July 02, 2005
Ospreys & their Bird Friends
Nest News: We apparently have another form of life living on the platform -- our annual spider has returned (although it's obviously a different spider). Every year we have a spider who manages to get out onto the tiny camera and weave a web around the lens. Fortunately, it's not visible during most of the day and neither is its web, but at night during the summer, it's very visible, as this photo testifies. It will likely be with us for a while, so we thought we'd warn you in case you wonder what that white stuff is on the lens. Unfortunately, ospreys don't eat spiders, so it's pretty safe there for now.
Speaking of ospreys and other wildlife, an interesting topic that we haven't mentioned in the web logs before is the relationship that ospreys have with the rest of the bird world. While we've looked at the often adversarial relationship that ospreys have with bald eagles, we haven't really mentioned the rest of the bird community.
Some sources (including one of our cam watchers) have testified to the fact that when fish are not readily available, ospreys will eat non-fish items such as ducklings, frogs, turtles, and even small mammals. Although ospreys are considered rather strict fish-eaters, there is evidence that they will broaden their diet if they must to survive.
But on the norm, ospreys are among the least threatening raptors in the avian world. In fact, there are quite a few bird species that are so comfortable around ospreys that they have been known to nest within the osprey's own nest or in its supporting structure.
A report that appeared in the Wilson Bulletin (an ornithological publication)* in 1994 discussed observations made during the 1991 and 1992 osprey breeding seasons in both Michigan and Ontario where other birds nested inside osprey homes. The scientists reported that in the Great Lakes basin, they found six species of small birds breeding within or immediately beneath occupied osprey nests. Breeding inside the nests were common grackles, tree swallows, European starlings, and house sparrows; and breeding in the structure supporting the nest were northern flickers and barn swallows.
The scientists reported that the ospreys laid eggs in all but one of these nests and they saw no hostility between the ospreys and their tenants. Also worth noting was that the frequency of the hitchhiker birds was higher at natural nest sites rather than at artificial platforms -- mainly because artificial platforms were often too shallow to provide space for additional birds.
So why did these tenant birds choose to nest in close proximity to such a powerful raptor? The scientists stated in their paper that the smaller birds probably benefited in several ways; first, the osprey nest made up for a lack of suitable nesting habitat in the area; second, the smaller birds benefited from a decreased risk of predation because the ospreys would attack any menacing crows, raptors, or mammals near the nest; and third, the smaller birds could eat the insects that were attracted to the fish remains in the nest.
As for the ospreys, they benefited from this symbiotic relationship by using the smaller birds as added security guards. The neighbor birds could sometimes see predators before the ospreys did and alert them to danger.
In addition to songbirds, other birds found nesting within osprey homes include smaller herons and ducks. Melanie Lynch, from Chesapeake Bay Magazine, recently went on an osprey banding trip on our local Patuxent River and took this photo of an osprey platform built atop a wood duck box (click on the thumbnail). Melanie said a wood duck flew out of the box before they reached it, so it appears it was active. Apparently the ducks did not have a problem living directly below a raptor family.
While all these reports make for enjoyable web-of-life stories, the fact is some small birds are not always welcome at an osprey nest. At the Woods Hole Oceanographic Institute's Osprey Cam, their current mother osprey has found a neighboring red-winged blackbird that likes to sit on her tail and make a nuisance of itself. You can see a photo of the pesky little bird here. It's likely that he's attracted to the insects at the nest, but the mother osprey must wonder why he feels compelled to sit on her tail and add to her troubles when she has three hungry chicks to raise. There's one in every group. :-)
Until next time,
Lisa - webmaster
* "Birds breeding in or beneath Osprey nests in the Great Lakes basin" Wilson Bull., 106(4), 1994, pp. 743-749
Posted by Webmaster at July 2, 2005 08:20 AM