Changes between first and second atlases
Some species confirmed as breeding during the first Atlas were not confirmed during the second Atlas, and vice versa (Table 4). Some were found during both atlases but could not be confirmed during one of the atlases. Species confirmed during the second Atlas include those that have been present in Vermont at low numbers and have been gradually disappearing from the state (e.g., Barn Owl, Short-eared Owl, Red-headed Woodpecker), as well as species that have declined substantially (Common Nighthawk) or respond to fluctuations in food resources (Cape May Warbler, Bay-breasted Warbler).
Table 4. Species confirmed exclusively during the first or second Atlas.
|Confirmed in Atlas 1, not Atlas 2Northern Pintail
Cape May Warbler
|Confirmed in Atlas 2, not Atlas 1Bald Eagle
Great Black-backed Gull
The main focus of this second generation Atlas is change. Vermont Atlas data provide an overview of which species have had an expanded, diminished, or relatively unchanged distribution and number of occupied blocks. Known or hypothesized reasons behind changes in block occupancy for each species are discussed in their respective accounts.
Species that showed dramatic increases in block occupancy since the first Atlas (Fig. 9 a,b) have undergone range expansions and/or increases in abundance in the state due to ecological factors or intentional management programs.
- Increase of Canada Goose, Wild Turkey and some waterfowl species due to programs to increase populations.
- Passerines such as Tufted Titmouse, Carolina Wren, Pine Warbler and Northern Cardinal have shown range expansion due to ecological factors and/or feeding stations.
- Increases in Cooper’s Hawk, Barred Owl, and Winter Wren possibly due to maturation of Vermont forests.
Figure 9. Species that increased most based on a) number of blocks gained and b) percent of increase in block occupancy between the first and second atlases. Gadwall, Red-breasted Merganser, and Wilson’s Warbler, which increased from 1 block to 2, 3, and 2 blocks, respectively, are not included.
Figure 10. Species that decreased most based on a) number of Priority 1 blocks lost and b) percent of decrease in Priority 1 block occupancy between the first and second atlases. “Species that were present in less than 10 blocks in the first atlas are not included (Northern Pintail, Spruce Grouse, Northern Bobwhite, Black Tern, Red-headed Woodpecker, Loggerhead Shrike, Cape May Warbler, Grasshopper Sparrow).
The list of top decreasing bird species between atlases is dominated by species associated with shrub and grassland habitats, as well as Nightjars and some aerial foragers (Fig. 10 a,b).
Changes in PO, PR, and CO records between atlases
Each Atlas Project must evaluate the extent to which protocols from the previous Atlas will be followed. The Atlas advisory committee discussed, based on data from this Atlas, whether all three breeding codes (PO, PR, CO) need to be recorded in order to obtain quality Atlas data. To evaluate this question, we compared the changes between the first and second atlases based on the different categories. In general, for all but the rarest species, changes in CO and PR closely followed changes in PO, PR, and CO records combined from the first to the second Atlas (ρ = 0.94). In other words, omitting PO records would not result in dramatically different interpretations of the data, generally speaking. In addition, changes in CO records were highly correlated (ρ = 0.91) with changes in PO, PR, and CO records combined, suggesting that using only CO records would in general yield similar results to using all three breeding codes.
Despite the general correlation among results from the different breeding codes, there were exceptions for which omitting a breeding code would change the interpretation of the data. For example, there were fewer CO records but more PO and PR records for Wood Thrush during the second Atlas compared to the first Atlas. Possible explanations include: a) there was a real decrease in productivity; b) Wood Thrush abundance had decreased, reducing the likelihood of recording breeding evidence; c) changes in average observer ability between atlases resulted in fewer CO records for this species. Regardless of the underlying reason for the apparent discrepancy, omitting PO records could yield misleading results.
For some species, it may be more meaningful to evaluate changes between atlases using only one of the breeding codes. For example, the distribution of waterbirds may be best represented using CO records. The strict enforcement of safe dates during the second Atlas may have resulted in fewer PO and PR codes for waterbirds, which are often found in pairs while still migrating. For example, Common Goldeneye had the same number of CO records between atlases, but fewer PO and PR records during the second Atlas. Likewise, for swallows and other species with predictable nest sites, or nests that are easy to find, changes in CO records between atlases may reflect a real change in nesting habitat or nest density. For Bank Swallow, Cliff Swallow, and Purple Martin, all of which showed decreases in both CO and PO records between atlases, CO records provide meaningful data. In the case of species for which breeding evidence is difficult to obtain, however (e.g., Turkey Vulture), CO codes would not likely suffice. Using PR plus CO codes may be adequate in many cases, but eliminating PO codes does not reduce field work time appreciably and may result in missing or underestimating the presence of rare species.
Given that certain breeding categories provide better data some species than others, eliminating the use of any of the breeding codes would require careful thought, and would likely need to be tailored to each species. Wholesale elimination of a category from Atlas field work protocols may increase efficiency, but would likely compromise the quality of information for some species.