I recently read on another website that Ospreys are monogamous and mate for life. This is a commonly held belief about Ospreys and people often ask me about this aspect of osprey behavior. I have documented repeatedly that they are not monogamous and do not necessarily mate for life. A perfect example is our 21 year old male this year. He has been with the same mate for 13 years. This is the longest pair bond I have documented in this population of ospreys. He had at least one other mate prior to this one. However, I have observed this male engaging in many extra pair copulations. He was with another female during the early days of this season before his mate returned. One year I witnessed him copulating with three different females before his long term mate returned! And yet, they remain together year after year. They have been very successful, producing 26 chicks together! This is why they stay together, the biological imperative and driving force is to reproduce. Several of their offspring have returned and nested successfully also. I have also documented many "divorces". I hate this term as it is a legal term which does not really apply to birds, but it refers to the situation when a mated pair of ospreys are still alive and well, but split up and mate with other partners at other nests. It almost always follows a failed breeding season. My earliest observation of extra pair copulation, which led to the published paper, was seeing a female copulating with a different male than her long term mate for at least a week, but then she returned to her old mate and nest to lay her eggs. He was probably seen as a good provider in a better territory. When the chicks were all predated that year, she left him the following year and mated with the "boyfriend" of the previous year. I have observed several males and females this year who are on different nests, with different mates, than they were last year. In all cases, their nests failed last year. Not all ospreys split after a failed season however. The flip side is that producing chicks successfully together creates a bond with the partner and the nest...but extra pair copulations still occurr when the opportunity arises!
I co authored a paper that included some of this research that was published in 2008 in the Journal of Raptor Research. As our population of Ospreys has increased, I am seeing more extra pair copulations and divorces. Hard for me to understand why anyone still believes they are monogamous! But it takes long term study, good field work, with banded birds to prove these behaviors. I am certain it occurs more than we think among unbanded birds.