Tennessee Breeding Bird Survey -- Summary

For me, major point to be gotten from 45 years of the Breeding Bird Survey in Tennessee is that the birds are doing pretty well, overall. Most species have shown increases in their numbers, and the non-native invaders remain a small component of the avifauna in most areas. The two most coherent trends are large increases in diurnal birds of prey and in a large suite of forest-dwelling birds that do not winter primarily in the neotropical regions. The former I would expect to be attributable to direct conservation efforts and increased cultural awareness; the latter strongly suggests a fundamental and substantial improvement in the amount and quality of forest habitat within the state. Even beyond these suites of species, trends average modestly upwards in most cases (even for the neotropical migrants).

Behind this happy picture there is a small suite of species that appear to be in serious trouble. Landscape-level habitat changes probably underlie many of these large declines; the same improvement in forest habitat will be linked to a reduction in edge and agricultural habitats. The exact mechanisms underlying these declines are doubtless unique to each species and are not always obvious. Some of these drops seem more drastic than simple habitat changes can account for.

One lesson I would also take from these data is: "Conservation works." Many of these optimistic trends really took off in the late 1970s; it hardly seems coincidental that this comes on the heels of the major advances in environmental legislation and programs that began in ernest in the early 1970s. Of course this can make avian conservation a victim of its own success -- why do we need all these programs when the birds are doing so well? This same ironic conundrum affects all successful programs. It still does not mean that we should overlook or downplay the successes and try to gloss over the fact that in total our state's breeding bird populations appear substantally healthier than they were 45 years ago.

The final lesson would be: "Past performance is no guarantee of future results." Many of these upward trends show hints of abating in the last decade, though it is too early to tell for certain. This may just be the wobbles of this varied and nicely-rebounded avifauna as it settles down into a new quasi-steady state in our improved, avian-friendly landscape. But it might not. Nothing can be safely taken for granted in this arena.

As a closing note, I should point out that I do not see any suggestion of a climate change effect in these trends. There is no pattern of species spreading preferentially northwards, southwards, eastwards, or westwards, as would be expected in response to large-scale shifts in climate patterns. This is not surprising, as the actual climate change signature in Tennessee's own climate records remains very small and difficult to detect statistically if it is there at all. One would expect these effects to show earliest and strongest in species that nest in areas experiencing the greatest changes, such as the far north. We of course only see those birds in Tennessee in migration and in winter.

Whatever the future may hold, we in Tennessee have headed in to the Third Millenium with a breeding avifauna that is on the whole diverse, robust, and thriving. Let's work to keep it that way.

Tennessee BBS index:
Introduction
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
Habitats
Inflection Points
Summary

Tennessee BBS: Inflection Points

As I scrolled through all those graphs for the individual species, it became clear that 45 years was not only long only enough to show trends, it was long enough to show trends in trends -- the second derivative, not just the first. Few of the graphs show a simple up or down trend over the whole period, even when allowance is made for the year-to-year wiggles. American Goldfinch provides a stark example of this; Chipping Sparrow shows another. There appear to be three periods around which these inflection points cluster. I have had trouble making sense out of some of these; perhaps readers with more knowledge of the changes in land use, legislation, and other possible impacts in the mid-south during this era might shed some light.

Turning Number 1: Late 1970s-1980

Something clearly happened around this time. I discussed this earlier as the Great Sparrow Shift; it involves other species as well, from Goldfinches to Orioles to Herons. This is the time at which several of the woodpeckers and raptors began their impressive increases. Overall, for the species that seem to show a notable shift in the direction of their trend lines at this time, the large majority of these shifts are towards the positive. Many of these are weedy and edgy species, though a marked exception is the Gray Catbird, which began a sharp drop around 1980.

Turning Number 2: Around 1990

About a decade later, there was another shifting in the trend lines for a variety of species. This is a very mixed bag, with some increasing trends flattening while other flatlines started increasing and some species entirely changed directions. As with the first turning, the majority of the shifts were positive, but less dominantly so in this case. Wood Ducks, Chimney Swifts, Great Horned Owls, and Red-winged Blackbirds are among the species that began declines at about this time.

Turning Number 3: Early 2000s?

As it is close to the end of the current dataset, this third turning is less definite. Most of these suggested trends remain hard to distinguish from ordinary short-term wobbles. Still, there are indications of another shift in the trend lines for many species in the last decade. Unfortunately, most of these possible shifts appear to be in the wrong direction, or at least the flattening out of a long-term increase. There are some positives, including the surges in Tree Swallows and Mississippi Kites. The coming decade will tell whether most of these hinted-at changes are sustained.

I would expect these turnings to primarily be associated with landscape and habitat shifts. These could come about from official programs, economics, or many other factors. I would be curious to hear suggestions as to what these factors might have been.

Tennessee BBS index:
Introduction
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
Habitats
Inflection Points
Next: Summary

Tennessee BBS: It’s the Habitat

As alluded to in the last post, the population trends of birds indicated by numbers from the Breeding Bird Survey in Tennessee appear to be related in some degree to the habitat requirements of each species. I looked at this in more detail by categorizing each species according to very general, landscape-level habitat preferences. The categories I chose were:

Forests -- relatively continuous and closed canopy forests. Occurs throughough the state, but most extensive in the eastern mountains and hill 'n' holler portions of the Highland Rim.

Edge/Mosaic -- the mosaic of woodlots, smaller farm and pasture areas, edges, roadsides, successional clearcuts, lower-density houses, and other mixed habitats typical of rural and less dense suburban areas that archetypally characterize much of Tennessee. Widespread statewide, and especially typical of the Highland Rim, hillier parts of the Coastal Plain and Central Basin, and the less rugged areas of eastern Tennessee.

Open country -- large-scale acriculture, with most land area in croplands and pasture. Characteristic of much of the Coastal Plain and Mississippi Alluvial Valley, the flatter inner Central Basin, and the Barrens areas of the Highland Rim. Small areas of this sort of landscape occur in most regions of the state wherever flatter ground occurs.

Residential/suburban/urban -- higher population density areas with lawns, “landscaping,” parks, commercial areas, etc. Includes centers of small towns as well as suburbs of large metro areas.

For each species, I checked off one or more of these categories based on general pereferences, not strict obligate requrements. For instance, Broad-winged Hawks are scored as “forest” and Horned Larks as “open,” but I have seen and heard both species from the exact same spots on our “edge/mosaic” farm in a Highland Rim hollow.

I also added a fifth category, both because it is of much interest in the conservation community and because a quick inspection seemed to confirm its significance. I checked off species that are neotropical migrants -- those migrants for which the majority of the population winters south of the U.S.

And as a final detail, when I calculated averages and other statistics on the percent changes, I used a logarithmic transformation in the form of X = ln(N_f/N_i) where N_i is the average count in the initial 10 years (1966-1975) and N_f is the average count in the final 10 years (2001-2010). This straightens out and balances the skewed nature of these ratios and gives them a more approximately normal (statistically speaking) distribution. Using the log metric, if one species has dropped to half while another has doubled, they will average out to zero. Using the straight percentages, they would average out to +25%. To give the final numbers, I converted these log transfomed averages back to regular percentages.


Averages by habitat type

When treated as described above, the following average and median changes come out for birds of each habitat preference:

Habitat	No. Species	Median	Mean
Forest	36	+74%	+101%
Edge/Mosaic	59	+35%,	+24%
Open	27	+15%	+32%
Residential	20	+67%	+71%

Though statistical tests do not make these trends significant, they confirm the impression that forest species are doing better than average. This gives a strong suggestion that one of the main driving factors for the rise in numbers of most Tennessee birds has been an improvement in the quantity and quality of forest habitat across the state.


The Neotropical Effect

When looked at in isolation, the difference between the neotropical migrants (NMs) and the non-neotropical migrants (non-NMs) was not significant. The 48 NM species showed an average increase of 21% (median 25%); the 53 non-NM species averaged 70% (median 75%). This appears to be a substantial difference, but because of the very large variability between species this difference is not outside the 95% confidence zone. Remember that individual species range from a drop of 99.8% to a rise of 69400% (excluding the three species that were not recorded in the 1966-1975 period and hence have an infinite percentage increase).

It is more revealing to look at the interaction between the neotropical effect and the general habitat preferences. Here are the median percent changes for the species in each category, with the number of species in parentheses:

Habitat	NM	non-NM
Forest	+20% (20)	+143% (16)
Edge/Mosaic	+32% (29)	+55% (30)
Open	-5% (7)	+43% (20)
Residential	+231% (4)	+69% (16)

In the first three habitats the neotropical migrants all show lower median changes; this reverses in the final category but there are very few neotropical migrant species that prefer residential habitats.

Looking at the average (rather than median) change, in most cases the pattern is similar with larger increases for the non-NM species in all cases:

Habitat	NM	non-NM
Forest	+43%	+208%
Edge/Mosaic	+13%	+35%
Open	-13%	+48%
Residential	+40%	+79%

In the case of the averages, statistics can be applied; the one pattern that is (highly) significant is the very large increase among the non-neotropical migrant forest species in comparison to all other categories. In the cases of the other three habitats, the smaller differences between the NM and non-NM trends are not quite statistically significant. It is interesting, though, that they all fall in the same direction.

Overall, the predominant pattern is large increases (an average of three-fold) in non-neotropical migrant forest birds, and a smaller general increase in other species (including the neotropical migrants). Of course a large number of these species do not show simple monotonic trends up or down over the entire 45 year history of the BBS; this is the topic for the next post.

Tennessee BBS index:
Introduction
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
Habitats
Next: Inflection Points
Summary

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.

Downs

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
Whip-poor-will
Orchard Oriole
Cerulean Warbler
Chuck-will's-widow
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
Ovenbird
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.

Ups

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
Killdeer
Great Crested Flycatcher
Acadian Flycatcher
Yellow-throated Vireo
Red-bellied Woodpecker
Worm-eating Warbler
Broad-winged Hawk
Downy Woodpecker
Dickcissel
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:
Introduction
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
Summary

Tennessee BBS 9: Icterids to House Sparrow

Dickcissel

35 year change: +1.90 (+70%) ± 1.65

Numbers of Dickcissels tend to bounce up and down more than those of many other species, but the overall trend has been a modest and significant increase.


Red-winged Blackbird

35 year change: +3.57 (+11%) ± 6.26

The overall net change in Redwings for the BBS era is essentially zero. The graph suggests this was the end result of about 20 years of modest increases followed by a similar decline.


Eastern Meadowlark

35 year change: -26.50 (-61%) ± 3.68

Meadowlarks have declined steadily through the BBS era, with an overall drop of roughly threefold.


Common Grackle

35 year change: -55.30 (-58%) ± 18.82

Common Grackles have also dropped steadily and substantially through the period, again showing roughly a three-fold decline overall. At the beginning of the period this was by far the most abundant species on the BBS in Tennessee; it has now descended into the pack.


Brown-headed Cowbird

35 year change: +2.96 (+32%) ± 1.18

Cowbird numbers have shown a fairly modest but statistically significant increase over the BBS period.


Orchard Oriole

35 year change: -4.34 (-52%) ± 0.48

The more common of Tennessee's two oriole species has declined steadily, with recent numbers being less than half of those at the beginning of the survey.


Baltimore Oriole

35 year change: +0.39 (+125%) ± 0.17

Baltimore Orioles are less widely distributed, with the largest counts coming from the MAV routes in northwest Tennessee. Their numbers have shown a steady increase over the BBS era.


House Finch

35 year change: +4.56 ± 1.02

After first appearing in 1987 numbers of House Finches rose sharply over the next decade. Recent data suggest a possible decline from this peak, but more years are needed to assess whether or not this is real.


American Goldfinch

35 year change: +7.98 (+220%) ± 1.75

The overall change in Goldfinch numbers is a substantial increase of more than three-fold. This appears to be constructed from a low flat line through 1980 followed by a steady climb through 2000, then a modest decline in the last decade.


House Sparrow

35 year change: -23.80 (-70%) ± 5.29

House Sparrows have declined by nearly a factor of four since the beginning of the BBS. The rate of decline appears to have moderated in the last decade or so. It is interesting to note that this decline appears to have been underway before the arrival of the House Finch as a breeding species in 1987, but the steepest drop in House Sparrows does coincide with the time of most rapid increase in House Finches.

Tennessee BBS index:
Introduction
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
Next: Ups and Downs
Habitats
Inflection Points
Summary

Tennessee BBS 8: Towhees to Buntings

Eastern Towhee

35 year change: -1.42 (-12%) ± 1.17

Towhee numbers have remained rather steady on Tennessee BBS routes. The small decline is statistically significant but is a minor change in context.


Chipping Sparrow

35 year change: +5.14 (+80%) ± 2.33

There seems to have been a "Great Sparrow Shift" around 1980 in Tennessee. Before this time, all the common species were showing rapid changes in abundance; when The Shift came their trends switched to the opposite direction or stabilized. In the case of the Chippie the initial trend was a substantial decline, which then shifted to a sustained increase. Overall the numbers rose significantly for the period.


Field Sparrow

35 year change: -3.05 (-18%) ± 3.45

For Field Sparrows, the Great Sparrow Shift was preceeded by a rapid decline amounting to a drop of over 50% in less than 10 years. After The Shift numbers gradually recovered, so that by the end of the period the net change was not significantly different from zero.


Lark Sparrow

35 year change: +0.01 (+8%) ± 0.07

Though they have been found on several different BBS routes, the data for Lark Sparrows are too limited to allow any trend to be seen.


Grasshopper Sparrow

35 year change: -0.06 (-6%) ± 0.45

This is another sparrow that shows a rapid decline of 50% or more in the beginning of the BBS period, followed by a recovery after The Great Sparrow Shift around 1980. The data are "bumpier" than those for some of the other sparrows, probably because of the loosely colonial nature of the species and the somewhat ephemeral nature of its habitat. Again, the net change for the total period was not significantly different than zero.


Song Sparrow

35 year change: +4.39 (+84%) ± 2.04

The Song Sparrow reverses the pattern shown by the other common sparrows. Its numbers rose rapidly in the early decades of the BBS, then stabilized after The Great Sparrow Shift. The overall increase for the BBS period was nearly two-fold.

Three other species of sparrows were tallied as presumed breeders on a few Tennessee BBS routes and on a very few occasions: Savannah, Vesper, and Bachman's. In all cases the data were far to skimpy to provide much information. Another species, Henslow's Sparrow, has been found in recent years to be highly local breeder at a number of sites spread over a large area of Tennessee. It has yet to be recorded on any BBS route in the state, however.


Summer Tanager

35 year change: +0.41 (+9%) ± 0.61

Summer Tanager numbers have remained stable over the BBS period, with no significant change.


Scarlet Tanager

35 year change: +1.46 (+111%) ± 0.42

In contrast, Scarlet Tanager numbers have more than doubled since the beginning of the BBS. Most of this increase happened before 1993, with fairly stable counts since then.


Northern Cardinal

35 year change: +1.34 (+4%) ± 2.84

Cardinals are abundant, ubiquitous, and show stable numbers over the BBS era.


Rose-breasted Grosbeak

35 year change: +0.02 (+628%) ± 0.02

This species has been tallied in quite variable numbers and on only a small number of BBS routes. Hence even a 628% increase is not statistically significant.


Blue Grosbeak

35 year change: +3.35 (+162%) ± 0.63

Blue Grosbeaks have increased steadily throughout the period, more than tripling in numbers from 1966 to 2010.


Indigo Bunting

35 year change: +4.374 (+11%) ± 3.08

Another of the most abundant and ubiquitous species on Tennessee BBS routes, Indigo Buntings have shown a statistically significant but fairly small increase. Overall their numbers appear to have been stable

Tennessee BBS index:
Introduction
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
Next: 9: Icterids to House Sparrow
Ups and Downs
Habitats
Inflection Points
Summary