The apparent drop in May may be an artefact due in particular to an absence of observations (observer) in much of May, 2005-2008. However, all other years affirm that osprey sightings are fairly even from April to May (while the birds are establishing their nest sites and families) and increase through June to July and August, with high frequency of feeding sorties by both parents, and later "training flights" by the newly fledged juveniles, which are often seen perched on utility poles within sight of the nest platform.
It is interesting to note that the migration of ospreys through Presqu'ile provincial park, located roughly 40 km south of the town on the north shore of Lake Ontario, is later than in the Trent valley breeding area, both in spring and in the fall. Ospreys are rare at Presqu'ile in summer, and are uncommon migrants from mid-April to mid-May and from early September to mid-October, with extreme sightings on 04 April and 28 October (LaForest, 1993, p.99).
To the northwest, beyond Rice Lake in Peterborough county, the "fish hawk" is typically seen from 02 April to 04 October, or exceptionally between 29 March and 03 December (Sadler, 1983, p.58).
LaFOREST,SM (1993) Birds of Presqu'ile Provincial Park. Friends of Presqu'ile Park / Ontario Ministry of Natural Resources, 436pp.
SADLER,D (1983) Our Heritage of Birds: Peterborough County in the Kawarthas. Peterborough Field Naturalists / Orchid Press, Peterborough, ON, 192pp.