August 05, 2005
Siblings & Foraging
Nest Update: We've seen some great shots of the oldest chick flying about the nest the last couple days. Here's a two-shot series where you see the oldest approaching the nest from the right and making a nice landing.
The youngest has been inspired by all the flying and has begun flapping more ambitiously now that the oldest is airborne. Hopefully it won't be long before the littlest is out of the nest, too, and working on its flying skills.
As the chicks achieve their independence, it's natural to wonder what will happen once they begin flying regularly. First, it's important to note that the osprey chicks will not completely disappear once they're both airborne. It's true that we'll see less of them over time, but in the past we've been able to observe the juveniles right up until they left Blackwater Refuge, around late August or early September, to migrate south.
Even though the chicks are now gaining independence, it's likely they will continue to use the platform for eating, landing practice, resting, and reuniting with one or both parents. Also the ospreys will continue to defend the platform from the eagles.
Although the chicks are too big to be taken by eagles at this point, we know from past experience that the eagles like to use the cam platform as a place to perch once the ospreys migrate, so the eagles will likely show an interest in the platform, especially if the chicks are there by themselves and seem vulnerable.
Last year we saw that after the osprey parents had migrated and the two chicks were on their own, they often seemed agitated, and we believe the bald eagles were harassing them. The day after the osprey young left for migration, the eagles immediately took over the platform as their river perch.
The eagles will also likely show an interest in any of the juvenile ospreys that catch fish, as eagles occasionally steal fish from ospreys, and the inexperienced juveniles might be easier mugging victims.
Something else to keep an eye out for is our chicks bringing their first fish catches back to the nest. Ornithologists have seen juvenile ospreys successfully catch a meal as soon as 2-3 days after fledging, although normally it takes about 2 weeks, and they should definitely be catching their own fish within 3 weeks.
The fact that our osprey chicks have a partner sibling might be initially beneficial to both of them. Back in 1985-86, a study was done in northern Florida where biologists learned that osprey siblings will forage together after they have fledged. The biologists believed that the social learning which took place when the siblings hunted together made them more efficient fishermen. In fact when compared to single young, the siblings had a better catch rate earlier in their fledgling days.
The biologists stated that while some fishing behaviors are instinctual, some are obviously learned, and osprey siblings integrated factors such as glare, wind direction, dive height, and water clarity into their foraging strategy at a faster pace than did single juvenile ospreys.
The biologists reported, however, that by the end of the season, the osprey young had similar success rates when foraging, so the juvenile ospreys that grew up in a one-chick nest were ultimately not penalized for not having any siblings.
Our osprey cam chicks will fly and hunt until the end of August or the beginning of September, then it will be time for them to migrate. We'll be talking more about migration soon, but basically in the last four years of our Osprey Cam, the mother osprey left first in August, and the father remained to watch over the chicks; then the father left and the chicks took care of themselves until it was time to go. At this point, our current father osprey has been a somewhat absent dad, so we're not sure how closely he'll look after the young once the mother osprey migrates.
Once the siblings leave for migration they may start out flying together, but ultimately all the family members will migrate separately. If they all survive migration, they will be scattered throughout Central or South America for the winter. The young will stay down south for about 3 years until they mature, but the adults will be back with us next March.
Being primarily eaters of fresh fish, ospreys must migrate to areas where they can fish in shallow waters throughout the winter. Cold temperatures can freeze water or drive fish to greater depths, so that is why the ospreys must migrate to a warmer climate. Eagles at Blackwater Refuge do not need to migrate because in the winter they can turn to waterfowl and small mammals as alternative foods.
Until next time,
Lisa - webmaster