Wednesday, June 30, 2010

Down Home Breeding Birds

Over the last five years, I have been running Breeding Bird Survey-style roadside routes across my home turf of Lewis County, Tennessee; a few routes every year. My goal was to cover the vast majority of the public roads in the county, allowing for the half-mile nominal spacing of the stops. This year I finished the 13th route, completing my coverage. Next year I'll begin them all again, running two or three a year, so I will keep a 5-year moving average estimate of countywide bird numbers.

My 13 routes include an unofficial run of the same roads covered by the official Lewis Forest BBS route, which is contained entirely within Lewis County and has been run for many years by the same skilled observer. Unlike the real BBS, I did all my coverage after sunrise, generally using the first 4 hours of daylight. I also did not necessarily complete a route in a single day, frequently splitting them between two mornings. My strategy for laying out the routes was mostly to keep turning left until I was blocked (by the county line, the end of the road, or areas I had already covered), then turning around and backtracking (without making additional stops) until I could turn left again. I also added small side trips etc. as needed to avoid leaving road segments "orphaned," unsurveyed but surrounded on all sides by roads that had already been covered. This more or less made a crooked clockwise spiral in towards the center of the county; as I got down to the last few routes I needed to do more picking and choosing of strategy to fill the remaining holes. Also unlike the BBS, a single "route" is not necessarily continuous, but includes skips and backtrack sections where I did not make stops. This is also a reason for not trying to cover an entire route in one morning; the skips and backtracks take up quite a bit of time and doing 50 stops in 4 hours was not usually practical. In the end, I did succeed at covering nearly every public road in the county, totaling 650 three minute roadside point counts.

My totals for 13 routes over 5 years (2006-2010):

Canada Goose 35
Wood Duck 3
Northern Bobwhite 83
Wild Turkey 12
Great Blue Heron 7
Green Heron 11
Black Vulture 12
Turkey Vulture 86
Cooper's Hawk 2
Red-shouldered Hawk 24
Broad-winged Hawk 7
Red-tailed Hawk 3
American Kestrel 5
Killdeer 38
Rock Pigeon 37
Eurasian Collared Dove 7
Mourning Dove 268
Yellow-billed Cuckoo 85
Eastern Screech-Owl 1
Barred Owl 2
Chimney Swift 59
Ruby-throated Hummingbird 8
Belted Kingfisher 4
Red-headed Woodpecker 21
Red-bellied Woodpecker 202
Downy Woodpecker 62
Hairy Woodpecker 23
Northern Flicker 13
Pileated Woodpecker 100
Eastern Wood-Pewee 131
Willow Flycatcher 2
Acadian Flycatcher 127
Eastern Phoebe 96
Great Crested Flycatcher 100
Eastern Kingbird 43
Loggerhead Shrike 2
White-eyed Vireo 243
Yellow-throated Vireo 137
Blue-headed Vireo 1
Warbling Vireo 1
Red-eyed Vireo 336
Blue Jay 182
American Crow 456
Northern Rough-winged Swallow 19
Purple Martin 176
Tree Swallow 6
Barn Swallow 105
Cliff Swallow 183
Carolina Chickadee 134
Tufted Titmouse 581
White-breasted Nuthatch 99
Carolina Wren 434
House Wren 21
Blue-gray Gnatcatcher 297
Eastern Bluebird 181
Wood Thrush 131
American Robin 237
Gray Catbird 30
Northern Mockingbird 181
Brown Thrasher 101
European Starling 509
Cedar Waxwing 25
Blue-winged Warbler 23
Northern Parula 112
Yellow Warbler 3
Yellow-throated Warbler 78
Pine Warbler 79
Prairie Warbler 162
Cerulean Warbler 1
Black-and-white Warbler 15
Prothonotary Warbler 4
Worm-eating Warbler 38
Ovenbird 31
Louisiana Waterthrush 27
Kentucky Warbler 99
Common Yellowthroat 182
Hooded Warbler 49
Yellow-breasted Chat 282
Eastern Towhee 208
Chipping Sparrow 207
Field Sparrow 212
Henslow's Sparrow 2
Grasshopper Sparrow 5
Summer Tanager 201
Scarlet Tanager 176
Northern Cardinal 655
Blue Grosbeak 102
Indigo Bunting 455
Dickcissel 4
Red-winged Blackbird 115
Eastern Meadowlark 217
Common Grackle 178
Brown-headed Cowbird 187
Orchard Oriole 88
Baltimore Oriole 3
House Finch 62
American Goldfinch 180
House Sparrow 138

Total -- 98 species, 11,107 individuals, five highest counts (descending order): Northern Cardinal, Tufted Titmouse, European Starling, American Crow, Indigo Bunting.

There are five additional species of nocturnal birds that I know nest in the county: Great Horned Owl, American Woodcock, Common Nighthawk, Chuck-will's-widow, and Whip-poor-will. These would bring the total up to 103 species.

Very roughly, these 650 point counts probably detected about 1% of the total number of birds in the county -- more for conspicuous and roadside-loving birds, less for inconspicuous and nocturnal species, and in many cases much more likely to detect males than females. Still, to an order if magnitude, if you multiply my 5-year totals by 100 you get a very crude first-draft ballpark estimate of the total breeding population in the county. This very loose ratio is based on an extremely rough estimate that a BBS stop will effectively detect the birds in an area of about 1 ha (0.01 km2, a square 100m on a side or a circle 56m in radius), the total of 650 stops (giving 6.5 km2 covered), and the total area of the county of 730 km2.

It is interesting to contemplate that a bird I only found once in theory likely has about 100 other undetected individuals in the county (100 Blue-headed Vireos and 200 Willow Flycatchers? The mind boggles...). That leaves me pondering what the birds might be that there really are only a few pairs of, making it very likely I would have missed them. Some reasonable candidates might be Bewick's Wren and Bachman's Sparrow, a few of which could be lingering in the many clearcuts I do not have access to. There are almost certainly some American Redstarts in the area as well, as I get them regularly on BBS routes in surrounding counties. A few pairs of Sharp-shinned Hawks spread over 730 km2 could be very easy to overlook. But what of all the other, less obvious possibilities? A pair of Hooded Mergansers on a pond I can't get to, maybe? Perhaps a Black-throated Green or a Swainson's Warbler hiding in a deep hollow somewhere?

As is usual, if you take those counts, rank them, and plot them from lowest to highest on a log scale, the left (lower) part of the graph roughly approximates a straight line. This suggests that there should be approximately as many species between 0.1 and 1 as there are between 1 and 10, and the same for 0.01 and 0.1. A total of 0.01 is statistically about what you might expect for a species that there is only one of in the entire county. As there are 23 species on the list with single digit counts, this seems to be suggesting that there might be 40 more species than the 98 I actually found, in the county, at any given moment in late May or June. Clearly this sort of extrapolation is fraught with problems and easily could greatly overestimate the real number of missed species. I can't come up with 15 additional species that would not be considered extremely unusual, even unprecedented occurrences in this area at this season.

This makes you really start to ponder the mega-rarities -- Tropical Parula anyone? How about some bizarre mesoamerican flycatcher? Thing as strange as this have happened (Tropical Parula in downtown Fort Collins CO, Variegated Flycatcher in Tennessee). As my regular readers know, the fact that there are many more birds out there than what gets detected and identified by birders is a recurring theme of mine. It is a safe bet that at any given time, any given area likely contains a surprising number of mega-rarities living there quietly unnoticed. The thought that there may be dozens of birds at this very moment in my own small home county that I have never recorded here makes me just want to drop everything else and go out birding! Odds of me finding even one of them: very low; odds of this causing major adverse impacts on the rest of my life: rather high. Sigh.. still, remember that the vastness of the unknown remains much greater than the tiny fraction of the world that we are able to actually get our eyes, ears, and binoculars on.


At 12:15 AM, Blogger Unknown said...

wonder what the birds thought of you Bill ?


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