FOREST BIRDS AND WOODY PLANTS: BROAD-SCALE HABITAT RELATIONS IN THE NORTH MAINE WOODS
Randall B. Boone, Department of Wildlife Ecology, and
William B. Krohn, Maine Cooperative Fish and Wildlife Research Unit, University of Maine, Orono
FIGURES 

Transition zones have been identified for woody plants in Maine: 


INTRODUCTION 
     McMahon (1990) showed that range limits of woody plants (n=240) in Maine exhibit two transition zones.  A north-south zone occurs across south-central Maine, where many northern plants have southern limits, and many southern plants have northern limits.  A second zone was shown, running north-south, where many species have east-west limits.  Research has shown repeatedly that birds respond to numbers of tree species at the local level (e.g., MacArthur and MacArthur 1961; Rice et al. 1984; O'Connor et al. in press). We hypothesized that birds may also be responding to woody plant distributions at a broad scale. We quantitatively compared bird richness to woody plant ranges, and qualitatively the ranges of birds using different habitats to the plant transition zones. 
Bird ranges and richness in Maine were developed and compared to woody plant distribution patterns: 

 

 METHODS  
    The ranges of birds breeding in inland Maine were defined based upon observations, literature, personal communications, and expert review. Breeding Bird Survey data - not used in defining ranges - were used to test ranges. Bird ranges agreed well (4 to 8% mean disagreement). Woody plant ranges were  efined using McMahon et al. (1990), recent literature and expert review. Richness maps were created for plants and birds based upon 18 x 18 km blocks.  
     Bird richness was modeled using plant distributions, with multiple linear regression and tree regression, which includes nonlinear relations. After dividing birds into five groups based upon habitats used, visual comparisons were made between their richness and plant transition zones.
Correlation analyses identified nonlinear relations between birds and woody plants: 

 

 RESULTS  
     In linear correlations of woody plant richness on bird richness, 67% of variation was unexplained. With tree regression (which includes nonlinear relations), only 22% of variation was unexplained. Thus 45% of variation in bird richness was related nonlinearly to plants, much higher than for amphibians (26%), reptiles (3%), and mammals (26%). When the residuals from the linear regression were plotted, they suggested the plant transition zones. 
Richness grouped by habitat showed strong relations to the vegetation transition zones: 

 

      We hypothesized that birds associated with one type of forest (conifer or deciduous, but not both) would be more closely associated with the distribution of woody plants than birds that used other habitats. For example, forest specialists would have more range limits in Maine than generalists, because of the plant transition zones.  When the 186 birds were divided into five groups based upon primary habitats used: 
 
Habitats
Limits in Maine
Statewide
Percent with limits
Barren, Urban
5
11
31
Early successional
15
17
44
Wetland, Water
27
26
51
Forest generalist
5
34
13
Forest specialist
27
17
61
Forest specialists had more (P < .001) range limits in Maine (61%) than others. Forest generalist were almost as numerous as specialists, yet only 13% of generalists had limits in Maine.  
    We plotted richness for each bird group and compared the maps to woody plant transition zones. Forest specialists were strongly spatially coincident with the north-south transition zone, and early successional birds were coincident with the east-west transition zone.  
Avian transition zones occur in Maine and eastern North America: 

 

 DISCUSSION  
     We show that birds exhibit transition zones in Maine. Woody plants display a replacement series across the zones, with richness not elevated in the overlap (McMahon 1990; Currie and Paquin 1987) but richness for birds is higher within the zones.   
     High bird richness also corresponds with the north-south plant transition zone at a continental scale (Boone 1996). The plant transition zone extends across eastern North America at about 45 degrees latitude. Richness from the Breeding Bird Survey (Sauer et al. 1996) were compared to the vegetation transition zone (not shown). The area of highest bird richness corresponded reasonably well with the vegetation transition zone (see Boone 1996). A avian transition zone exists across eastern North America, with bird ranges being determined in part by broad-scale habitat relations.  
LITERATURE CITED 

Boone, R.B.  1996.  An analysis of terrestrial vertebrate diversity in Maine.  Ph.D. Thesis, University of Maine, Orono. 

Currie, D.J. and V. Paquin.  1987.  Large-scale biogeographic patters of species richness of trees. Nature 24:326-327. 

MacArthur, R. and J. MacAruther.  1961.  On bird species diversity.  Ecology 43:594-598. 

McMahon, J.S., G.L. Jacobson, Jr., and F. Hyland.  1990.  An atlas of native woody plants of Maine: a revision of the Hyland maps.  Maine Agricultural Experiment Station, University of Maine, Orono. Bulletin 830. 

McMahon, J.S.  1990.  The biophysical regions of Maine:  patterns in the landscape and vegetation. M.S. Thesis, University of Maine, Orono. 

Rice, J., B.W. Anderson, and R.D. Omhart.  1984.  Comparison of the importance of different habitat attributes to avian community organization. Journal of Wildlife Management 48:895-991.  

Sauer, J.R., S. Schwartz, B.G. Peterjohn, and J.E. Hines. 1996. The North American Breeding Bird Survey home page, version 95.1. Patuxent Wildlife Research Center, Laurel, Maryland