Blog #382
The rain finally stopped after a cold day in the 40s. Ursula had sat in the nest for the majority of the day, moving to a nest perch when the sun came out. Orpheus flew from the trees, mated with Ursula on the perch, and flew over the lake. Minutes later, an unknown Osprey flew out of the east and sailed lazily up the shoreline before turning. Ursula looked up, chirped in alarm, and, with wings shaking. stood erect on the edge of the nest. Her short, clear alarm whistles became more rapid and excited the closer the intruder got to her nest. Despite Ursula’s efforts, the intruder entered her territory—the air space around the nest—and flew directly at it. In a test of wills, Ursula flashed talons at the intruder. Orpheus came from a tree screaming and forcing it away on the wing. The intruder took off up the lake with Orpheus in pursuit.
Ursula defends her nest as an intruder approaches, courtesy of Karel and Cindy Sedlacek.
Little is known about the life history of intruders (also known as floaters) in migrant Osprey populations since they are not anchored to a nest. Much of what is known has been extrapolated from studying more than 40 species of birds from a wide range of taxonomic groups. One thing is for sure, floaters do far more than lead a wandering, subordinate existence with no fixed areas of residence or social organization.
Darwin (1871) was the first to describe the occurrence of wandering male birds in populations, but he had not tied the idea to the territorial spacing of breeding pairs. Until recently, nonbreeders in a population were essentially considered unsuccessful competitors who had been excluded from breeding territories by higher-ranked individuals. On the contrary, many floaters are young non-territorial males who have returned to their natal areas to mature and gain the experience necessary for initiating breeding activities. These juveniles may routinely end up breeding as they mature and are only temporarily excluded from breeding. It is thought that competition for nest sites limits the number of breeders and forces some individuals to become floaters when all suitable breeding habitats are occupied. However, floaters also include adults that do not breed for part or all of their lifetimes. There is no clear evidence that size, condition, or testosterone level explain floater status or whether all floaters are non-reproductive as certain male floaters have occasional extra-pair copulations with unmated or mated females. Floaters may also be individuals who reject vacant breeding locations, preferentially waiting for access to a higher quality site to increase their reproductive fitness.
Ursula repelling an intruder diving at her nest, courtesy of Karel and Cindy Sedlacek.
A floater may shift from being a secretive presence near an occupied nest to becoming an active intruder that seriously threatens the owners by sapping their energy with frequent chases. These intrusions cost the nest owners time they could spend fishing or guarding the nest. More often than not these intrusions turn into clashes of wills and not talons, but some fights result in injury and death.
In addition, sexually mature individuals that control a nest can also shift from breeder status to floater status if evicted from their nest by an intruder. This could be Ophelia’s fate, Orpheus’s mate of the last 9 years after she was expelled from her nest last week. Alan Poole (2019) estimates that for every 50 breeding pairs in a colony there are probably 25 floaters vying for nests. Non-breeding floaters represent a considerable percentage of the population, especially among males, and are important for providing new blood for the colony when needed.
Eyes to the sky!
Candace
Candace E. Cornell
Friends of Salt Point, Lansing, NY
Cayuga Lake Osprey Network
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