Pitfalls in Osprey Identification

By Jerry Liguori

Plumage variations occur within all raptor species.  Birds such as the Red-tailed Hawk and Swainson’s Hawk exhibit enormous differences between races or morphs whereas species such as the American Kestrel and Prairie Falcon exhibit only slight variation in overall plumage.  “Variation” is not meant to define the differences between the races, ages, or sexes in a particular species, but the degree of difference within those specific races, ages and sexes.  Understanding overall plumage variation will help in accurately identifying, aging, and sexing raptors in the field. 

For instance, most people have learned that the only way to determine the sex of an Osprey is to observe its bib (or necklace) or lack thereof.  Birds with bibs are deemed female (See Figure 1) and birds without are deemed male (see Figure 2). While this is the norm, using the bib as way of determining sex is not without its faults.  There are many male Ospreys that show bibs (See Figure 3) and females that do not (See Figure 4).  (Note: While conducting nesting productivity surveys with the NJ Endangered Species Program, I photographed these (4) Ospreys where they were sexed on the basis of their behavior; i.e. egg laying, incubation, position during copulation, etc. In fact, Figure 2 and Figure 4 are a mated pair.  Perhaps, if the birds in Figure 3 and Figure 4 were observed on migration, they would be sexed incorrectly?

The issue becomes even more difficult considering that almost all juvenile Ospreys show a somewhat prominent bib.  So, before attempting to sex an individual Osprey, it must be aged correctly.  Aging Ospreys can be very difficult in itself.  There are plumage differences between adult and juvenile Ospreys.  However, most of these differences are minor, and almost impossible to see in the field.  In most cases, even a more obvious distinction such as the pale-tipped upperwing coverts of juveniles versus the all dark upperwing of adults is difficult to observe.  The most obvious feature on juvenile Ospreys is the bright, rufous wash to the underwings.  Adults may show a hint of rufous to the underwings, but it is faint at best.

Since almost all juvenile Ospreys remain on their wintering grounds for a year or more, where they acquire their adult plumage, spring Osprey migration consists exclusively of adult birds (with rare exceptions possible).  This information may help in determining the age of Ospreys in spring, but does not help to definitively determine the sexes of Ospreys during migration.  In conclusion, while it is fairly accurate to assume the sex of an Osprey by the lack or presence of a bib, it can be fallible in many instances.