I posed my question to the "experts" at the Cornell Lab of Orn Hi All,
I posed my question to the "experts" at the Cornell Lab of Ornithology, and here are three responses I received back from them. And they all make good sense! 1) Assume that Pine Grosbeaks are producing two viable young/year -- this is a pretty reasonable estimate, since natural populations need to raise an average of two young to fledging age to keep a stable population. Obviously there is year-round mortality and not all birds will breed. So that means that 25% of birds will be adult males, and 75% will be female-like. Then, if Pine Grosbeaks had a really good breeding year, and produced four young per pair instead of two, that drops to 1/6 as adult males. Also, with many birds, the southernmost individuals (like those in Minnesota) tend to be immature birds, since they are pushed to more marginal parts of the range and are taking a bigger risk in migrating. The "best" areas might be claimed by adults closer to the center of the range. All those factors probably combine to give Pine Grosbeaks (and Northern Harriers, and a lot of other birds) fairly low proportions of adult males, and it really isn't all that odd when you think about it. 2) Pine Grosbeaks have delayed plumage maturation — without getting too detailed it takes birds longer to reach adult plumage than most passerines. So you see young males, young females and adult females compared to just one small group of identifiable adult males. Note these young birds are technically not juveniles, since they have undergone a preformative molt. The southern edge of the range of Pine Grosbeaks in winter, particularly during an irruption, is usually dominated by young birds. Adults typically can stay farther north and adult males have a tendency to stay farthest north. This is true of most species — the percentage of adult Snowy Owls, even during an irruption is dominated by young birds and adult males are rarest. This is not always the case, but it's typical. Of course, there are exceptions — there is an adult male White-winged Crossbill in Georgia right now. 3) It’s good logic. I have noticed that the FeederWatch Cam in northern Ontario seems to have primarily male Pine Grosbeaks attending the feeder. This single data point suggests that adult males are staying farther north, a pattern that is rather common in other species but not something I’ve heard suggested for this one. Betsy Beneke St. Cloud ---- Join or Leave mou-net: http://lists.umn.edu/cgi-bin/wa?SUBED1=mou-net Archives: http://lists.umn.edu/archives/mou-net.html
