August 26, 2005
The Need for Migration
Nest Update: We saw Lightning bring in his first fish on Friday. After landing he tried to keep the fish away from Thunder, and while his efforts worked for a while, Thunder eventually came over and took what was left.
One of our cam watchers pointed out that this now means the ospreys are appropriately named because as Mark Twain said, "Thunder is good, thunder is impressive; but it is lightning that does the work." :-)
Now that Lightning is bringing in food, we hope he will soon learn that sometimes it's better to eat his food in a tree rather than bring it home to where his hungry sister is waiting for him.
Besides seeing Lightning fly in with a meal, we also saw a nice photo of the chicks watching their neighbors fly by -- this was a neat shot.
And we had something interesting happen on Thursday, in that some turkey vultures visited the nest while the chicks were out. Turkey vultures are also raptors, and they love to scavenge the leftover fish pieces the ospreys leave in the nest. Although the vultures are eagle-sized, they would never directly challenge the ospreys for the nest, as ospreys -- with their sharp talons and beaks -- would not be an easy match.
To see the vultures in more detail, click on the thumbnail to see a great photo of two turkey vultures residing at another Maryland refuge -- Eastern Neck NWR. Turkey vultures are carrion eaters, and they often place their heads inside carcasses to feed, so their heads are featherless to eliminate the chance of bacteria growth and disease. You can learn more about turkey vultures here.
Regarding our osprey chicks, we've had a couple cam watchers ask if there is any chance that Lightning -- our reluctant flyer -- would stay for the winter rather than migrate. While it's likely that Lightning will migrate a little late -- since the family nested late -- we do know that both chicks will eventually migrate.
First, it's important to point out that not all ospreys have to migrate. Those that live in milder climates, such as Florida, Mexico, and the Caribbean, will not migrate; but most ospreys from North America, Europe, and northern Asia will migrate south in the fall (ospreys from North America generally go to Mexico, Central America, and South America, and ospreys from Europe and northern Asia generally go to Africa, India and southeast Asia).
The ospreys at Blackwater have to migrate, and the main reason they will migrate is food. Ospreys could likely survive the winter temperatures in the Chesapeake Bay area -- our ospreys' current home -- but the cold weather would either drive the fish too far underwater for the ospreys to reach or would freeze over the water completely. Check out this photo to see what the Blackwater River can look like in the winter.
Ultimately, our osprey fledglings will be driven to leave because of instinct. Those ospreys that are born in regions where migration is necessary have a built-in instinct to move south to the area where their parents have gone before them. The chicks may not go to the exact place where their parents have gone, but they'll be in the same general region.
A natural question at this point might be: Since it's warm down south and the ospreys can fish all year, why don't they just stay there during breeding season? The answer is because the southern wintering grounds do not have the space and resources required to support all breeding ospreys. Many raptors need space when raising a family (some need a LOT of space) and spreading the ospreys around the world means they're not in competition with large numbers of fellow ospreys looking for the same food sources and choice nesting sites during the crucial breeding season.
As for knowing when to leave, current research indicates that birds have a genetic component that tells them when they should leave and what direction they should travel. The sense of "when to go" can be triggered by several different factors, but is most likely triggered by the amount of sunlight -- known as the photoperiod. The photoperiod triggers other aspects of a bird's life as well, such as feather molting and production of sexual hormones, so the photoperiod is like an external clock that is in sync with the bird's internal biological clock.
While it's true that Blackwater Refuge is beginning to lose its osprey population as fall approaches, that doesn't mean that the Refuge will be losing all its birds during the cooler months.
Blackwater Refuge was created in 1933 as a refuge for migratory waterfowl. We are a major stop on the Atlantic Flyway, which is one of the avian "highways" that birds use during migration. Many types of waterfowl will either stop here for the winter or refuel here before heading further south. So while the ospreys will be heading out, many more birds will be coming in to take their place.
If you've ever thought about visiting Blackwater Refuge during the cooler, less buggy months, November and December are the months when the duck, swan, and geese populations are at their highest. It's also a great time to see bald eagles because during the cooler months, we host a large number of migrating eagles from other states. Who knows, maybe even Porthos -- our Eagle Cam eaglet that went to Vermont -- will come down here for part of her winter.
Until next time,
Lisa - webmaster