BIRD PLOT 



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CAT31115719 



A99.9 F764U 

 no.431 

 1990. 



variable-radius point sample compared to total 

 fixed-area snag counts is the subject of a separate 

 study and will not be addressed here. 



Forest Survey does not measure foliage volume 

 directly. Crown volume was estimated from the 

 Forest Survey data base using measurements of 

 tree height, crown ratio, minimum and maximum 

 crown widths, and crown shape. Foliage volume 

 was then estimated by discounting the crown vol- 

 ume estimates to bring them in line with other esti- 

 mates reported in the literature for ponderosa pine 

 forests (Cunningham and others 1980). These foli- 

 age volume estimates represented the relative foli- 

 age volume differences between the plots and made 

 it possible to separate other tree foliage volume 

 from ponderosa pine foliage volume. Cunningham 

 and others (1980) found that bird use of foliage may 

 be better explained if foliage volume is partitioned 

 into ponderosa pine and other tree volume. 



Foliage volume was also estimated using an index 

 of food availability developed by Schroeder (1983) as 

 part of a Habitat Suitability Index (HSI) model for 

 assessing potential habitat of the black-capped 

 chickadee {Parus atricapillus) — another cavity- 

 nesting gleaner species. This index is an assess- 

 ment of food availability (the term "availability" was 

 substituted for "suitability" in this paper) computed 

 from a measure of average height of overstory trees 

 and tree canopy closure. These variables are each 

 assigned a rating, then combined in an equation 

 (fig. 2) to arrive at an overall food availability rank- 

 ing between and 1.0. 



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25 50 75 



Percent Cover 



J 6 9 17 15 



Average Height of Overstory (m) 



Figure 2 — Overall food availability index 

 equation with individual cover and height 

 indices (Schroeder 1983). 



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