Friday, September 30, 2011

Tennessee BBS 7: Wood Warblers


35 year change: -0.36 (-18%) ± 0.43

Ovenbird numbers on Tenneessee BBS routes have fluctuated, but the overall change is not significantly different from zero.

Worm-eating Warbler

35 year change: +0.24 (+69%) ± 0.17

Worm-eating Warblers have shown a fairly steady increase in Tennessee over the BBS era.

Louisiana Waterthrush

35 year change: +0.17 (+22%) ± 0.18

Though the overall trend is not significant, Louisiana Waterthrush numbers appear to have dropped substantially between 1966 and 1980 then recovered steadily.

Blue-winged Warbler

35 year change: +0.12 (+35%) ± 0.17

The numbers of Blue-wings have remained relatively stable over the BBS period. The upswing in the last few years is due primarily to large counts on a few of stops on one route (Collinwood) where a large area of old clearcuts has reached the proper successional stage. It does not seem to be part of a large-scale trend.

Golden-winged Warbler

35 year change: -0.14 (-89%) ± 0.08

Never widespread, Golden-winged Warblers have declined steadily to the point that they have not been recorded at all since 2007.

Black-and-white Warbler

35 year change: -0.42 (-41%) ± 0.17

Numbers of Black-and-white Warblers have declined substantially over the BBS period. These data also illustrate the "Appalachian bump" that will be a prominent feature in the data for several more species below. In the early 1990s there was a sharp increase in the detection of many woodland species on eastern Tennessee routes. This appears to have been more of an observer effect than a real change in populations. It complicates the interpretation of trends for species that occur primarily or exclusicely in the eastern mountains.

Swainson's Warbler

35 year change: +0.01 (+83%) ± 0.02

Swainson's Warblers have been recorded in low numbers throughout the period, with no statistically significant trend discernable.

Kentucky Warbler

35 year change: +0.11 (+5%) ± 0.46

Though not abundant, Kentucky Warblers are widespread and have shown no significant trend in numbers over the BBS era.

Common Yellowthroat

35 year change: -3.16 (-23%) ± 0.93

Yellowthroats have shown a steady modest decline throughout the period. Even with the decline they have been one of the two most numerous warblers on Tennessee BBS routes throughout the period.

Hooded Warbler

35 year change: +0.83 (+74%) ± 0.37

Though Hooded Warbler counts have shown a statistically significant upward change, it appears that this may primarily a function of the "Appalachian bump" in the early 1990s, with flat numbers before and after the bump.

American Redstart

35 year change: -0.60 (-65%) ± 0.36

Redstart numbers have shown a substantial drop in spite of a significant "Appalachian bump" effect.

Cerulean Warbler

35 year change: -0.39 (-48%) ± 0.24

Similarly, Cerulean numbers have also dropped significantly even with a large "Appalachian bump." The data from 2009 and 2010 show a drastic drop; hopefulyl this will be reversed with 2011 data.

Northern Parula

35 year change: +2.93 (+881%) ± 0.62

Parula numbers appear to be growing exponentially, with about a 10-fold increase over the BBS period. They have now climbed to the point that they are the third most abundant wood warbler on Tennessee BBS routes, after Common Yellowthroat and the (probably not really a warbler) Yellow-breasted Chat.

Blackburnian Warbler

35 year change: +0.01 (+153%) ± 0.03

Blackburnians have been recorded in small numbers on very few routes. No significant trend is discernable from the limited data.

Yellow Warbler

35 year change: -1.78 (-87%) ± 0.26

Yellow Warblers have shown a steady, severe decline over the period, with a drop of nearly 10-fold.

Chestnut-sided Warbler

35 year change: +0.08 (+90%) ± 0.05

This is perhaps the most confusing instance of the "Appalachian bump." Numbers appeared to decline, then rose sharply in the "bump" time, with additional declines after the bump. The overall change is significantly positive, but one suspects that the bump is in fact masking an underlying long-term decline. This is where the much more refined analyses used by the BBS researchers are far more powerful than my simple approach. In the Appalachian region of Tennessee there are just not enough routes to average out the observer effects and other biases, which do appear to come out in the wash for more widespread statewide species.

Black-throated Blue Warbler

35 year change: +0.03 (+28%) ± 0.04

Black-throated Blues were found in too small numbers and on too few routes to discern a significant trend.

Pine Warbler

35 year change: +1.05 (+418%) ± 0.19

Pine Warbler numbers rose steadily through 1990. with an especially rapid rise beginning in 1985. Since 1990 numbers seem to have been roughly stable. This increase would seem likely connected to pine conversion forestry practices.

Yellow-throated Warbler

35 year change: +2.28 (+194%) ± 0.44

Numbers of this species rose steadily and substantially until the late 1990s, and appear to have stabilized since then.

Prairie Warbler

35 year change: -1.10 (-37%) ± 0.53

Prairie Warblers showed s steady, moderate decline until about 1990, with stable or slightly increasing counts since then.

Black-throated Green Warbler

35 year change: +0.46 (+104%) ± 0.24

Similar to the Chestnut-sided, this is another case where a large "Appalachian bump" seems to be obscuring what may be an overall declining trend. Numbers appear to have fallen off a cliff in 2010; the data from 2011 will illuminate whether this was just a fluke or a real event.

Canada Warbler

35 year change: +0.36 ± 0.18

Canada Warblers were found only on the Fish Springs route and only in the "Appalachian bump" period. Though the increase is statistically significant, it is probably not meaningful.

Yellow-breasted Chat

35 year change: -5.10 (-31%) ± 1.09

This anomalous warbler (which is probably not really a warbler) shows a pattern very similar to that shown by the Prairie Warbler, with which it shares many habitat preferences. Counts declined modestly until about 1990, then stabilized or perhaps rebounded somewhat. Even with the decline, Chats have been one of the two most abundant warblers on the Tennessee BBS throughout the period.

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
Next: 8: Towhees to Buntings
9: Icterids to House Sparrow
Ups and Downs
Inflection Points

Tennessee BBS 6: Gnatcatchers to Waxwings

Blue-gray Gnatcatcher

35 year change: +8.13 (+267%) ± 1.12

Still another common species that has shown a large increase in numbers on Tennessee BBS routes. Gnatcatcher counts have increased steadily by roughly a factor of four.

Eastern Bluebird

35 year change: +9.73 (+146%) ± 1.54

Though the first decade or so of the BBS appeared to show a decline in Bluebirds in Tennessee, in 1979 this turned and the overall trend for the BBS era has been strongly upwards.


35 year change: +0.01 ± 0.01

Recorded only on the Fish Springs route and only in a few years, the Veery data provide little information.

Wood Thrush

35 year change: -2.25 (-24%) ± 1.02

Wood Thrushes have declined modestly since the inception of the BBS, with most of this drop taking place between 1975 and 1990. Numbers have been quite stable in recent decades.

American Robin

35 year change: +13.21 (+113%) ± 4.36

Robin numbers increased steadily until about 1990 and have remained approximately stable since then. This is now one of the most widespread and abundant species on Tennessee BBS routes.

Gray Catbird

35 year change: -3.04 (-71%) ± 0.42

Catbirds have declined throughout the BBS period, with an especially steep drop during the 1980s. Present day counts are barely a quarter of what they were in the 1960s.

Northern Mockingbird

35 year change: -4.32 (-16%) ± 1.59

There has been a small but statistically significant drop in Mockingbird numbers, most of which took place before 1980. Mockingbirds remain abundant and widespread with stable counts in recent decades.

Brown Thrasher

35 year change: -1.15 (-21%) ± 0.68

Thrashers have also shown a small but statistically significant decline through the period.

European Starling

35 year change: +8.14 (+15%) ± 9.78

Though Starling numbers appear to have fluctuated somewhat, the change across the entire BBS era is not statistically significant. They were abundant in the 1960s, and remained just as abundant in the 2000s.

Cedar Waxwing

35 year change: +2.28 (+871%) ± 1.12

Waxwing numbers underwent a rapid shift during the late 1980s and early 1990s from a low, stable level to a much higher and more variable plateau.

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
Next: 7: Wood Warblers
8: Towhees to Buntings
9: Icterids to House Sparrow
Ups and Downs
Inflection Points

Tennessee BBS 5: Larks to Wrens

Horned Lark

35 year change: -0.34 (-25%) ± 0.45

Numbers of Horned Larks recorded on the BBS in Tennessee appear to have fallen, then risen, and ended up not significantly changed from where they started.

Northern Rough-winged Swallow

35 year change: +1.87 (+141%) ± 0.79

All five species of swallows recorded on the BBS in Tennessee have shown increases in numbers during the period. In the case of Rough-wings, the increase happened between 1980 and 2000, with some small retreat suggested since then.

Purple Martin

35 year change: +5.33 (+121%) ± 1.43

The significant increase in Purple Martins has happened primarily since 1995.

Tree Swallow

35 year change: +0.99 ± 0.25

As presumed breeders, Tree Swallows were unrecorded on the BBS in Tennessee until 1986; there was one 1973 report that was presumed to be late migrants. Numbers stayed low through 2000, then spiked upwards dramatically to the point that this is now an expected species on many BBS routes across the state.

Bank Swallow

35 year change: +0.10 (+2063%) ± 0.21

Bank Swallows are found infrequently and on only a very few BBS routes in Tennessee. The trend in counts has been upwards but this is not statistically significant.

Barn Swallow

35 year change: +6.30 (+26%) ± 3.53

Barn Swallows are abundant and ubiquitous on Tennessee BBS routes. Their numbers have increased less dramatically than the other swallow species in the area. Though the increase is statistically significant, overall the numbers have generally undulated around a fairly stable level.

Cliff Swallow

35 year change: +6.24 (+798%) ± 2.96

The increase in Cliff Swallows has mostly happened since 1988 and especially since 1998. As their favored habitat in this area is bridges, the BBS is generally good at detecting them. However, they are also very clumpy in distribution and subject to large shifts from year-to-year in response to bridge construction.

Carolina Chickadee

35 year change: +5.90 (+117%) ± 1.51

The roughly 2-fold increase in Chickadee numbers occurred entirely before 1990. Since then counts have been roughly stable with perhaps a slight decline

Tufted Titmouse

35 year change: +8.29 (+75%) ± 1.44

Titmouse numbers have shown a steady modest increase since about 1980.

Red-breasted Nuthatch

35 year change: +0.12 ± 0.11

Red-breasted Nuthatches have only been recorded on the Fish Springs BBS route. The marginally significant increase is not necessarily of any larger meaning.

White-breasted Nuthatch

35 year change: +3.20 (+756%) ± 0.47

White-breasted Nuthatches have increased spectacularly and steadily over the entire BBS period. The average count for the late 20-Aughts was 10-fold higher than the average for the late 1960s.

Brown-headed Nuthatch

35 year change: +0.02 ± 0.03

Brown-headed Nuthatches have been spreading dramatically through the Tennessee Valley in eastern Tennessee In recent decades. However, Tennessee BBS routes have only record the species in very small numbers so far. A major spike occurred in 2010; it will be interesting to see what 2011 data show.

Brown Creeper

35 year change: +0.003 ± 0.006

Creepers have only been recorded on the high elevation Fish Springs BBS route, and only in a few years.

Carolina Wren

35 year change: +14.58 (+154%) ± 2.31

The Carolina Wren is another common species that has shown a dramatic and sustained increase in numbers in Tennessee during the BBS era, especially since about 1980. It is now one of the most abundant and numerous species on the BBS in this state.

Bewick's Wren

35 year change: -0.66 (-100%) ± 0.19

The collapse of eastern Bewick's Wren populations region wide is well known, and the BBS data show it clearly. The species was already in a precipitous decline when the survey began in 1966; by 1979 numbers were reduced to just a small relict. The remaining populations continued a slower decline, last being found on a BBS route in Tennessee in 2005 (which was itself the first record in 7 years). Outside the BBS only a very few Tennessee nesting sites are known to still exist in a small region of the eastern Central Basin.

House Wren

35 year change: +0.63 (+786%) ± 0.18

House Wren numbers rose very slowly in the early decades of the BBS, then began a rapid climb after 1990.

Winter Wren

35 year change: +0.01 (+188%) ± 0.02

Winter Wrens are another high elevation species that have been found only on the Fish Springs route. No meaningful trends can be extracted from this meager data.

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

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