rose-breasted grosbeaks (a new state record count), 86 bobolinks, and 261 purple finches. Additionally, there were hundreds of Swainson’s thrushes heard moving overhead before dawn.

Between 8/15 and 9/7, an amazing season total of 79,825 non-raptors were counted, a much higher total than normal for this period. Most of that number was due to an incredible August flight of 42,916 common nighthawks, with 30,874 being seen just on August  21st alone (the second-highest state count ever). The second most common migrant has been cedar waxwings, with 25,383 counted so far, including over 6,000 waxwings seen on two days in a row, August 20 and 21. The counters also observed a single flock of 156 American white pelicans swirling in unison on September 3, and a single flock of 56 red-necked phalaropes landing on a completely calm Lake Superior on August 20.

Banding of raptors also takes place on Hawk Ridge, but August numbers were just average with just 85 birds banded. The September numbers have shown some improvement, but are still lower than average likely due to the weather: Duluth had many days of extremely hot weather, followed by cold, moist days with east winds which discouraged migration.

The North Shore of Lake Superior is one of the premier migratory routes for raptors and passerines in North America. Fall migration counts of raptors and passerines at Hawk Ridge and during a recent North Shore study are estimated to be in the hundreds of thousands to millions. Reasons for this concentration of birds along the North Shore are many. As migrating birds fly south they seek out cover and food along their routes, neither of which Lake Superior provides.  If birds approach the Lake during daylight hours they change their course to follow the shoreline.  If birds that fly during the nighttime hours find themselves over the lake at dawn, they reorient to the nearest shoreline.  This movement coupled with the prominent ridgelines that parallel Lake Superior that act as funnels, cause a massive congregation of birds within the Lake Superior coastal region.

One particular variable has also stood out in importance: closeness to the shoreline itself.  Data collected at three perpendicular distances from shore has shown that migrating birds, especially nonraptors (mainly songbirds), are concentrating within 1 mile of the Lake Superior coastline.

Mid-to-late September is primetime for the largest hawk flights of the year. Watch the weather, and if a west or north wind is forecast with no rain, I recommend you get to Duluth’s Hawk Ridge as fast as you can – you might just catch one of the days when tens of thousands of raptors are flying over.

Upside-down Hummers

Diane Steele sent me this email along with two photos of a hummingbird hanging upside from her feeder: “This is the first time I've ever witnessed this hummingbird behavior. It must be a young one? It stayed upright at the feeder for a long time, and would go into its acrobatic upside-down routine whenever another hummer came around. Was it acting submissive? Wanting to be fed? The other hummers tended to buzz around it once or twice and then ignore it.”

Well, the explanation I’ve found doesn’t quite fit Diane’s scenario, but it does fit scenarios I’ve been told about by other folks. When hummingbirds sleep, or when temperatures drop to near freezing, they go into a hibernation-like state called torpor. Their metabolism slows to one-fifteenth of normal, their body temperature drops to nearly hypothermic, and their heart rate drops to about 50 beats per minute (down form an average 500 beats per minute). Their breathing also slows to the point that it looks like they have stopped breathing, all of which saves up to 60% of their available energy.

When hummingbirds are in the torpor state, they occasionally hang upside-down and may even appear to be dead. It then takes anywhere from 20 minutes to an hour for a hummer to fully recover from torpor, but once they are fully recovered from torpor, they immediately seek out food, eating 25% of their daily intake.

This explanation works fine for sleep and cold weather, but Diane’s observation indicates an active process of hanging upside down whenever another hummer approached. Any thoughts on this from readers out there?

Sightings

On 9/9, Kathy and John Jolin in Minocqua observed an evening grosbeak at their feeder. Given that this date seems too early for any wintering movement of evening grosbeaks, I suspect they have a nesting pair somewhere in the area. We are at the southernmost edge of their breeding range, so they are quite uncommon here but not rare.

Linda Johnson sent me a photo of what she thought might be some sort of sphinx moth caterpillar, and noted, “There were 8-10 on the fireweed in our yard.” She was right – they appear to a hornworm/hawkmoth called the Galium sphinx (Hyles gallii).

Missy Drake on Round Lake sent me a photo of a couple of Jell-O-like blobs and asked, “Just wondering if you can identify this disgusting thing growing on our lake ladder. I'm so glad I never stepped on it!” Her blobs are bryozoa, each one an extremely odd, but fascinating colony of “moss-animals” in the animal phylum Ectoprocta, a group whose fossil record extends back to the Upper Cambrian, 500 million years ago.

            They are very strange looking things, resembling a stiff, clear-gray Jell-O that one could easily imagine as a giant brain. The gelatinous colony is 99 percent water, but firm and slimy to the touch. The surface appears divided into tiny rosettes, each with 12 to 18 “zooids.” Each microscopic zooid has whorls of delicate feeding tentacles that sway slowly in the water and capture food, feeding on small microorganisms, including diatoms and other unicellular algae. In turn, bryozoans are preyed on by grazing organisms such as fish, and are also subject to competition and overgrowth from sponges and algae.

            Massive colonies may exceed 2 feet in diameter, although typical sizes are 1 foot or less. The colonies form on submerged logs, twigs, even wooden docks. They’re harmless and usually indicate good water quality.

Celestial Events

            Summer officially ends on 9/22, the autumn equinox, as daylight and darkness achieve equal presence and the sun crosses the equator into the southern celestial hemisphere. The sun will rise nearly due east and set nearly due west, and then on 9/26, we’ll experience our first night longer than the day since March 16.