Wednesday, March 12, 2014
By Herb Wilson
This column continues the discussion of osprey biology from my last column. I wrote about the dependence of osprey on live fish for food. Ospreys are not picky about the fish they prey upon in either salt- or freshwater environments. As long as the fish are in shallow water or within three feet of the surface of deeper water, they are at risk from an attack from above by a feathered menace.
Ospreys are effective fishers. They succeed in getting a fish on 24 percent of their dives. The average hunting time to catch a fish is 11.8 minutes.
Ospreys may nest in loose colonies. A critical feature for a nest site is proximity to fishing habitat; ospreys don't mind commuting 10 miles to fish. They also like nest sites that are open.
A study in Nova Scotia showed the cleverness and even cooperation of ospreys that were nesting close together. At that site, ospreys were feeding on marine fish. Sometimes they fed on fish that tend to concentrate in shoals: alewives, pollock and smelt. At other times they took fish like winter flounder that tend to be more randomly distributed.
Ospreys at their nests watched other ospreys returning to their nest with food. If the returnee had an alewife or pollock, other ospreys would immediately head off in the direction from which the successful forager came. A fish shoal had been found and there should be plenty of fish for all!
If the forager came back with a flounder, the ospreys did not respond in the same way. Information on the location of a nonschooling fish like a flounder has little value for later foragers.
One might think that ospreys taking a cue from other ospreys with prey is a form of parasitism. However, there is evidence that an osprey returning with a smelt or other shoaling fish gives a distinctive flight display to alert other ospreys to the fish shoal. This behavior seems to be a cooperative behavior, reminiscent of the bee waggle dance, where bees communicate the presence of nectar-rich plants to other bees.
Ospreys have had a mixed relationship with humans. Persecution caused the demise of ospreys in Scotland and southern and central California in the early part of the 20th century. This persecution came in the form of egg collecting for private collectors, hunting to obtain stuffed specimens for people's parlors and irrational killing of birds of prey by ignorant shooters.
The removal of large trees along the shores of Lake Huron to create space for housing developments led to a precipitous decline of ospreys in Minnesota.
Of course, the greatest human-related scourge on ospreys was the widespread use of DDT over 20 years beginning around 1950. This "wonder insecticide" found its way into our marine and freshwater systems. The DDT was absorbed by small animals and was difficult to eliminate. As a result, DDT biomagnified as it passed up the food chain, becoming more concentrated as it passed from prey to intermediate predators to top predators like the osprey.
The major effect on the osprey was to cause eggshell thinning; the eggs were easily broken by an incubating parent. Some adult mortality from exposure also occured. Over 90 percent of the ospreys from New York to Massachusetts perished.
Fortunately, the banning of DDT has led to a recovery. By 2000, most osprey populations had recovered to pre-DDT levels.
And the provision of nesting poles has led to dramatic increases in local osprey populations.
Herb Wilson teaches ornithology and other biology courses at Colby College and perches in South China. He welcomes reader comments and questions at: