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1
An assessment of PIF population estimates for
forest birds in Northern Maine
Randy Dettmers1, Mitch Hartley1, and Tom
Hodgman2 1U.S. Fish Wildlife Service, Hadley,
MA and 2Maine Dept. Inland Fisheries Wildlife,
Bangor, ME

• Introduction
• Continental population estimates of breeding
  landbirds (Rosenberg and Blancher 2005) were
  developed for the North American Landbird
  Conservation Plan (Rich et al. 2004)
• Uncertainty and low confidence in PIF estimates
  exists due to difficulties estimating their
  accuracy and precision and few efforts to
  validate them at different scales
• We compared PIF estimates to an intensive study
  by Hagan et al. (1997), who used a robust
  sampling framework to count birds in a 1,270 km2
  forested landscape in northern Maine

• Results
• Density and population estimates calculated by
  Hagan et al. (1997) were higher than PIF
  estimates for 94 of species (n70)
• Median density and population estimates by Hagan
  et al. were gt9 times higher than corresponding
  PIF values
• The mean ratio of Hagan PIF populations was gt25
  (density ratio 21)
• Hagans estimates were gt10 times higher than PIF
  estimates for 44 of species For 10 of species,
  Hagans estimates were gt50 times higher than PIF
  values
• Hagans mean (median) population estimate was
  20,110 (10,527) birds the PIF mean (median) was
  2,272 (971) birds
• BBS routes sampled habitat types in rough
  proportion to their occurrence in the study
  landscape

Discussion Hagan and PIF estimates differed by an
order of magnitude for half the species examined
and differed most for a suite of boreal
(Three-toed Woodpecker, Yellow-bellied
Flycatcher, Boreal Chickadee, Gray Jay, Blackpoll
Warbler, Lincolns Sparrow) and sub-boreal bird
species (Blackburnian Canada Warbler). For
that group Hagans estimates were typically 100
times higher than PIF estimates. Most of that
group are PIF species of continental
importance. Given the robust sampling framework
and greater sampling intensity, Hagans estimates
are presumably more accurate than PIF estimates
for this landscape. Biased sampling of habitat
types did not appear to drive observed
differences, which were most likely due to
1) Higher detection rates by Hagan et
al. (10-min counts, two visits) 2) Probable
overestimates of abundances in 50m- radius
circles by Hagan et al. With preliminary
correction methods applied, Hagan estimates were
typically three times higher than PIF estimates
for most species.

• Methods
• Hagan et al. (1997) estimates based on two visits
  to 387 point counts (50m fixed-radius)
  distributed across forest types and age-classes
• Data from six Breeding Bird Survey routes in the
  same landscape used to estimate densities and
  populations according to PIF methods (Rosenberg
  and Blancher 2005)
• To correct biases in both methods, BBS values
  were adjusted upwards to account for 3- versus
  10-minute point counts for 15 species with
  published data (Buskirk McDonald 1995)
• Hagan et al. (1997) values adjusted downward to
  account for overestimates of density in 50m
  fixed-radius point counts (Simons et al. 2007)
• Land cover around BBS routes was compared to
  habitat occurrence across the entire area

Conclusions Though PIF population estimates are
largely untested and imprecise, they have been
used to set population and/or habitat objectives
at continental and regional scales. Such
objectives will be more meaningful if we can
better understand the accuracy of these estimates.
Figure 1. Ratio of Hagan et al. (1997) PIF
population estimates. Bars represent number of
species in each category.
Literature Cited Buskirk, W.B. and J.L. McDonald.
1995. Comparison of point count sampling regimes
for monitoring forest birds. Pages 25-34 in USDA
Forest Service PSW-GTR-149. Albany, CA. Hagan,
J.M, P.S. McKinley, A.L. Meehan, and S.L. Grove.
1997. Diversity and abundance of landbirds in a
northeastern industrial forest. J. Wildl. Manage.
61718-735. Rich, T.D., C.J. Beardmore, H.
Berlanga, P.J. Blancher, M.S.W. Bradstreet, G.S.
Butcher, D.W. Demarest, E.H. Dunn, W.C. Hunter,
E.E. Iñigo-Elias, J.A. Kennedy, A.M. Martell,
A.O. Panjabi, D.N. Pashley, K.V. Rosenberg, C.M.
Rustay, J.S. Wendt, and T.C. Will. 2004.
Partners in Flight North American Landbird
Conservation Plan. Cornell Lab of Ornithology.
Ithaca, NY. Rosenberg, K.V. and P.J. Blancher.
2005. Setting numerical population objectives for
priority landbird species. Pages 57-67 in
Proceedings of the Third International Partners
in Flight Conference 2002. Volume 1. USDA Forest
Service PSW-GTR-191. Albany, CA. Simons, T.R.,
M.W. Alldredge, K.H. Pollock, and J.W. Wettroth.
2007. Experimental analysis of the auditory
detection process on avian point counts. Auk
124986-999.
Correction Factors Ratios of adjusted HaganPIF
densities were 32 of original ratios (41 to
1.41 median). However, most species in this
analysis (eastern deciduous birds) were quite
uncommon in the study area.
Figure 2. Hagan and PIF density (birds/50 ha)
estimates as calculated and adjusted for likely
biases.
For Further Information Contact
randy_dettmers_at_fws.gov or mitch_hartley_at_fws.gov
This poster can be obtained at
www.acjv.org/pif_poster.htm