Nesting ecology of grassland birds following a wildfire in the southern Great Plains.
Two simultaneous wildfires burned [mayor que o igual a]360,000 ha of predominantly private rangelands east of Amarillo, Texas, in March 2006 (Centers for Disease Control, 2007). These wildfires, known as the East Amarillo Complex (EAC) fires, provided an opportunity to examine grassland bird response to a wildfire and to develop insights into the ecological influences of wildfire on grassland communities. The EAC fires potentially resulted in substantial, short-term, negative impacts on many grassland species because of vegetation loss, including local populations of grassland-obligate songbirds that breed in the Southern High Plains such as Cassin's sparrows (Peucaea cassinii), grasshopper sparrows (Ammodramus savannarum), and horned larks (Eremophila alpestris).
We studied the nesting ecology of grassland birds in the areas affected by the EAC fires during the second and third summers following the fires. We had no prefire nesting data for the area, so we compared populations on burned sites with populations on nearby, similar, unburned sites. Our objectives were to examine nesting success and nest site selection among burned and unburned sites 2 and 3 y following the EAC fires. We hypothesized that individual species responses would follow known habitat associations after the fires. Therefore, species that prefer sparse vegetation (e.g., horned lark) would have increased nest success following fire while species that prefer dense vegetation (e.g., Cassin's sparrow) would have decreased nest success.
METHODS--Study Area--We conducted our study on private ranches in Roberts, Gray, and Donley counties in the Texas Panhandle from May through August in 2007 and 2008. This area is a transition zone between short-grass prairie and mixed-grass prairie types in the southern High Plains. The landscape is characterized by rolling hills leading to flat plains and is interspersed with ephemeral wetland depressions (Williams and Welker, 1966). Climate in this area is characterized by hot summers and cold to mild winters. Vegetation in this area was mainly blue grama (Bouteloua gracilis) and buffalo grass (Bouteloua dactyloides), with lower-laying lands and sandy soils containing more little bluestem (Schizachyrium scoparium), switchgrass (Panicum virgatum), and indiangrass (Sorghastrum nutans).
We selected 16 study plots of 12 ha each based on access to private property and burn history. Burn status due to the EAC was established by talking with landowners and local officials and looking for fire scars on woody vegetation such as catclaw mimosa (Mimosa aculeaticarpa) or sand sagebrush (Artemisia filifolia). We equitably designated the 16 plots into burned and unburned plots categories. Plots were at least 1 km apart and we minimized potential edge effects by insuring plot perimeters were 0.5 km from known fire boundaries, roads, or other vegetation or topography changes. We visually compared among burned and unburned areas to choose sites that had similar dominant vegetation and topography. We were not able to regulate grazing on our plots, and various grazing intensities were used throughout the study. All plots were grazed in at least 1 y of the 2 y of our study.
Nesting Success and Characteristics--We established a nest search area of 4 ha (200x200 m) in each of the 16 plots. Within these areas, we conducted focused searches for ground-nesting bird nests using the search-line method (Berthelsen et al., 1989) along with opportunistic searches based on bird behavior (Winter et al., 2003). We did not conduct nest searching during precipitation to avoid the risks of missing nests, putting nests at risk by flushing females during inclement weather, and damaging wet vegetation (Winter et al., 2003). We recorded nest locations with a global positioning system and visually marked locations with flagging [mayor que o igual a]5 m north and south of nests. Subsequent monitoring occurred every 3-4 d until nest outcome was established (Martin and Geupel, 1993). We considered a nest successful if [mayor que o igual a]1 nestling of the host species fledged.
Vegetation Sampling--We measured nest site vegetation within 5 d of determining nest fate. We measured percent cover of grass, forbs, litter, bare ground, and woody plants using a modified (50x50 cm) Daubenmire frame (Daubenmire, 1959) at the nests and 1 m from nests in each cardinal direction. To estimate vertical obstruction, we used a modified vegetation profile board (Nudds, 1977) read from 4 m in each cardinal direction at 1 m height. The profile board was 2 m tall by 10 cm wide and was separated into sections of 1 dm. We estimated percent coverage at each decimeter as well as recording the highest point reached by living or dead vegetation and the nearest woody plant species. Nests located in woody plant species were recorded as 0.01 m to the nearest woody vegetation.
Data Analysis--We calculated Mayfield nest success (Mayfield, 1975) over the entire breeding season and we followed Johnson (1979) to calculate standard errors for nest success rates. We calculated nest success for individual species when sample sizes were sufficient for analysis ([mayor que o igual a]5 nests per treatment) and for all passerines combined to represent the suite of species utilizing southern Great Plains areas. We used three species to represent passerine nest success on our plots. These were the mostcommon nesting species and had similar nesting periods; Cassin's sparrows, lark sparrows (Chondestes grammacus), and grasshopper sparrows. To estimate nesting success, we used an incubation period of 10 d, nestling period of 9 d, and an overall nesting period of 19 d. We obtained duration of each nest stage for each species from Baicich and Harrison (2005). Small sample sizes did not allow for individual plot estimates of nest success and hence standard hypothesis tests were difficult to use. Instead, we used standard errors to place a confidence interval around the difference of the mean nest success to test for differences between burn condition and year (Zar, 1999). This is similar to a t-test in that the confidence interval does not include zero if results are significantly different. For example, if a difference is statistically significant, the following is listed after the results: (0.211 < 0.219 < 0.228). This confidence interval presents the size of the difference between two treatments in proportional nest success.
We analyzed nest site vegetation data using analysis of variance in Program R (The R Foundation, Vienna, Austria, http://wwwR-project.org/) with year and burn condition as independent variables after confirming that data met the assumption of homoscedasticity. In all cases a probability level of 0.05 was used to determine significance. General nest site characteristics of these species have been previously described and, in the context of this paper, the differences between nests built on burned and unburned areas is of greater ecological relevance. Analysis of Daubenmire frame data and vertical densities at each decimeter used an average of the center and the four cardinal directions. We restricted analysis of vertical cover to estimates of 50 cm and below due to negligible data from higher measurements. Examination of all vegetation differences can be found in Roberts (2009).
RESULTS--We found 221 nests of eight species of birds over two field seasons (Table 1), with Cassin's sparrows, grasshopper sparrows, lark sparrows, and western meadowlarks (Sturnella neglecta) accounting for 84% of all nests monitored. Total passerine nest success was greater on burned than on unburned plots (Fig. 1) in both 2007 (0.052 < 0.059 < 0.066) and 2008 (0.073 < 0.082 < 0.090). Nest success was significantly lower in 2008 than in 2007 on both burned (0.146 < 0.152 < 0.158) and unburned (0.165 < 0.175 < 0.184) plots.
Due to sample sizes of nests, we could only justify comparing Mayfield nest success rates for Cassin's sparrow, grasshopper sparrow, and lark sparrow. Cassin's sparrow nest success during this study ranged from 0.202 on unburned sites in 2007 to 0.529 on burned sites in 2007 (Fig. 1). In 2007 Cassin's sparrow nest success was greater on burned than unburned plots (0.269 < 0.328 < 0.387), and burned plots had greater nest success in 2007 compared to 2008 (0.188 < 0.214 < 0.239). Nest success of grasshopper sparrows ranged from 0.320-0.756 during our study. Grasshopper sparrows had greater success in 2008 compared to 2007 on both burned (0.303 < 0.319 < 0.337) and unburned plots (0.509 < 0.557 < 0.605). In 2008, grasshopper sparrow nests were more successful on burned plots compared to unburned plots (0.357 < 0.404 < 0.451). Lark sparrow nest success ranged from 0.297-0.424. Lark sparrow nest success was greater on burned sites in 2007 compared to 2008 (0.070 < 0.088 < 0.107).
Sample size of nests limited analysis of nest site vegetation to Cassin's sparrows (n = 42), grasshopper sparrows (n = 60), and lark sparrows (n = 68; Table 2). Differences in vegetation at nest sites of Cassin's sparrow occurred between years in the litter and bare-ground categories. Nests were found in areas with a higher percentage of litter in 2008 compared to 2007 ([F.sub.1,39] = 18.33, P < 0.001) and lower bare-ground cover in 2008 than in 2007 ([F.sub.1,39] = 13.65, P < 0.001). The single difference of Cassin's sparrow nests between plot types was the finding that there was a higher percentage of woody vegetation ([F.sub.1,39] = 6.28, P = 0.017) around nests in unburned plots compared to burned plots. Three ground cover variables differed between years around grasshopper sparrow nests. Grass ([F.sub.1,57] = 8.75, P = 0.005) and forb ([F.sub.1,57] = 38.29, P < 0.001) cover was higher at nests built in 2007 compared to 2008 while litter cover ([F.sub.1,57] = 63.60, P < 0.001) exhibited the opposite pattern. There were two differences between grasshopper sparrow nests on burned and unburned plots. Grass cover was higher ([F.sub.1,57] = 17.68, P < 0.001) and litter cover was lower ([F.sub.1,57] = 18.72, P < 0.001) around nests in unburned plots compared to burned plots. Four of the five ground cover variables we measured were different between years around lark sparrow nests. Forbs ([F.sub.1,65] = 14.64, P < 0.001), woody vegetation ([F.sub.1,65] = 5.32, P = 0.024), and bare ground ([F.sub.1,65] = 19.07, P < 0.001) were higher around nests in 2007 than in 2008 while litter cover was higher ([F.sub.1,65] = 139.96, P < 0.001) around nests in 2008 compared to 2007. In addition, woody vegetation cover was higher around lark sparrow nests on unburned plots compared to burned plots ([F.sub.1,65] = 6.53, P = 0.013). There were no differences in vertical coverage measurements between years or burn history for any of the three species.
DISCUSSION--Nesting success across the study area and among species was similar to that reported in other studies in the Texas Panhandle (Berthelson and Smith, 1995; Thompson et al., 2009; Conway and Johnson, 2011) but greater than was reported in other grassland bird studies (Rohrbaugh et al., 1999; Fondell and Ball, 2004). For example, Conway and Johnson (2011) found 30% nest success for Cassin's sparrows (n = 15). Rohrbaugh et al. (1999) found grasshopper sparrow nest success ranging from 10-20% over two treatments for 38 nests. Texas Panhandle grasslands are relatively continuous over large scales compared to other areas in the Great Plains, allowing wildfire to potentially impact entire landscapes but also providing many microhabitats within the landscape. This may result in greater nest success than observed in smaller patches of grasslands. In general, nest success was higher on burned plots, suggesting the alteration of vegetation improved nesting conditions for a variety of species.
Differences of nest placement between burned and unburned sites were few but were consistent with changes in a postfire grassland landscape. Cassin's sparrow and lark sparrow nests were built in areas with less woody vegetation on burned plots, consistent with fires removing woody debris. Lark sparrows nest in woody cover more than most grassland birds (Lusk et al., 2003), and Cassin's sparrows use woody perches for their breeding displays and nest sites in many areas of the southwestern united States (Dunning et al., 1999). The intensity of the EAC fires likely removed woody vegetation on burned plots, leaving fewer song perch sites for Cassin's and lark sparrows. After a prescribed fire in Arizona, researchers found a decreased abundance of small mesquite plants and Cassin's sparrows (Kirkpatrick et al., 2002). After a fire in sagebrush grasslands in Washington State, long-term lark sparrow abundances decreased along with the decrease in woody vegetation (Earnst et al., 2009). Thus, nests placed in woody vegetation may have been more common on areas that had not been burned due to more woody material being available. Alternatively, sand sagebrush, a common shrub on our sites, is more resilient to fire, and structural characteristics recover in 3-4 y after prescribed fires (Winter et al., 2011). Our findings are consistent with those of Bock and Bock (1987), who found lark sparrows avoided areas with little or no woody vegetation after a wildfire in Montana. During the breeding season immediately following the EAC fires in 2006, there was sparse rainfall. However, above average rainfall during 2007 resulted in similar biomass production between burned and unburned plots by the end of the 2007 growing season (Rideout-Hanzak et al., 2011). Vegetation measurements around the nests of the three sparrow species revealed lower litter cover and more bare ground in 2007 before the biomass fully recovered. Subsequently, there were few differences in ground cover variables in 2008, after which vegetation seemed to recover (Wester et al., 2014).
An examination of a larger suite of vegetation differences found that there were more differences between nest sites and random sites than between burned and unburned areas (Roberts, 2009). This suggests that individual species were selecting a specific site within the landscape, and those site types were available on both burned and unburned areas by the second summer after the EAC wildfires. This was also seen in studies of vegetation response to the EAC where ground cover was fully recovered 3 y after the fires (Wester et al., 2014). Vertical structural cover seemed to be important for all bird species, as nests were placed in taller, denser vegetation such as little bluestem or three awn (Aristida species) rather than the dominant blue grama and buffalo grass. Lark sparrows showed the greatest association with dense vertical vegetation (Roberts, 2009).
Fire effects on birds are likely confounded by fire severity, and the time since fire and responses by individual species likely vary by fire intensity (Smucker et al., 2005). As early as two breeding seasons after the wildfires, it seems there had been little effect on nest success or nest placement strategies by individual species, possibly as a result of increased vegetation growth due to higher than average rainfall. Rideout-Hanzak et al. (2011) found similar standing crop biomass after the second growing season between burned and unburned plots associated with the EAC wildfires, and this pattern has been seen elsewhere in the short-grass prairie (Schein-taub et al., 2009). This follows a similar timeline presented by Launchbaugh (1964), who found vegetation recovery from dry season fire in the southern Great Plains typically requires no longer than 3 y.
Ultimately, it appears that despite the size and intensity of the EAC fires (Centers for Disease Control, 2007), those variables presented few if any persisting negative influences on the nesting grassland birds in our study. Rohrbaugh et al. (1999) came to a similar conclusion that burning does not negatively affect reproductive success of many grassland birds. The combination of varying grazing regimes and periodic prescribed fire in the Texas panhandle would provide a mosaic of grassland patches in varying stages of recovery from disturbance (Herkert, 1994; Rohrbaugh et al., 1999; Fuhlendorf et al., 2006). Varying applications of the historic disturbance regimes of drought, grazing, and fire may attract a variety of grassland birds with a gradient of habitat associations (Coppedge et al., 2008). Drought and grazing are already present on this landscape but fire has yet to return to historic levels. Prescribed fire has been used to mimic wildfire effects and reduce wildfire potential (Pattison, 1998); increasing its application in the Southern Great Plains may serve to replicate the historic pattern of disturbance to grassland communities.
Submitted 23 December 2015. Accepted 31 January 2017. Associate Editor was Michael Scott Husak.
This work would not have been possible without the cooperation of the following landowners: H. Boone, D. Burger, K. Burger, K. Flowers, S. Hale, J. Hutchinson, B. Ragsdale, J. Rhoades, and J. Shaw. Funding for this study was provided by the Natural Resources Conservation Service and the Texas Parks and Wildlife Department. We thank S. Piedra for review. We also thank N. Cook and R. Mayo for assistance with collecting field data. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
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ANTHONY J. ROBERTS, * CLINT W. BOAL, AND HEATHER A. WHITLAW
Department of Natural Resources Management, Texas Tech University, Lubbock, TX 79409 (AJR)
U.S. Geological Survey, Texas Cooperative Fish and Wildlife Research Unit, Lubbock, TX 79409 (CWB)
U.S. Fish and Wildlife Service, Lawton, OK 73552 (HAW)
Present address of AJR: Division of Migratory Bird Management, U.S. Fish and Wildlife Service, Laurel, MD 20708
* Correspondent: firstname.lastname@example.org
TABLE 1--Number of nests found and percent composition (in parenthesis) in 2007 and 2008 for nests monitored on grassland plots associated with the East Amarillo Complex wildfires of 2006 in the Texas panhandle. 2007 Burned Unburned Mourning dove (Zenaida macroura) 2 (4.3) 4 (7.1) Common nighthawk (Chordeiles minor) 1 (2.1) 2 (3.6) Scissor-tailed flycatcher 1 (2.1) 2 (3.6) (Tyrannus forficatus) Horned lark (Eremophila alpestris) 3 (6.4) 0 (0) Cassin's sparrow (Peucaea cassinii) 6 (12.8) 8 (14.3) Grasshopper sparrow (Ammodramus savannarum) 12 (25.5) 20 (35.7) Lark sparrow (Chondestes grammacus) 20 (42.6) 18 (32.1) Western meadowlark (Sturnella neglecta) 2 (4.3) 2 (3.6) 2008 Burned Unburned Mourning dove (Zenaida macroura) 5 (5.8) 1 (3.1) Common nighthawk (Chordeiles minor) 1 (1.2) 1 (3.1) Scissor-tailed flycatcher 0 (0) 2 (6.3) (Tyrannus forficatus) Horned lark (Eremophila alpestris) 6 (7) 3 (9.4) Cassin's sparrow (Peucaea cassinii) 24 (27.9) 4 (12.5) Grasshopper sparrow (Ammodramus savannarum) 24 (27.9) 4 (12.5) Lark sparrow (Chondestes grammacus) 18 (20.9) 12 (37.5) Western meadowlark (Sturnella neglecta) 8 (9.3) 5 (15.6) Total Mourning dove (Zenaida macroura) 12 (5.4) Common nighthawk (Chordeiles minor) 5 (2.3) Scissor-tailed flycatcher 5 (2.3) (Tyrannus forficatus) Horned lark (Eremophila alpestris) 12 (5.4) Cassin's sparrow (Peucaea cassinii) 42 (19) Grasshopper sparrow (Ammodramus savannarum) 60 (27.1) Lark sparrow (Chondestes grammacus) 68 (30.8) Western meadowlark (Sturnella neglecta) 17 (7.7) TABLE2--Nest site characteristics of Cassin's sparrows, grasshopper sparrows, and lark sparrows on burned and unburned grassland sites associated with the East Amarillo Complex wildfires of 2006 for 2 y postfire. Ground cover estimates were taken using a modified Daubenmire frame. Grass Forbs Woody Cassin's sparrow nn Mean SE Mean SE Mean SE 2007 Burned 6 24.3 2.6 15.8 2.9 10.0 4.6 Unburned 8 19.1 2.2 16.6 3.8 22.0 4.6 Total 14 21.3 1.8 16.3 2.4 16.9 3.6 2008 Burned 24 24.1 1.6 13.9 1.3 12.9 1.9 Unburned 4 32.0 7.4 5.3 1.6 22.0 10.4 Total 28 25.2 1.7 12.7 1.2 14.2 2.1 2007 Burned 12 42.1 5.3 27.1 4.1 2.5 1.6 Unburned 20 50.7 5.3 13.4 1.4 3.0 1.4 Total 32 47.5 3.9 18.5 2.1 2.8 1.0 2008 Burned 24 25.0 1.5 10.3 0.8 4.6 1.1 Unburned 4 41.8 3.1 4.2 2.1 1.3 1.3 Total 28 27.4 1.8 9.5 0.8 4.2 1.0 2007 Burned 20 25.4 2.8 14.6 1.9 5.2 1.4 Unburned 18 30.1 3.0 10.7 0.7 7.8 2.0 Total 38 27.6 2.0 12.7 1.1 6.4 1.2 2008 Burned 18 29.4 1.8 7.7 0.9 0.9 0.5 Unburned 12 35.7 3.4 6.9 1.7 5.6 1.3 Total 30 31.9 1.8 7.4 0.9 2.8 0.7 Litter Bare ground Cassin's sparrow Mean SE Mean SE 2007 Burned 11.8 2.6 42.6 6.2 Unburned 16.4 0.9 27.2 4.1 Total 14.5 1.3 33.8 4.0 2008 Burned 25.4 1.9 26.4 1.9 Unburned 25.5 2.4 16.7 3.5 Total 25.4 1.6 25.0 1.8 2007 Burned 11.4 1.6 20.0 4.7 Unburned 13.3 1.6 22.2 4.0 Total 12.6 1.2 21.4 3.0 2008 Burned 33.6 2.3 28.7 2.6 Unburned 30.8 2.3 20.3 5.2 Total 33.2 2.0 27.5 2.4 2007 Burned 7.7 0.9 50.2 3.7 Unburned 12.4 1.6 42.1 4.6 Total 9.9 0.9 46.3 2.9 2008 Burned 29.9 1.8 30.9 3.7 Unburned 27.3 2.0 25.6 4.6 Total 28.8 1.4 28.8 2.9 FIG. 1--Mayfield nest success (proportion and standard deviation) of Cassin's sparrows, grasshopper sparrows, lark sparrows, and the three species combined (Passerines) during 2007 and 2008 on grassland plots associated with the East Amarillo Complex wildfires of 2006 in the Texas Panhandle. Numbers at the base of each bar are sample sizes of nests in the relevant category. Cassin's Sparrow Proportion net success 2007 2008 Burned 3 17 Unburned 4 5 Lark Sparrow Proportion net success 2007 2008 Burned 12 10 Unburned 12 7 Grass hopper Sparrow Proportion net success 2007 2008 Burned 6 13 Unburned 12 4 Passerines Proportion net success 2007 2008 Burned 21 40 Unburned 28 16 Note: Table made from bar graph.
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|Author:||Roberts, Anthony J.; Boal, Clint W.; Whitlaw, Heather A.|
|Date:||Mar 1, 2017|
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