This month’s From the Board piece comes from one of our new Board Members Doug MacCarter. FAS wishes to thank Doug for his work with the ospreys of the Flathead Valley. We are proud to have him join the FAS Board.
By Doug MacCarter
This summer with the help of Shawn Richmond, I surveyed the osprey population in the Flathead Valley. The boundaries I selected for the survey area included Bigfork on the east, Kalispell on the north, and Somers on the west. This area included the lower Flathead Valley and the area around the northern tip of Flathead Lake.
Flathead Lake ospreys usually return from their wintering areas in late March or early April. The males generally arrive first, and the females typically follow two or three days later. The annual courtship consists chiefly of a nuptial display of aerial gymnastics by both sexes, including soaring, swooping, and hovering in wide circles. A male will often carry a fish in its talons during courtship displays. Males will copulate with their female mates numerous times throughout the courtship. After 30-35 days of incubation, the young ospreys usually hatch during the first week of June and fledge approximately eight weeks later, sometime during the second week of August. In late September or early October, the Flathead ospreys typically return to their wintering areas, which include southern Texas and Mexico.
In the Flathead Lake area, ospreys currently appear to favor platform nesting structures. Eighty-six percent of the nest structures this summer were active platform nests and fourteen percent tree nests. During my osprey study over the past 40 years, the vast majority of ospreys have utilized dead Ponderosa pine and cottonwood snags as their nesting sites; however, many of these potential osprey nesting sites have been cut down over the
years, so the birds have taken to using utility poles or nesting platforms erected for the exclusive use of ospreys.
Twenty-one osprey nests were located during the study period. However, only 19 nests were utilized in 2015. Of the occupied nests, 18 were active. Six nests contained one nestling; nine nests contained two chicks; and three nests contained three chicks. There were a total of 33 nestlings, 29 of which successfully fledged (an 88 percent success rate). The number of fledglings per occupied nest is typically expressed in terms of productivity. Thus the productivity of ospreys in the Flathead Valley was 1.5 per active nest site.
Bailing twine has become a serious problem for ospreys in the Flathead Valley. Ospreys seem to prefer lining their nests with grass, fishing line, bailing twine (or colored mesh) and other likely materials. As a result, adults and/or chicks may become entangled and often die as a result. Dr. Eric Green, a professor at the University of Montana, has frequently observed adult and/or young ospreys dangling upside down and dead from a length of baling twine from the nest sites. I have also personally observed this in the Flathead Valley. Dr. Green reports than an osprey nest that blew down in the Missoula area contained more than a quarter-mile of bailing twine within the nest. This summer, my brother Don also a biologist and osprey researcher discovered a dead, half-grown osprey chick in its nest when he was assisting in the banding of young ospreys; the chick was completely tangled in the twine.
Ospreys seem to be drawn to pick up brightly colored, discarded bailing twine from the fields. My summer osprey survey showed six nest sites out of 21 had signs of bailing twine hanging from the nests; other osprey nests may also have had bailing twine within the bowl of the nest that could not be seen by observers.
My goal this winter will be to remove the visible, trailing strings of bailing twine from six of the previously active osprey nests. However, I can only do this at the nesting sites which are not on active power poles; only the local power company is permitted to perform any work on osprey nests on active poles.
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