I think that most birders would agree that identifying adult gulls, especially those in alternate plumage, is relatively straightforward. That is far from the case, however, when it comes to identifying birds in various stages of subadult plumage.
This is the time of year when many of these challenges present themselves especially given the number of organized trips to gull hot spots such as the Niagara River, the north shore of Lake Ontario and even the local hot spots here in the Kitchener/Waterloo area. I am amazed at how many birders seem to say with absolute certainty “second year Iceland Gull,” “first year Lesser Black-backed Gull” and so on.
The two principal references I use are Gulls of Europe, Asia and North America (Klaus Malling Olsen and Hans Larsson) and Gulls of the Americas (Steve N.G. Howell/Jon Dunn), and I have several other references on my shelves.
Let me just quote several statements from these substantial, comprehensive and first class works. First from from Olson/Larsson:
- The large 'four-year' gulls show great variation in the time they take to develop into adult plumage. Age classes between first summer and adult should be regarded as generalisations only. (Bold type in the original).
- Grey and brown tones look different under different light conditions.
- Only by direct comparison – best in pairs – is sexing advisable.
- ...head and bill in the large gull complex have been introduced as characters separating subspecies within the Herring Gull complex, but the use of characters remains tentative.
- In judging photographs – the angle of the bird relative to the photographer, the light, the film type (obviously now superseded), equipment used and even processing methods may each result in subtly different shades of grey
- Birds are categorized by cycle only, i.e. First cycle, second cycle etc.
- It should be accepted that the magnitude of variation in many large white-headed gulls – compounded in some cases by hybridization – means that a large proportion of large gulls cannot be identified to species (or parentage) in the field.
- Environmental factors may operate directly on the gull or may be indirect but affect the observer's perception.
- Photographs on page 254 “Presumed first-cycle Kumlien's Gull......"
- Photographs on page 254 “Probably not safely separable from Iceland Gull."
- Photograph on Page 159 “Small-billed birds like this can be confused with Thayer's Gull."
I could cite other examples but I think that the point is made. Identification is far from easy and subject to a percentage of error which is probably significant from birders lacking sufficient expertise, whether they acknowledge it or not. I have been present when birders claim to identify gulls in myriad plumages as easily as they identify robins and chickadees in their yard.
Clearly this is impossible and I take all of the reports with a very substantial grain of salt. I think that there is as much alchemy, supposition, auto suggestion and the simple desire to correctly identify everything as there is precise identification of species.