FOLIAGE USE BY BIRDS OF THE OAK-JUNIPER WOODLAND AND PONDEROSA PINE FOREST IN SOUTHEASTERN ARIZONA RUSSELL P. BALDAl Department of Zoology University of Illinois Urbana, Illinois 61801 Bird populations obtain their requisites from METHODS the resources available to them in a number While conducting breeding-bird counts in various of different ways. Species within the same plant communities of the Chiricahua Mountains of community may use different configurations southeastern Arizona ( Balda 1967), two areas were of the habitat, or the same configurations in a selected for study of foliage use by the nesting birds. In the oak-juniper woodland (36-acre plot) and pon- different manner or in different proportions. derosa pine forest (38-acre plot) trees and saplings This tends to minimize or eliminate interspe- were measured for volume of foliage in conjunction cific competition. Habitat utilization by various with a sampling plan to obtain relative density, relative species of nesting birds is often a main portioIz frequency, relative dominance, and number of individ- of autecological studies (Stenger and Falls ual trees per acre. I used the plotless point-quarter method of Cottam and Curtis (1956) to sample trees 1959), or of studies dealing with the interac- with a DBH of three inches or more in both plots. In tions of a few species from a given avian com- each study area a series of points was established and munity. at each point the surrounding area was divided into Recent studies by Morse (1967) and Mac- four quarters. In each quarter the name of the tree Arthur (1958) have shown that volume of closest to the point and its distance from the point were recorded. The DBH was also measured on these four foliage may be an important factor in limiting trees per point. Quantitative analysis of these data the density of some species of birds. However, was made using the standard formulas (Phillips 1959; few attempts have been made to quantify the footnotes, table 1). In addition, the following data foliage portion of a plant community by spe- were recorded for the four trees sampled at each point: total tree height (from ground to highest point), cies of trees and the height distribution of this height from ground to lowest branches, and crown foliage on the trees. This information can be diameter. The shape of the crown for each tree was of value when correlated with total nesting- recorded as being either perfectly conical or perfectly bird use. MacArthur (1958) also showed how cylindrical. An Abney level was used for height mea- five species of warblers which overlap in their surements. A total of 30 points ( 120 trees) in the oak- juniper woodland plot, and 25 points (100 trees) in use of foliage may exploit this feature of the the ponderosa pine forest plot was sampled. habitat in different ways. Saplings were counted within circular plots of 0.01 Emlen (1967) has proposed a method for acre (radius of 3.6 m) and measured for volume as de- obtaining the canopy cover value of a plant scribed above for the trees. The tree and sapling data were expressed in terms of community by recording the presence or ab- total volume of foliage per species of tree and distribu- sence of foliage above the observer at predeter- tion by height from the ground. These values were mined points. This method is of value for then converted to a per acre total for each species of comparative purposes, but neither gives the ab- tree for a composite picture of the entire community. solute volume of foliage nor shows its height In addition to recording the location of individual singing males by the spot-map method (Kendeigh distribution above the ground in a plant com- 1944), the species of tree in which each bird was ob- munity. Both aspects of volume of foliage are served and its height from the ground were recorded. important in bird utilization. When possible, information was gathered as to be- The objective of this study is to depict the havior of each species. At least 12 complete daylight counts were made on each area. Counts were made height distribution and volume of foliage in during May, June, and July 1965. two different plant communities and to show I analyzed the interaction of available foliage and how the total avifauna utilizes this dimension observed bird-use in the following two ways. In of their habitat during the breeding season. the first analysis, which yielded the use index value, When possible, foliage use by individual bird the per cent of total volume of foliage contributed by each species of tree (including saplings) was calculated species is discussed to show their contribution from tables 2 and 4. Using this figure. an exoected to the overall pattern of use. Data on utiliza- number of bird observationsper species of‘ tree,-based tion are expressed in terms of volume of foliage on percentage of the total foliage, was calculated. For rather than area. example, if one species of tree contributed 84 per cent of the total volume of foliage in the plant community, then the expected number of bird observations would Present‘ address: Department of Biological Sciences, Nmth- em Arizona University, Flagstaff, Arizona 86001. be 84 per cent of the total observations recorded. This K+QQl The Condor, 71:39Q-412, 1969 400 RUSSELL P. BALDA FIGURE 1. Oak-juniper woodland study area showing Quercus sp., Pinus ZeibphylZu, and Juniperus deppeanu. expected number of bird observations was then com- In the second analysis the total volume of foliage for pared with the actual number of bird observations in each species of tree was subdivided by height classes each species of tree using the standard chi-square pro- of 5-ft intervals from 0 to 30 ft and lo-ft intervals cedure and dividing the total obtained by the number above 30 ft. The volume of foliage for each species of of categories used (different species of trees) minus tree as it was distributed by height was then combined one (degree of freedom). While the procedure de- to give a composite picture of the foliage distribution scribed is identical to that used in determining chi- for the entire plant community. The change in scale square, the observations were not independent (H. W. above 30 ft allowed for the decreased accuracy in de- Norton, pers. comm.) and the total thus obtained termining the height of birds at these elevations. An could not be treated for statistical significance. Rather, expected number of bird observations was calculated the values, after modification as shown below, were in the manner described above. The per cent of total utilized as the use-index. bird observations at each height class was then plotted There is an inverse linear relationship between the against the per cent of total volume of foliage at each total chi-square figure and the number of observa- height class, and a use-index value calculated. tions if the same proportion of the observations occurs in each category. Thus, the larger the number of ob- RESULTS AND DISCUSSION servations, the lower the chi-square value. Since differ- ent numbers of observations were obtained per bird OAK-JUNIPER WOODLAND species, it was necessary to adjust the numher of ob- This study area was located within a large servations per species to correspond to the lowest number gathered for any one species. This was done tract of relatively homogeneous vegetation that by assigning each observation a number and then extended along the south side of Cave Creek randomly removing a number of observations. For ex- about 0.10 miles south of the Southwestern Re- Ltmple, 45 observations were made on Graces’ Warbler search Station. and 125 on the Black-throated Gray Warbler. Thus, 80 The elevation of this area ranged from 5470 observations had to be randomly removed from the latter species. The figure obtained from these calcula- ft at its northern edge up to about 5560 ft at tions the southern limits. A large rocky outcropping Adjusted Expected 3 in approximately the center of the plot had an V - Adjusted Observed ) elevation of 5580 ft. The southern two-thirds + Degrees of freedom of the area is on a north-facing slope of about 1 Adjusted Expected 1 6”. No grazing or fires have disturbed the area will be referred to hereafter as the use-index value. The in the past 10 years (fig. 1). lower the use-index value, the better the goodness-of- fit between expected bird use of available foliage Juniperus deppeanu was the most abundant and observed use of the foliage. of the eight species of trees present. Its impor- FOLIAGE USE BY BIRDS IN SOUTHEASTERN ARIZONA 401 TABLE 1. Composition of trees in the oak-juniper woodland community. Relative Relative Relative Species density* dominame~ frequencyc Junipers &ppeanu 38.3 60.1 (60.1)” 33.3 Pinus leiophylla 21.7. 17.9 (18.2) 19.4 @emus emoqi 20.8 10.7 ( 10.9) 23.6 Quercus a&o&a 17.4 11.0 (10.4) 20.9 Pinus cembroides 1.8 0.3 ( 0.4) 2.8 Total 100.0 100.0 100.0 8 Number of individuals of the species x 100. Number of individuals of all species b Total basal area of the species x 100. Total basal area of all species c Number of points of occurrence of the species x 100. Number of points of occurrence of all species d Sum of relative densitv. relative dominance. and relative frequency. e Calculated from foliag~vchme‘ data (table 2).‘ tance value (sum of relative density, relative and saplings per acre.