Besnard Lake is a lake in the Canadian province of Saskatchewan, north of Saskatoon and west of La Ronge. It supports a large population of bald eagles that have been the subject of ongoing studies since the 1960s. The lake is from tip to tip with of shoreline. It has a surface area of and a maximum depth of, at an elevation of about. At a narrows it is crossed by a bridge carrying provincial highway 910. This road was completed in 1973 and increased access to recreational fishing. There are over 250 islands on the lake, and the lakebed is rocky, making navigation difficult but creating a good habitat for fish. There are cabins, campgrounds and fishing camps on the lake. There is a airstrip south of the lake. Besnard Lake is within the Churchill Riverdrainage system. While much of the lake is in the Canadian Shield, its southern portion is in the boreal forest where the Mercer River brings nutrients which leads to healthy invertebrate populations that provide plenty of food for the fish which are the mainstay of the eagles' diet. They fish mainly for white sucker and cisco, and occasionally pike, walleye and burbot. Compared to the nearby and similarly sizedNemeiben Lake which is entirely on the shield, the eagle population has a higher density and stability.
The bald eagle community at Besnard Lake has been studied continuously for 50 years following their discovery there by Jon Gerrard. In that time eagle population dynamics, behaviour, growth and migration have been studied, resulting in the publication of more than 50 scientific papers and the 1988 book The Bald Eagle: Haunts and Habits of a Wilderness Monarch. Over 800 eagles have been banded, and have been found to winter mainly between Iowa and Missouri, Wyoming and Montana, but some have been found as far south as Texas, Arizona and Southern California. In the 1960s it was considered that bald eagles were rare in Canada away from the Atlantic and Pacific coasts. In 1966 Gerrard found 18 nests with 27 young in the area. A more extensiveaerial survey of the region in 1974 found 14,000 eagles. Besnard Lake was found to be a perfect area to study bald eagles in these borealbreeding grounds. The lake has provided habitat for a stable population of about 100 eagles since the 1970s. One discovery was that the sex of immature eagles could be accurately and efficiently determined by measuring the size of the foot and length of the culmen on the beak. This led to the discovery that the sex of the young seems to be regulated by the quantity of food available, with more females hatching when food is plentiful. On other lakes where food is less plentiful a higher ratio of males hatch.
