Microhabitat features associated with the song perches of Painted and Indigo Buntings (Passeriformes: Cardinalidae) in northeast Texas.
Indigo Buntings (Passerina cyanea) are common passerines that breed through much of North America east of the Great Plains. In central and west Texas, the Indigo Bunting is largely replaced by the congeneric Painted Bunting (Passerina ciris). There is a broad zone of overlap between the two species that extends across much of Oklahoma, east Texas and Louisiana (Sauer et al. 1997).
Painted Buntings are neotropical migrants and, like many neotropical migrant birds, they have recently experienced population declines (Sauer et al. 1997). There are many possible reasons for these declines, but factors associated with reduced breeding performance have been cited as critically important (Terborgh 1992). For Painted Buntings, it has been suggested that, at least in some parts of their distribution, population declines may, in part, be due to competition with Indigo Buntings (Cely 1997). This seems like a plausible hypothesis because much of the available information on these two species suggests that they are ecologically equivalent (Sprunt 1968; Taber & Johnston 1968). In the popular literature (e.g. Peterson 1947; Robbins et al. 1983) both Indigo and Painted Buntings are described as occupying the same breeding habitats; these being brushy areas, river and streamside thickets and forest edges.
On the other hand, Parmelee (1959), who described the breeding behavior of Painted Buntings in southern Oklahoma, seldom observed Painted and Indigo Buntings in the same areas. This suggests that, at least in western sympatric populations, the two species might occupy different habitats and, hence, should not compete directly for resources. Unfortunately, there is no quantitative data available to test this idea. In fact, while Indigo Buntings have been well studied with respect to their association with habitat features (Posey 1974; Conner et al. 1983; Yahner 1986; Best et al. 1995), ecological gradients (Johnston & Odum 1956; Shugart & James 1973), and habitat alteration (Stauffer & Best 1980; Strelke & Dickson 1980; Triquet et al. 1990; Yahner 1993), there is no quantitative data on the habitats occupied by Painted Buntings.
The objective of the current study was to provide a quantitative description of microhabitat features associated with the song perches of territorial male Indigo and Painted Buntings in northeast Texas. In so doing, this study provides the first quantitative description of microhabitat features associated with the breeding territories of Painted Buntings. The data are contrasted between the two species to test the hypothesis that, in northeast Texas, Indigo and Painted Buntings are segregated according to habitat.
MATERIALS AND METHODS
Study area. -- The study was conducted on the agricultural property of Texas A & M University-Commerce near the city of Commerce in Hunt County, Texas. The study area consisted of 5.5 [km.sup.2] of land that varied considerably in elevation and habitat types, but which reflected variation typical of this region. About 60% of the area was composed of managed and unmanaged pasture which contained a variety of wooded streamsides, bottomland woods, upland woods and fencerows. These woodland inclusions varied considerably and represented a wide variety of secondary and mature growths. Cropland, active and idle, made up another 15% of the area. These fields contained no woody vegetation, but were bordered by fencerows. About 10% of the area was managed and unmanaged hay fields. These areas were bounded by fencerows and, in one case, contained a streamside thicket and isolated stands of trees. The remainder of the study area was composed of bottomland woods and idle land in various stages of succession. Dominant trees on the study area were black oak (Quercus velutina), hickory (Carya sp.), green ash (Fraxinus pennsylvanica), sugarberry (Celtis laevigata), pecan (Carya illinoinensis), cedar elm (Ulmus crassifolia), bois d'arc (Maclura pomifera), honey locust (Gleditsia triacanthos), post oak (Quercus stellata), red cedar (Juniperus virginiana), slippery elm (Ulmus rubra) and persimmon (Diospyros virginiana).
Data collection and analyses. -- Song perches of male Indigo and Painted Buntings were located from 5 May 1996 through 25 June 1996. Singing males were identified and the species of tree in which the song perch was located was recorded. To discriminate individuals and to verify that adjacent perches represented different individuals a spot-mapping technique was used (Robbins 1970). Thus, when a song perch was identified, the location of the perch was plotted onto an aerial photograph of the study area along with the locations of immediate neighbors. These simultaneous registrations (Robbins 1970) ensured that adjacent song perches did not belong to the same individuals. At least three visits were made to each song perch to verify the locations of the owner and his neighbors. In this way, each individual could be represented on the photographs as a cluster of points. However, spot mapping can cause some individuals to be overlooked while others are duplicated (see review in Verner 1985). Thus, additional measures were taken to eliminate the possible duplication of individuals in the sample. Notes were made of the song types of each male and his neighbors (distinctive syllables, phrases and accents). Painted Buntings do not share song types with neighbors (Forsythe 1974) and some individuals could be recognized this way. Also, as has been observed in South Carolina and Georgia (Forsythe 1974), many Indigo Buntings sang recognizably distinctive songs that differed from those of their neighbors. Notes on song characteristics were consulted to corroborate the identity of the resident male on subsequent checks to ensure that adjacent song perches did not belong to the same individuals. Thus, using a combination of simultaneous registrations, recognition of individuals when possible and corroborating multiple observations of singing males with the locations of specific song perches, all the song perches used were determined to be those of different individuals. Song perches of 26 Indigo Buntings and 24 Painted Buntings were used for analyses.
Ten sample points distributed in random directions within 30 m of each song perch were located by using a random number generator to assign distances and compass directions. Data on vegetation characteristics were then collected at these sample points. Because the song perches were invariably adjacent to habitat edges, the sample points either fell within fields or wooded areas. Fields were those locations where the dominant vegetation was grasses and forbs and there was no overhead cover greater than 2 m in height. Wooded sites were locations where the dominant vegetation was woody and there was overhead cover greater than 2 m in height. The type of data collected varied according to whether the sample point represented a field site or a wooded site. At field sites, data were collected within a 1 [m.sup.2] quadrat. Data were collected on the percent cover and maximum height of grasses and forbs, the percent of bare ground, and the number of woody and thorny plants present. For each song perch the data from all of the field sites were averaged to provide a single mean value for each variable at each song perch.
When a sampling site was wooded, data were collected on vegetation features within a circular area 4 m in diameter and centered at the sample point. In this case, data were collected on the identity of the nearest tree, the height and distance to the nearest shrub and tree, and the number of vines and thorny plants present within the sample area. For this study, trees were defined as any woody species with a diameter at breast height (dbh) of 5 cm or more, shrubs were woody plants with a dbh of less than 5 cm. Vegetation height was measured using a tape measure or, in the case of taller plants, by using a clinometer. For each song perch the values collected at all the wooded sites were averaged to provide a single value for each variable for each song perch.
Variables were tested for normality using Wilk's Statistic (SAS Institute Inc. 1990). Because most of the variables were not normally distributed, Wilcoxon's Rank Sum Tests were used for making comparisons between species (Pratt & Gibbons 1981, Sokal & Rohlf 1995). In cases where frequency tables were evaluated, Fisher's Exact Multiway Tables were used to determine significance (SAS Institute Inc. 1990).
Analyses of the data collected at random field sites are presented in Table 1. There were no significant differences between Indigo and Painted Buntings with regard to the coverage or height of grass, the height or coverage of forbs, the amount of bare ground exposed, or the number of woody and thorny plants present. Thus, the vegetation characteristics of the open areas surrounding Indigo and Painted Bunting song perches were essentially the same.
Analyses of data collected at the random wooded sites are presented in Table 2. Shrub density, measured as the distance to the nearest shrub from the sample point, and tree density, measured as the distance to the nearest tree from the sample point, were not found to differ for wooded areas associated with Indigo or Painted Buntings (Table 2). There was also no difference between bunting species with regard to overstory height (measured as tree height) or understory height (measured as shrub height) (Table 2). Similarly, Indigo Buntings and Painted Buntings did not differ with respect to the number of vines or thorny plants present in the sample areas surrounding their song perches (Table 2). Thus, the vegetation characteristics of the wooded areas surrounding Indigo and Painted Bunting song perches also appeared to share the same physical structure.
Despite the lack of differences associated with the physical structure of the fields and woods surrounding the song perches of the two species, there were differences in the types of trees associated with Indigo and Painted Buntings. Indigo Buntings were observed singing from nine different species of trees (Table 3), but most commonly were observed in honey locust, green ash and bois d'arc (Fig. 1). These three species accounted for 65.4% of all the song perches observed for this species. Painted Buntings sang from 12 different species of trees (Table 3), but were most commonly observed singing from sugarberry trees (Fig. 1). These trees accounted for 33.3% of all the song perches observed for this species. Use of the five most common song perch tree species was found to differ significantly between Indigo and Painted Buntings (Fisher's Exact Multiway Table, P = 0.0029).
Trees found in the general vicinity of the song perches of Indigo and Painted Buntings were identified from the randomly selected wooded sites. As with the song perches, there were differences in the species of trees associated with each species of bunting (Fig. 2) and these differences were parallel to those observed for song perches. Thus, Indigo Buntings were most frequently associated with honey locust and green ash, whereas Painted Buntings were most often associated with sugarberry, and to a lesser extent, with pecan and cedar elm. These differences were statistically significant (Fisher's Exact Multiway Table, P = 0.0027). Eighteen tree species were found in areas around the song perches of Indigo Buntings, whereas only 11 species of trees were found in areas around the song perches of Painted Buntings (Table 3).
Both Indigo and Painted Buntings are birds characteristic of areas where there is a mixture of wooded and field habitats. In Oklahoma, Parmelee (1959) described Painted Buntings as occurring in areas where there was a mixture of wooded stands interspersed among fields and in areas where there were wooded ravines in otherwise open habitats. Painted Buntings were found in similar areas during the current study. Indigo Buntings have been described in a wide variety of habitats. For example, in Iowa, they were documented in 12 different types of habitat, ranging from tilled row crops and herbaceous fence rows to upland and bottomland forests (Best et al. 1995). On an east Kansas flood plain, Indigo Buntings were found in cropland, oldfields, and mature hardwood forests (Zimmerman & Tatschl 1975). In northwestern Arkansas, Indigo Buntings occurred in xeric forests, woodland edges, oldfields, shrubby fields and mesic forests (Shugart & James 1973). In Illinois, Indigo Buntings were found in early successional shrub habitats, late successional shrub habitats, bottomland forests and a mature upland forest (Karr 1968).
The preceding description suggests that Indigo Buntings are highly adaptable and capable of occupying a wide range of physical habitat. Indigo Buntings, therefore, have the potential to be competitors with Painted Buntings for habitat. The observation that the two species did not differ relative to microhabitat features would tend to support this hypothesis. However, the current study also demonstrated that Painted Buntings and Indigo Buntings were associated with different species of trees, suggesting that some form of habitat segregation, possibly based on floristic associations, may be occurring. As such, Indigo Buntings and Painted Buntings may not be ecologically equivalent. This finding is consistent with the observation by Rotenberry (1985) that, within a given general habitat type, plant species composition may be more important in explaining the local distribution and abundance of bird species than is the physical structure of the vegetation. However, much more detailed study would be needed to determine whether the differences in tree species associated with Painted and Indigo Buntings found in this study truly represent habitat segregation based on floristic associations or whether they represent more subtle differences in microhabitat correlated with the occurrence of the different tree species.
It is also possible that the tendency for Indigo Buntings in northeast Texas to be associated with honey locust and green ash and the tendency for Painted Buntings in the same area to be associated with sugarberry may be correlated to differences in habitat structure on a scale other than at the microhabitat level. For example, at the mesohabitat level, it is reported elsewhere (Crist 1998) that in the same study area, Indigo Buntings occurred more frequently in disturbed bottomlands whereas Painted Buntings occurred more often in more heterogenous, less disturbed, sites. However, given that Indigo Buntings are observed to occupy so many different physical habitat types in other areas, including uplands, bottomlands, disturbed, and undisturbed sites (Taber & Johnston 1968), it seems unlikely that they should be strongly constrained by physical aspects of vegetation structure in northeast Texas. Clearly, more detailed study of the physical and floristic environments associated with Indigo and Painted Buntings is necessary to fully explain the results of the current study.
In summary, this study found no evidence of microhabitat differences relative to vegetation structure surrounding the song perches of Indigo and Painted Buntings in northeast Texas. However, the two species were found to be associated with different species of trees, suggesting that some form of habitat segregation is occurring. For this reason, it seems unlikely that Indigo and Painted Buntings are strong competitors. Therefore, at least in northeast Texas, it is unlikely that recent population declines of Painted Buntings can be attributed to competition with Indigo Buntings.
Table 1. Microhabitat variables measured in 1 [m.sup.2] quadrats at randomly selected field sites located within 30 m of the song perches of Indigo and Painted Buntings. Measure Indigo Bunting Painted Bunting P(a) Mean [+ or -] D Mean [+ or -] SD Percent cover--grasses 28.8 [+ or -] 16.1 36.0 [+ or -] 11.0 0.074 Height of grasses (cm) 75.1 [+ or -] 32.5 67.2 [+ or -] 21.4 0.634 Percent cover--forbs 35.2 [+ or -] 16.9 32.4 [+ or -] 15.4 0.580 Height of forbs (cm) 76.6 [+ or -] 34.5 66.0 [+ or -] 26.3 0.420 Percent cover--bare ground 36.6 [+ or -] 17.8 31.1 [+ or -] 10.5 0.449 Number of woody plants 0.49 [+ or -] 0.45 0.64 [+ or -] 0.63 0.531 Number of thorny plants 0.49 [+ or -] 0.58 0.50 [+ or -] 0.44 0.561 (a) Probabilities based on Wilcoxon's Rank-Sum Tests. n = 26 for Indigo Buntings, n = 24 for Painted Buntings. Table 2. Microhabitat variables measured at randomly selected wooded sample points located within 30 m of the song perches of Indigo and Painted Buntings. Measure Indigo Bunting Painted Bunting P (a) Mean [+ or -] SD Mean [+ or -] SD Distance to nearest shrub (cm) 53.9 [+ or -] 29.9 53.4 [+ or -] 22.1 0.977 Height of nearest shrub (cm) 158.8 [+ or -] 89.5 141.7 [+ or -] 69.5 0.717 Distance to nearest tree (cm) 131.0 [+ or -] 42.6 115.0 [+ or -] 55.5 0.438 Height of nearest tree (cm) 581.8 [+ or -] 312.7 675.5 [+ or -] 287.9 0.387 Number of vines within 2 m 1.1 [+ or -] 0.64 1.2 [+ or -] 0.57 0.847 Number of thorny plants within 2 m 1.1 [+ or -] 0.73 1.2 [+ or -] 0.70 0.138 (a) Probabilities based on Wilcoxon's Rank-Sum Tests. n = 26 for Indigo Buntings, n = 24 for Painted Buntings. Table 3. Species of Trees used as song perches by Indigo and Painted Buntings and species of trees found in randomly selected wooded sites surrounding the song perches of Painted and Indigo Buntings. Numbers represent the number of observations. Tree Species Song Perches Wooded Sites Indigo Painted Indigo Painted Bunting Bunting Bunting Bunting Honey Locust (Gleditsia triacanthos) 7 2 16 7 Green Ash (Fraxinus pennsylvanica) 7 0 13 6 Bois d'Arc (Maclura pomifera) 3 2 3 0 Pecan (Carya illinoensis) 2 2 2 6 Sugarberry (Celtis laevigata) 2 8 5 13 American Elm (Ulmus americana) 1 0 1 0 Black Willow (Salix niger) 1 0 1 0 Post Oak (Quercus stellata) 1 2 0 0 Dead Tree 2 1 0 0 Hercules Club (Zanthoxylum clava-herculis) 0 2 1 2 Cedar Elm (Ulmus crassifolia) 0 1 1 7 Slippery Elm (Ulmus rubra) 0 1 6 0 Bumelia (Bumelia lanuginosa) 0 1 0 1 Hawthorn (Crataegus sp.) 0 1 0 2 Red Cedar (Juniperus virginiana) 0 1 0 0 Water Oak (Quercus nigra) 0 0 0 3 Winged Elm (Ulmus alata) 0 0 0 2 Persimmon (Diospyros virginiana) 0 0 0 1 Elderberry (Sambucus canadensis) 0 0 1 0 Redbud (Cercis canadensis) 0 0 1 0 Possomhaw (Ilex decidua) 0 0 1 0 Black Oak (Quercus velutina) 0 0 1 0 Burr Oak (Quercus macrocarpa) 0 0 1 0 Hickory (Carya sp.) 0 0 1 0 Buckthorn (Rhamnus caroliniana) 0 0 1 0 Chinaberry (Melia azedarach) 0 0 1 0 Percent Frequency Indigo Bunting Painted Bunting Honey Locust 7 2 Green Ash 7 0 Bois d'Arc 3 2 Sugarberry 2 8 Pecan 2 1 Other 5 11 Figure 1. The five most common species of trees used by Indigo and Painted Buntings as song perches. Data are expressed as percent frequency; numbers over bars represent actual frequency. For scientific names of species see Table 3. Note: Table made from bar graph. Percent Frequency Indigo Bunting Painted Bunting Honey Locust 16 7 Green Ash 13 6 Sugarberry 5 13 Pecan 2 6 Cedar Elm 1 7 Other 19 11 Figure 2. The five most common species of trees observed in randomly selected wooded sites surrounding the song perches of Indigo and Painted Buntings. Data are expressed as percent frequency; numbers over bars represent actual frequency. For scientific names of species see Table 3. Note: Table made from bar graph.
We thank Dr. Don Cawthon of the Department of Agriculture at TAMUC for allowing us to use the agricultural property for this research. This study was funded in part by a TAMU-C faculty minigrant to JGK.
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Jeffrey G. Kopachena and Christopher J. Crist
Department of Biological and Earth Sciences
Texas A & M University-Commerce
Commerce, Texas 75429
JGK at: email@example.com
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|Author:||Kopachena, Jeffrey G.; Crist, Christopher J.|
|Publication:||The Texas Journal of Science|
|Date:||May 1, 2000|
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