Hi all:
This has come up before in this venue, but it bears periodic repetition. Sabine's Gull (SAGU) is the longest-distance migrant gull breeding in North America, wintering in the Humboldt Current along western South America and at the southern extremity of Africa. As such, like many another very-long-distance migrant species, they conduct all of their molting on the winter grounds -- there just isn't enough time in the Arctic to manage that. Sabine's Gull is a two-year gull, that is, it becomes an adult in its second year when roughly 17-19 months old. Many/most first-cycle SAGUs stay on the winter grounds during their first summer, but, regardless, all these birds molt on the winter grounds. Additionally, those that do head north in their first spring/summer, spend that time at sea, not necessarily near the breeding grounds. While occasional second-cycle SAGUs don't quite get to complete definitive plumage in their second year, they get very close and are not particulalry easily distinguished as such in the field, without close scrutiny of their outer primaries. With all of the above, one can infer that the vast majority of the SAGUs traversing Colorado in fall would be juveniles (2-3 months old) and adults (>25 months old). Additionally, there should only be two plumage classes: Juvenal (= First Basic) and Definitive Alternate (adult breeding). Should CO be lucky enough to host a one-year-old, it would be in Formative plumage. The bottom line: virtually all (if not all) younger-than-adult SABUs occurring in CO are juveniles and all adults there are in Definitive Alternate plumage. Finally, as I've noted before in this venue, the term "immature" should really be reserved for situations in which it is/was impossible to determine the precise age of a bird younger than an adult. Since all such SAGUs occurring in CO are juveniles (in Juvenal = First Basic plumage), we should call them juveniles. Additionally, any non-juvenile SAGU that is not obviously in Definitive Alternate in CO is not only worthy of exclamation points in any post to Cobirds, but is also easily termed a one-year-old or a bird in Formative plumage. Since the determination of a SAGU as a juvenile is so straightforward, when one calls a SAGU an immature in CO, some of us might leap to the inference that it is not a juvenile and we might just rush right out to look at the beastie, with the resultant annoyance when it turns out to be "just" another juvenile. Sincerely, Tony Leukering Villas, NJ -- You received this message because you are subscribed to the Google Groups "Colorado Birds" group. To post to this group, send email to cobi...@googlegroups.com. To unsubscribe from this group, send email to cobirds+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/cobirds?hl=en.
