Monday, January 14, 2008

Eagle Survey Continues, with Ruminations on Telling Eagles Apart

And I'm not talking about telling Balds from Goldens, either. Part of the task in surveying for anything is to avoid double-counting - you're always asking, did I count that bird before or is it a new one? Happily, Bald Eagles have gotten to the point in southern NJ where there are so many of them that the day is continually spent re-assessing that question.

Highly skilled birders get in the habit of not only identifying a bird to species, but then noticing the differences between the bird in front of them and others of the same species they've seen before. Start doing this, and you'll notice there are pale first-year Herring Gulls and lighter ones, for example, or Mallard hens with more or less orange on the bill, or Hermit Thrushes that are exceptionally heavily spotted below, or waxwings that seem bigger and have white spots on the wing. . ."hey, wait a minute!" You get the idea - once more we learn the value of the old adage, know your common birds well. It's also just plain fun to get to know an individual bird, so you can it's not a redtail, it's that redtail I saw perched there yesterday.

I'm quite confident that Bald Eagles can tell each other apart just fine, but for humans it's not such a simple matter. With immature eagles it's a bit easier. For the survey we note what plumage young birds are in, adopting the terminology used in the Peterson Hawks field guide and elsewhere: Immature, White Belly I and II, Transitional, and adult. Within the immatures, there is quite a range of variation in the amount and location of white in the plumage, as well as in molt, so for example on Saturday when I saw a transitional bird I noted it was relatively darker on the head than these birds often are - it had the Osprey-like eye stripe but the crown was tawny to chocolate-y, not gleaming white at a distance as it sometimes can be.

But how do you tell one adult Bald Eagle from another adult Bald Eagle? Answer: I don't know. I surveyed the Cohansey River yesterday, had over 30 individual sightings of adult Bald Eagles, and concluded based on timing and location (we plot the eagles on topo maps as we encounter them) that the sightings represented 12 individuals, which was still "too many." There are four known Bald Eagle nests in the area I surveyed, and I knew where each one was and accounted for each member of each pair, but then I saw another clear pair of adults sitting close to each other but away from any known nest, plus two other singles that I couldn't be sure were attached to a nest. "Our" Bald Eagles are on territory and have begun sprucing up their nests - I saw both members of one pair sitting in their nest together - but wintering birds from points north may well still be around.

Then you have distant sightings, like two adults in the scope chasing a White-Belly II, way out there but probably over one of the known nests. Then you drive back by a nest that had no eagles near it a half hour ago, and this time there are two there. The ones that had chased the immature? New ones? I guess it's a nice problem to have. And it could be worse - Chuck Slugg told me he had over 70 eagle sightings yesterday and had to sort them out.

My full list from the Cohansey yesterday is below. I will say it is a beautiful and bird rich area, particularly the north side of the river, where I'd never been in winter - a marvelous mix of agricultural lands and wetlands.

Location: Cohansey eagle survey area
Observation date: 1/13/08
Number of species: 61
Snow Goose 100
Canada Goose 500
Mute Swan 5
American Black Duck X
Mallard X
Northern Pintail X
Bufflehead X
Hooded Merganser X
Common Merganser 5
Wild Turkey 18
Great Blue Heron 3
Turkey Vulture 35
Bald Eagle 15
Northern Harrier 13
Sharp-shinned Hawk 4
Cooper's Hawk 1
Red-tailed Hawk 18
American Kestrel 1
Ring-billed Gull X
Herring Gull X
Great Black-backed Gull X
Rock Pigeon X
Mourning Dove X
Belted Kingfisher 2
Red-bellied Woodpecker 5
Downy Woodpecker 2
Hairy Woodpecker 1
Northern Flicker 5
Blue Jay X
American Crow X
Fish Crow X
Horned Lark 50
Carolina Chickadee X
Tufted Titmouse X
Red-breasted Nuthatch 1
White-breasted Nuthatch 5
Brown Creeper 2
Carolina Wren X
Golden-crowned Kinglet 5
Ruby-crowned Kinglet 2
Eastern Bluebird 10
Hermit Thrush 5
American Robin X
Northern Mockingbird X
Brown Thrasher X
European Starling X
Yellow-rumped Warbler X
Eastern Towhee 2
Field Sparrow 2
Savannah Sparrow 15
Song Sparrow X
Swamp Sparrow X
White-throated Sparrow X
Dark-eyed Junco X
Northern Cardinal X
Red-winged Blackbird X
Common Grackle X
Purple Finch 1
House Finch X
American Goldfinch X
House Sparrow X

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