Bird Diversity and Composition in Even-Aged Loblolly Pine Stands Relative to Emergence of 13-year Periodical Cicadas and Vegetation Structure
In southern Arkansas, l&year periodical cicadas (Magicicada spp.) were expected to emerge in late April and early May of 1998. Presence of a superabundant food source, such as periodical cicadas, may attract greater numbers of b i r d s a n d m o r e s p e c i e s o f b i r d s t h a n i s u s u a l l y p r e s e n t i n a p a r t i c u l a r a r e a . T h r e e e v e n a g e d l o b l o l l y p i n e (Pinus faeda L . ) stands were surveyed for birds by using a fixed-width transect before, during, and after emergence of the periodical c i c a d a s . E m e r g i n g c i c a d a n y m p h s w e r e t r a p p e d a n d c o u n t e d t o o b t a i n a n e s t i m a t e o f c i c a d a d e n s i t y . T h e d e n s i t y o f cicadas in these pine stands was considerably lower than densities reported for other forest stands in the Eastern U.S. Bird d i v e r s i t y w a s v a r i a b l e b e t w e e n m o n t h s a n d s t a n d s b u t d i d n o t a p p e a r t o b e s t r o n g l y i n f l u e n c e d b y t h e p r e s e n c e o f p e r i o d i c a l c i c a d a s . B i r d c o m p o s i t i o n i n t h e s e m a n a g e d l o b l o l l y p i n e s t a n d s w a s s i m i l a r t o t h a t i n u n m a n a g e d , e v e n a g e d p i n e a n d p i n e / h a r d w o o d s t a n d s o f s i m i l a r a g e s . INTRODUCTION Emergence of periodical cicadas (Homoptera: Cicadidae: Magicicada) is a highly predictable occurrence. The l3-year race of periodical cicadas that occurs in Arkansas is a group of three species including Magicicada tredecim (Walsh and Riley), M. tfedecassirti (Alexander and Moore), and M. tredecula (Alexander and Moore). Adult cicadas synchronously emerge from the ground every 13 years, ascend the nearest tree, and cast their nymphal skin. Adult males aggregate in the forest canopy and form chorusing centers to attract females for mating (Williams and Smith 1991). Adults complete their reproductive cycle and die within 4 to 6 weeks. Adult populations of periodical cicadas can be very dense. Populations in previous emergences have been estimated at tens of thousands to hundreds of thousands of individuals per acre (Dybas and Davis 1962, Rodenhouse and others 1997, White and others 1962). This type of ephemeral, high density occurrence of insects can provide a superabundance of food that may attract greater numbers and more species of insectivorous birds than is usually present in a particular location. In this study, we investigated the influence of 13-year periodical cicadas on bird diversity in even-aged stands of loblolly pine (Pinus taeda L.) in southern Arkansas where periodical cicadas were observed during a previous emergence (personal observation, M.D. Cain, 1985). The study also provided an opportunity to examine bird composition relative to vegetation structure of loblolly pine stands that were managed to maximize pine development. Others have examined bird composition relative to vegetation structure in unmanaged, even-aged pine and pine/hardwood stands (Childers and others 1986, Conner and others 1979, Conner and others 1983, Dickson and Segelquist 1979, Kellner 1996), but this information is lacking for managed stands. Recent concern over declining populations of neotropical migratory songbirds and an increasing interest in managing forests at the ecosystem level warrants an examination of bird communities inhabiting forests managed for timber production (Finch and Stangel 1993). METHODS Study Sites Three plots were surveyed for birds and cicadas on the Crcssett Experimental Forest, Ashley County, Arkansas. Merchantable-sized trees (a3.5 in. d.b.h.) on all plots were primarily loblolly pine, but a few shortleaf pine (Pinus echinafa Mill.) and assorted hardwood species were present. Plot 5 is a 1 O-acre, 25year-old stand measuring 1,320 ft long and 330 ft wide. This stand was precommercially and commercially thinned and periodically prescribe burned in winter (Cain 1996). Burning was initiated in the ninth year to control understory vegetation. The most recent bum was conducted in February 1998. Plot 5 also had patches of unmanaged pine and openings that supported grasses and forbs. Plots 37N and 37s are both 15-year-old loblolly pine stands. Plot 37N is 1,000 ft long and 165 ft wide (3.8 acres), and plot 37s is 660 ft long and 330 ft wide (5.0 acres). As part of one ongoing study, plots 37N and 37s have been similarly managed (Cain 1997). Both plots were precommercially and commercially thinned and competing vegetation was controlled with herbicides, manual removal, or both during the first 5 years of pine development. These two plots also had patches of unmanaged pine. All plot boundaries were well-defined by mowed fire lanes. Bird Survey Birds were surveyed using a fixed-width transect (Mikol 1980). A transect was established through the center of each plot. Bird surveys were conducted along each transect for 2 days each in early-April, mid-May, and mid-June of 1998. Each 2-day survey was completed within a 4-day period. All birds detected by sight or sound within the plot boundary were recorded and plotted on a map as the observer slowly traversed the transect. In addition, all birds detected within 80 ft outside the plot boundary were recorded and mapped for two reasons. First, birds were not confined to our artificially-defined boundaries. Second, birds in the periphery were observed using the study plots. ’ P a p e r p r e s e n t e d a t t h e T e n t h B i e n n i a l S o u t h e r n S i l v i c u l t u r a l R e s e a r c h C o n f e r e n c e , S h r e v e p o r t , L A , F e b r u a r y 1 6 1 8 , 1 9 9 9 . ’ Biological Science Technician and Research Forester, USDA Forest Service, Southern Research Station, P.O. Box 3516, Monticello, AR 71656,
VEGETATION CHARACTERISTICS IMPORTANT TO COMMON SONGBIRDS IN EAST TEXAS
Multivariate studies of breeding bird communities have used principal component analysis (PCA) or several-group (three or more groups) discriminant function analysis (DFA) to ordinate bird species on vegetational continua (Cody 1968, James 1971, Whitmore 1975). In community studies, high resolution of habitat requirements for individual species is not always possible with either PCA or several-group DFA. When habitat characteristics of several species are examined with a DFA the resultant axes optimally discriminate among all species simultaneously. Hence, the characteristics assigned to a particular species reflect in part the presence of other species in the analyses. A better resolution of each species' habitat requirements may be obtained from a two-group DFA, wherein habitats selected by a species are discriminated from all other available habitats. Analyses using two-group DFAs to compare habitat used by a species with habitat unused by the same species have the potential to provide an optimal frame of reference from which to examine habitat variables (Martinka 1972, Conner and Adkisson 1976, Whitmore 1981). Mathematically (DFA) it is possible to maximally separate two groups of multivariate observations with a single axis (Harner and Whitmore 1977). A line drawn in three or n-dimensional space can easily be positioned to intersect two multivariate means (centroids). If three or more centroids for species are analyzed simultaneously, a single line can no longer intersect all centroids unless a perfectly linear relationship exists for the species being examined. The probability of such an occurrence is extremely low. Thus, a high degree of resolution can be realized when a two-group DFA is used to determine habitat parameters important to individual species. We have used two-group DFA to identify vegetation variables important to 12 common species of songbirds in East Texas. METHODS
Depredation of artificial ground nests in a managed, forested landscape
The effects of forest fragmentation on avian nesting success have been examined experimentally using artificial nests in a number of studies (review by Paton 1994). Yahner and Scott (1988), for example, noted a direct relationship between amount of forest fragmentation created by clearcutting and rates of predation on artificial avian nests; conversely, Yahner et al. (1993) did not observe greater predation rates on artificial nests despite greater fragmentation resulting from additional clearcutting on the same study area. Furthermore, as a direct consequence of forest fragmentation the availability of suitable nest sites declines, perhaps enabling predators more readily to find nests located in the remaining forested tracts (patches or corridors) referred to as the ecological-trap hypothesis (Gates & Gysel 1978). Forest clearcutting for ruffed grouse (Bonasa umbellus) habitat in central Pennsylvania (U.S.A.) provided us with an ideal opportunity to test whether depredation of artificial ground nests differed (1) among those placed in contiguous forest, forested corridors, and forested patches and (2) between those placed at sites used repeatedly (fixed nest sites) versus random nest sites in these three habitats. To our knowledge, predation rates on artificial nests at fixed versus random sites as a function of size and shape of forested tracts have not been examined.