Sunday, October 09, 2011

Tennessee BBS: Ups and Downs

I have heard and read comments in the recent past suggesting that in another 50 years our North American avifauna will consist of a highly depauerate relict of exotics and a few generalists, with most of our native birds religated to isolated remainders if not outright extinction. If this doomsday future is going to come to pass, it certainly is not giving any hints of itself yet in Tennessee. These 45 years of Breeding Bird Survey data for Tennessee actually paint a picture of an avifauna that remains robust, diverse, and overwhelmingly native. Though there are some major red flags, and one native species that has been effectively extirpated in recent decades, the majority of Tennessee's breeding bird species have shown statistically significant increases over this time period. Exotics are not taking over the landscape; while there have been two new arrivals during the time frame, of the three other long establish invaders two have shown flat numbers while the third has declined rather drastically. The median for the 104 species that have sufficient data to establish a meaningful trend is a 66% increase between the 1966-1975 interval and the 2001-2011 period. This looks like a sign of Apocalypse Not rather than Apocalypse Now.


As mentioned, there are some "red flag" species showing ongoing major declines in numbers, and more that have experienced smaller drops. So to get to the bad news first, these 27 species showed statistically significant declines in BBS counts (biggest loosers listed first):

Bewick's Wren
Loggerhead Shrike
Golden-winged Warbler
Yellow Warbler
Northern Bobwhite
Common Nighthawk
Gray Catbird
House Sparrow
American Redstart
Eastern Meadowlark
Northern Flicker (Yellow-shafted Flicker)
Common Grackle
Orchard Oriole
Cerulean Warbler
Black-and-white Warbler
Chimney Swift
Belted Kingfisher
Prairie Warbler
Blue Jay
Yellow-breasted Chat
Wood Thrush
Common Yellowthroat
Brown Thrasher
Northern Mockingbird
Eastern Towhee

In this list and all the remaining analyses, I have not included species that occur only on small numbers of routes (generally high-elevation birds of far east Tennessee or wetland birds of far west Tennessee) or that were not recorded in enough years to display a meaningful trend. Many of these species are ones whose declines have attracted notice and concern, such as Bewick's Wren, Loggerhead Shrike, and Northern Bobwhite. But some were surprises to me. How much concern have you heard expressed about the disappearance of Northern Flickers as a breeding species in the region? Or Gray Catbirds? Or Orchard Orioles? It is interesting to note that the majority of these species are to a fairly large degree now dependent on human activities to create and maintain their habitats -- from the obvious (House Sparrows, Chimney Swifts) to the more subtle (Thrashers in early successional habitats, Belted Kingfishers on farm ponds). I will discuss habitat and landscape-level patterns in these trends more in a forthcoming post.

Holding Steady

These next 17 species showed trends that, though no identically equal to zero, where not significantly different from it. Again they are listed in order of their percentage change, even though these changes were not statistically significant:

Green Heron
Horned Lark
Field Sparrow
Eastern Screech-Owl
Grasshopper Sparrow
Eastern Kingbird
Northern Cardinal
Kentucky Warbler
Mourning Dove
Summer Tanager
Red-winged Blackbird
European Starling
Rock Pigeon
Louisiana Waterthrush
Blue-winged Warbler
Great Horned Owl

This fairly short list is quite a mixed bag of species.


Finishing with the good news, the list of gainers is quite long. These 60 species showed statistically significant gains over the period (largest increases listed last):

Indigo Bunting
Eastern Wood-Pewee
Barn Swallow
Brown-headed Cowbird
American Crow
Great Crested Flycatcher
Acadian Flycatcher
Yellow-throated Vireo
Red-bellied Woodpecker
Worm-eating Warbler
Broad-winged Hawk
Downy Woodpecker
White-eyed Vireo
Pileated Woodpecker
Hooded Warbler
Tufted Titmouse
Red-eyed Vireo
Chipping Sparrow
Song Sparrow
Barred Owl
Red-headed Woodpecker
Wood Duck
Scarlet Tanager
American Robin
Carolina Chickadee
Purple Martin
Baltimore Oriole
American Kestrel
Northern Rough-winged Swallow
Hairy Woodpecker
Eastern Bluebird
Eastern Phoebe
Carolina Wren
Blue Grosbeak
Yellow-throated Warbler
Blue-headed Vireo
Red-tailed Hawk
American Goldfinch
Ruby-throated Hummingbird
Blue-gray Gnatcatcher
Turkey Vulture
Sharp-shinned Hawk
Warbling Vireo
Pine Warbler
Black Vulture
White-breasted Nuthatch
Red-shouldered Hawk
House Wren
Cliff Swallow
Cooper's Hawk
Cedar Waxwing
Northern Parula
Wild Turkey
Great Blue Heron
Canada Goose
Tree Swallow
Eurasian Collared-Dove
House Finch

Though this is also a diverse list, two patterns jump out as you read through it. First, as I mentioned before, this list includes all of the diurnal birds of prey plus the two vultures. Even American Kestrels, subject of much concern regionally, showed a positive trend. Though it is just hypothesizing on my part, I have to suspect that this trend must be a combination of the effects of the DDT ban and reduced persecution because of better laws, law enforcement, and awareness. I know my own rural neighbors do not routinely shoot hawks; I suspect this would not have been the case 45 years ago!

The second pattern is a larger one which I will discuss more in the next post: Most of our forest birds are on this list. As Tennessee is principally a land of forests, both at present and in our ecological past, this pattern is the primary contributor to the overall message of good news for Tennessee's breeding birds coming out of these data. We still are home to extensive forests that are some of the most biodiverse temperate ecosystems on the planet, and our avifauna continues to strongly reflect this.

Tennessee BBS index:
1: Waterfowl to Herons
2: Vultures to Doves
3: Cuckoos to Woodpeckers
4: Flycatchers to Corvids
5: Larks to Wrens
6: Gnatcatchers to Waxwings
7: Wood Warblers
8: Towhees to Buntings
9: Icterids to House Sparrow
Ups and Downs
Next: Habitats
Inflection Points


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