Raptor Population Index

2013 Regional Trend Summaries and Conservation Assessments


Cite this page as:
    Brandes, D., D. Oleyar, S. Hoffman, and L. Goodrich. The Raptor Population Index, 2013 Regional Trend Summaries and Conservation Assessments. Available at http://rpi-project.org/2013/assessments2013.php


Overview

The summary data presented below are based on the latest 10-year (also displayed on the trend maps) and 20-year linear trends for count sites within each region. Longer-term trends can be found in the trend graphs for each individual site and have been previously assessed by Farmer et al. (2008). As in Farmer et al (2008), we considered trends to be significant if the probability of getting the stated estimate is < 5% (i.e., p < 0.05) when the actual trend equals zero. We considered trends to be near significant if 0.05 < p < 0.10, and non-significant (i.e. no trend) if p > 0.10. Sites included in the analysis had hourly data from 2002 through 2012 stored in Hawkcount.org.


The migration count sites are divided into four regions as shown in Figure 1 below, with the Eastern region sub-divided into Great Lakes and Atlantic/Appalachian sub-regions. The Mississippi River forms the boundary between the east and central region, and the Front Range of the Rocky Mountains forms the boundary between the central and west region. Note that there are no sites within the Central region; although there are several active hawk-watches in this region, their data have not met the minimum requirements for trend analysis.


Trend results are listed in the accompanying tables and are summarized by region with colored pie charts. The regional trend summaries and pie charts are based on sites with an average annual count of at least 20 of the given species (for example, if a site has an average annual count of 6 Rough-legged Hawks, the trend result would not be included in the regional RL summary pie chart). In the case of Prairie Falcon, we used 15 as the minimum average annual count since there are very few sites with this species.


We recognize that hawk-watch sites are not randomly nor uniformly distributed across these regions, and therefore the combined results as shown in the pie charts may not be truly representative of regional population trends. Furthermore, more work is needed to understand the "catchment areas" of the various sites and the potential sampling overlap between them. It is likely that there is sampling overlap in areas like the Great Lakes and northern Appalachians where there are multiple sites in close proximity. Conversely there are large geographic areas where there is no site coverage, for example much of the southeastern U.S. and the Central region.




Figure 1. Map of hawk-watch sites and regions (W = west, C = central, G = Gulf Coast, E-AA = east-Appalachian/Atlantic, E-GL = east/Great Lakes)


It is important to recognize that trends in migration counts are impacted by shifts in migratory behavior and changes in spatial distribution in addition to changes in populations. For example, it has been well-documented that Red-tailed Hawks have increased in population in the eastern United States at the same time that migration counts are showing decreasing trends (Bolgiano, 2013). Distributional shifts may be occurring with other species (Paprocki et al 2014), especially short-distance or partial migrants, and these may also affect the migration count trends. For complete or long-distance migrants (e.g. Osprey, Broad-winged Hawks, Swainson's Hawks), we can have greater confidence that the migration count trends represent true population trends. We are considering how to incorporate Christmas Bird Count (CBC) data as part of our 2015 assessment to better understand the observed migration count trends within the context of changing migration strategies. Breeding Bird Survey (BBS) data could possibly also be used, however the BBS protocol is not well designed for detecting raptors.



References

Bolgiano, N. 2013. Evidence for changed migration of Red-tailed Hawks in eastern North America. Hawk Migration Studies 38:16-24.

Farmer, C.J., L.J. Goodrich, E. Ruelas I., and J.P. Smith. 2008.Conservation Status of North America.s Birds of Prey. Pp. 303-420 in K.L. Bildstein, J.P. Smith, E. Ruelas I., and R.R. Veit (eds). State of North America.s Birds of Prey. Nuttall Ornithological Club and American Ornithologists. Union Series in Ornithology No. 3. Cambridge, Massachusetts, and Washington, D.C. [TP-07]


Paprocki, N., J.A. Heath, and S.J. Novak, 2014. Regional distribution shifts help explain local changes in wintering raptor abundance: Implications for interpreting population trends. PLoS ONE 9(1):e86814.


Regional Results

Overall Regional Summary - 10-yr trends

Overall Regional Summary - 20-yr trends

Pie Charts by Region

Eastern Region - Appalachian/Atlantic 10-yr trends
Eastern Region - Great Lakes 10-yr trends
Gulf Region 10-yr and 20-yr trends
Western Region 10-yr trends

Eastern Region - Appalachian/Atlantic 20-yr trends
Eastern Region - Great Lakes 20-yr trends
Western Region 20-yr trends

Tables of Trend Results by Site and Region

Black Vulture
Turkey Vulture
Bald Eagle
Osprey
Mississippi Kite
Swallow-tailed Kite
Northern Harrier
Northern Goshawk
Cooper's Hawk
Sharp-shinned Hawk
Red-tailed Hawk
Red-shouldered Hawk
Rough-legged Hawk
Swainson's Hawk
Broad-winged Hawk
Golden Eagle
Peregrine Falcon
Prairie Falcon
Merlin
American Kestrel
Zone-tailed and Gray Hawk

Authors and Reviewers: David Brandes, David Oleyar, Steve Hoffman, Laurie Goodrich.