August 21, 2006
Osprey Navigation
Nest Update: We had some interesting photos on the nest over the last couple days that seemed to show two adult male ospreys on the platform. The bird nearest the camera might have been a juvenile, but we did not see the tan-colored edges on its feathers. Both birds had white chests and looked smaller than a female, which would indicate they were male.
Normally an adult male would not look kindly on another adult male "hanging out" at the home nest, yet our father osprey seemed to be okay with it. So if both birds really were adult males, what does this mean? I'm not really sure, but one guess could be that the other adult male is one of the father's offspring. When mature offspring return to the breeding grounds, sometimes the parents will allow them on the nest, although there have also been reports of parents chasing off their mature young as if they were a threat. So it's hard to say what this was, but it was an interesting surprise for so late in the season.
In addition to the two male birds, we've seen several shots of what appeared to be the mother osprey, so for now she's still in town although she'll likely head south first. As for our young -- Hunter, Skye Diver, Wind Runner, and Cloud Dancer -- we haven't seen much of them recently, and we continue to be impressed with how independent they became so soon after fledging. The only downside to this independence is that it means they aren't at the cam nest much.
For those who missed my earlier post on the subject, we would expect the whole family to be gone by around the second week of September. A sure sign that the father and last young have migrated is if we see a bald eagle on the platform -- which means the osprey family is no longer defending the nest.
A cam watcher asked me about when the bald eagles will retake the Eagle Cam nest. The eagles will begin putting in regular appearances at the Eagle Cam nest around the beginning of December, although eggs will not appear until the third or fourth week of January. We're working on a new camera housing for the Eagle Cam with the hope that it will prevent the eagles from knocking the camera out of position as they've done before. During our first two seasons with the Eagle Cam, we've watched five eaglets grow and fledge, and it's been a great experience for everyone involved, so be sure to check back with us in December for another year with our fantastic eagle parents.
How Do Ospreys Navigate
In the last couple web logs, we've talked about why ospreys migrate and where they might go when they leave for Central or South America. In this next-to-last web log entry (we'll close the web log in the next post), I wanted to offer some information on how the ospreys find their way to their wintering or nonbreeding grounds.
Despite the fact that biologists and ornithologists have studied bird migration for many years, there is still a lot that humans don't really understand about how birds find their way when they migrate -- it's a field of study that still has some mystery about it.
But what scientists do know is that migratory ospreys 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 changing length of daylight -- 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.
Once ospreys make the decision to leave, they must then use their inherent navigational skills to locate their wintering or nonbreeding grounds. Since adult ospreys return to the same nonbreeding grounds each year, it's believed that on subsequent trips, adults will use landmarks to help guide them. But fledglings are "flying blind" -- without the benefit of having "learned" what landmarks they should look for -- so young birds must use other clues to navigate, such as the sun, the moon, and the stars. In addition to sky-reading skills, birds also have at their disposal certain skills that even humans don't possess -- such as sensitivity to the Earth's magnetic field and sensitivity to low frequency sounds created by wind and waves. During migration, ospreys frequently use their navigational tools in combination, so they can find their way through many challenging conditions.
Interestingly enough, radar technology has allowed humans to understand a great deal more about bird migration. Radar ornithology is the study of birds using radar, and it is a science that was started by the British Army during World War II. In the 1960s, radar ornithology caught on in North America, and since then it has helped the United States reduce the number of bird strikes by low-flying aircraft and has also helped bird conservation efforts by identifying popular stopover and roosting areas.
Today, both amateur and professional radar ornithologists use NEXRAD to study bird migration. NEXRAD stands for "NEXt generation RADar" and refers to the nationwide network of Doppler radar sites installed by the National Oceanographic and Atmospheric Administration, which provides almost complete radar coverage of the continental United States, Alaska, and Hawaii.
Click on the thumbnail to the right to see an animated collection of images from NEXRAD that shows a large burst of birds heading north (this was recorded in the spring) as they fly from Cuba over water into Florida -- leaving en masse under favorable flying conditions on one spring evening. And note that this is a path that our ospreys will take on their way back in the spring.
If you'd like to read more about radar ornithology and the use of NEXRAD, be sure to check out these websites:
Tracking Bird Migration with Radar from eBird
What is Radar from NJ Audubon Society
My next web log entry will be the last for this season, as the ospreys are getting close to migration time. Just as a reminder, we will be leaving the Osprey Cam on throughout the fall and winter, and we will continue to update the Gallery with photos of the different birds that visit the platform once the ospreys have left. So feel free to send in photos of anything interesting you see on the cam, and we'll put it in the Gallery and mention it on the cam page.
Until next time,
Lisa - webmaster
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