2013 SOUTHEASTERN NATURALIST 12(2):387–398 Nest Success and Reproductive Ecology of the Texas Botteri’s Sparrow (Peucaea botterii texana) in Exotic and Native Grasses Katherine S. Miller1,*, Erin M. McCarthy1, Marc C. Woodin2, and Kim Withers3 Abstract - Very little information is available for Peucaea botterii texana (Texas Botteri’s Sparrow) and nothing is known about its nesting ecology, in part due to its cryptic behavior and nesting strategies. Our goal was to examine the nesting ecology of Texas Botteri’s Sparrows and compare reproductive success between exotic and native grasslands. We searched for and monitored nests in 2004 and 2005 on the King Ranch in southern Texas. We found no relationship in reproductive effort, nest characteristics, and plant species richness around the nest between grassland types. Vegetation surrounding Texas Botteri’s Sparrow nests was significantly taller and denser in native grasslands than in exotic grasslands. Further research on nesting ecology for the Texas Botteri’s Sparrow is necessary to determine its habitat needs and its role as an indicator of grassland quality. Introduction Many North American grassland bird species are declining at faster rates than any other bird group, mostly due to habitat loss from fragmentation, deterioration from human urbanization, and agricultural conversion (Vickery and Herkert 2001, Winter and Faaborg 1999). Fragmentation can increase predation, the primary cause of nest failure for most ground-nesting passerines (Dion et al. 2000, Herkert et al. 2003, Jones 2003). Fragmentation can also lower reproductive success, the number of fertile offspring per mated pair. Exotic invasive species are another problem; monocultures of exotic grasses often have few native grass and forb species, which may reduce the abundance and diversity of insects, seeds, and cover (Bock et al. 1986, Flanders et al. 2006). Battin (2004) and Jones (2003) addressed ecological traps, where animals may prefer a low-quality habitat to a habitat of higher quality. Certain studies (Jones 2003, Lloyd and Martin 2005) have found grassland birds nesting in exotic grasses as often as in native grasses, which are usually assumed to be the higherquality habitat. Nearly half of the federally threatened and endangered species, including grasses, are at risk due to competition and predation from non-indigenous species (Wilcove et al. 1998). Although grassland bird reproductive success may be linked to population declines, this can be hard to determine because nests can be difficult to locate and monitor (Davis 2003). Nests of Peucaea botterii Sclater (Botteri’s Sparrow), a ground-nesting, tall-grass specialist (Webb and Bock 1996), are notoriously 1Department of Physical and Life Sciences, Texas A&M University-Corpus Christi, Corpus Christi, TX 78412. 2Tejas EcoServices, Inc., Corpus Christi, TX 78412. 3Center for Coastal Studies, Texas A&M University-Corpus Christi, Corpus Christi, TX 78412. *Corresponding author - Katherine.Miller@students.tamuk.edu. 388 Southeastern Naturalist Vol. 12, No. 2 difficult to find. Texas Parks and Wildlife Department lists P. b. texana (Texas Botteri’s Sparrow) as threatened (www.tpwd.state.tx.us), due to its restricted breeding range and its characterization as an indicator of an open, grassy, ungrazed environment (Marshall and Clapp 1985). The Texas Botteri’s Sparrow breeds in a coastal strip along the Gulf of Mexico, from the Rio Grande north to Copano Bay in San Patricio County (about 180 km), and south to Tampico, Veracruz, Mexico (about 400 km) (Marshall and Clapp 1985, Swanson 1985, Webb and Bock 1996). While a number of studies have focused on P. b. arizonae (Arizona Botteri’s Sparrow; Bock and Bock 1978, Jones and Bock 2001, Kirkpatrick et al. 2002, Maurer et al. 1989), very little research has focused on the nesting habits of Texas Botteri’s Sparrows. In a study conducted at Laguna Atascosa National Wildlife Refuge in extreme southern Texas, Marshall and Clapp (1985) primarily focused on singing males and breeding territories, and conducted density surveys. Although observations of its habitat (Harper 1930) and relationship to other species in the Aimophila (now Peucaea) genus (Wolf 1977) have been made previously, this is the first research to focus on the nesting ecology of the Texas Botteri’s Sparrow. With so little data available, our goal was to collect basic information on the nesting ecology of the Botteri’s Sparrow in southern Texas. Our objectives were to examine overall reproductive effort of Texas Botteri’s Sparrows, as measured by clutch size and nesting success, and to examine effects of nest placement by comparing reproductive effort, nest characteristics, and nest site microhabitat for nests in exotic and native grasses. Study Area Breeding habitat of Texas Botteri’s Sparrows is characterized by three ecological regions (Texas Parks and Wildlife Department 1995). Prairies and marshes of the lower Gulf Coast are dominated by Spartina spartinae (Trin.) Merr. ex A.S. Hitchc. (Gulf Cordgrass) and Borrichia frutescens (L.) DC. (Sea Oxeye Daisy). Coastal sand plains are characterized by Schizachyrium scoparium L. (Little Bluestem) interspersed with Quercus virginiana Mill. (Live Oak) and Prosopis glandulosa Torr. (Honey Mesquite). South Texas brush country, the area of which has dramatically increased due to overgrazing and fire suppression, is characterized by Honey Mesquite, Acacia spp. (acacia), and Opuntia spp. (prickly pear). Exotic grasses occurring in these areas include Cynodon dactylon (L.) Pers. (Bermudagrass) and Pennisetum ciliare (L.) Link (Buffelgrass). Currently, most of the breeding range for Texas Botteri’s Sparrows occurs in privately owned ranches and federal preserves (Laguna Atascosa NWR, Palo Alto National Historical Site, and tracts of the US Fish and Wildlife Service’s Lower Rio Grande Refuge system). This study was conducted on the Laureles and Norias divisions of the King Ranch, near Kingsville, TX. The ranch is 333,866 ha, and research was conducted in areas of the ranch where the majority of the Texas Botteri’s Sparrows 2013 K.S. Miller, E.M. McCarthy, M.C. Woodin, and K. Withers 389 had been detected within the last 10 years (T. Langschied, King Ranch, Kingsville, TX, pers. comm.). The Laureles Division, 52 km south of Corpus Christi on the north shore of Baffin Bay, consists of several habitat types: fallow agricultural fields, mesquite-thorn scrub, higher and drier grasslands with mesquite and prickly pear cactus; and extensive lowland Gulf Cordgrass flats. The Norias Division, 115 km south of Corpus Christi, has stands of cordgrass among mesquite thickets, prickly pear clusters, and Acacia farnesiana (L.) Willd (Huisache), but it lacks the extensive open cordgrass flats of Laureles. Two pastures frequently surveyed in the Norias Division had been root-plowed five to ten years earlier and were dominated by the exotics Bermudagrass and Buffelgrass, and these pastures were frequented by resident and migratory birds. Cattle, horses, and exotic ungulates intermittently graze on all ranch divisions. Methods We collected data over two field seasons, a pilot season from June to August of 2004, and a primary season from April to August of 2005. In 2004, we visited pastures to acquaint ourselves with the birds’ habits, behaviors, nests, and habitat preferences. In 2005, we visited the same pastures five to six days a week to search for and monitor nests. Nest-searching was accomplished by observing parental behaviors (Martin and Geupel 1993), systematic searching, and fortuitous flushes. We tied flagging to shrubs at least 10 m from the nest to mark the nest. We revisited the nest every 3–4 days to collect data on clutch size, nestling age, and nest fate. If a nest remained empty after two successive visits, it was considered inactive, and we proceeded to collect nest dimensions and microhabitat data. Overall reproductive effort We considered two factors important for reproductive effort: clutch size and nest success. We followed Webb’s (1985) description of clutch size: the total number of eggs laid by the female, either when found in laying or incubation, or inferred from the number of nestlings when found in that stage. We used a series of photographs from hatch day and subsequent days from a nest where hatch date was known, together with Webb and Bock’s (1996) descriptions, to estimate the age of nestlings for which we did not know the hatch date. These ages were used in calculating Mayfield nest success (Mayfield 1961, 1975). Nest characteristics At each nest, we collected data on primary nest vegetation, nest-cup dimensions, nest height, and concealment. We identified the primary nesting vegetation species in which the nest cup was built, and characterized it as exotic or native. We determined nest-cup dimensions by measuring the diameter (inside rim to rim) and depth (bottom to the top of the nest cup) in cm. We also compared the height of the nests in exotic and native grasses. For nests not directly on the ground, nest height was the distance (cm) from the ground to the bottom of the nest cup (Burhans 1997). 390 Southeastern Naturalist Vol. 12, No. 2 Nest concealment pertains to the amount of vegetation concealing the nest from view. We used Olympus digital cameras to take photos of the nest from a distance of 1 m above the nest and from each cardinal direction. We imported the photos into Adobe® Photoshop and divided the pixels covered by vegetation by total pixels to determine percent concealment (Ortega et al. 2003). A “concealment score” (all five values summed for each nest) and an average percent concealment were calculated. Nest site microhabitat We used the line transect method (Chase 2002) to determine nest site microhabitat, quantify herbaceous and woody plant species, and determine vegetation height and density within a 5-m radius surrounding the nest. We placed two 10-m rope line transects, marked in 1-m increments, in random directions with the nest at the center point of both ropes. We identified plants to species and use these to assess species richness (the number of species around the nest). At each meter along the transect, we placed a meter stick perpendicular to the transect line and measured the height of the vegetation. We determined vegetation density by recording the number of times the vegetation touched the stick in each 50-cm interval on the stick (Chase 2002). These measurements were averaged to calculate mean vegetation height. A higher mean number of hits showed greater vegetative density. Statistical analyses Overall reproductive effort. We defined a successful nest as one where at least one nestling survived to fledging. We compared clutch size for successful nests to that of failed nests. Clutch size was not normally distributed, therefore we used a Mann-Whitney rank sum test. We calculated overall nest success in two ways: apparent and Mayfield (Mayfield 1961, 1975). To calculate apparent nest success, we divided the number of successful nests by the total number of nests. We present apparent nesting success for comparisons to Arizona Botteri’s Sparrow and other grassland species; however, we acknowledge that it is a biased estimate. Therefore, we also used the Mayfield method to calculate daily survival rates for each nestling stage (incubation, hatching, and nestling periods), thus determining the probability that a nest will survive from incubation to fledging. To determine survival in incubation and nestling stages, the length of the nesting stage must be known. Using nest data from both seasons and data from Webb (1985), we used 12 days as the incubation period and 9 days as the nesting period. All nestlings shared the same hatch day; therefore, we calculated hatching success but did not factor in a time period for this stage. Standard deviations for the Mayfield probability were calculated using methods for a binomial distribution (Ott and Longnecker 2001, Winter 1999). Nest success was then compared between exotic and native grasslands, using a chi-squared test. Nest characteristics and nest site microhabitat. We wanted to determine if the type of grassland (exotic vs. native) had any effect on nest characteristics. We 2013 K.S. Miller, E.M. McCarthy, M.C. Woodin, and K. Withers 391 compared nest-cup diameter, depth, height, and concealment, and nest-site characteristics between exotic and native primary nesting vegetation using Student’s t-tests (α = 0.05). We used SigmaStat v. 2.03 to conduct statistical analyses. All values are presented as means ± SD. Results We found a total of 50 Texas Botteri’s Sparrow nests, 37 of which were found in an active nesting stage. Of those, 35 nests provided us with enough information for Mayfield calculations. Out of 15 nests that were successful, 11 were in exotic grasslands. Out of the 22 nests that were failures, 13 were in exotic grasslands. Overall reproductive effort Clutch size ranged from 1 to 4 eggs or nestlings, with 3 being the mode. Overall clutch size was 2.97 ± 0.81 (n = 36 nests). There was no significant difference in clutch size between native (3.00 ± 0.87) and exotic grasses (2.83 ± 0.76) (P = 0.30). Predators and flooding destroyed 4 of the 8 nests found in 2004, before data on nest characteristics were collected. The overall apparent nesting success for 2004 and 2005 combined was 41% (n = 37; Table 1). The Mayfield probabilities for incubation, hatching, and nestling periods, and for survival from incubation to fledging, are shown for both years (Table 2). Two nests from 2005 were destroyed before adequate data were obtained for Mayfield calculations. Due to the small sample size in 2004, we combined the data from both seasons before conducting statistical analyses. Apparent nest success was unrelated to grassland type (native vs. exotic: χ2 = 0.794, P > 0.05). There was no significant association in Mayfield probabilities of survival from incubation to fledging between native (0.41 ± 0.01) and exotic (0.30 ± 0.02) grasses (P = 0.43, α = 0.05). The primary cause of nest failure for Texas Botteri’s Sparrows in this study was predation. We were not able to determine type of predator for 13 of the nests because there were no signs of predation (e.g., broken egg shells, damaged nest cups). Solenopsis invicta Buren (Red Imported Fire Ants) also found three nests and killed the chicks, leaving the nest cup intact. In 2005, one nest that was built into a Red Imported Fire Ant mound was later abandoned. Table 1. Nest fate and apparent nesting success for Texas Botteri’s Sparrows on the King Ranch, southern Texas, 2004–2005A. 2004 2005 Native Exotic Total Native Exotic Total Overall Fate of nest Successful (n) 1 2 3 3 9 12 15 Failed (n) 2 3 5 6 10 16 21 Abandoned (n) 0 0 0 1 0 1 1 Total (n) 3 5 8 10 19 29 37 Apparent nesting success 37% 41% 41% ADue to the small sample size in 2004, we combined the data with 2005 for statistical analyses. 392 Southeastern Naturalist Vol. 12, No. 2 Nest characteristics Overall cup diameter averaged 6.9 cm ± 0.6, and overall cup depth averaged 6.6 cm ± 1.0. There was no significant difference in cup diameter between nests in native (6.89 ± 0.65) and exotic (6.91 ± 0.06) grasses (P = 0.88, α = 0.05). Cup depth did not differ significantly between native (6.41 ± 1.07) and exotic (6.68 ± 0.91) grasses (P = 0.26, α = 0.05). Only 8 nests, built into Gulf Cordgrass, were located above the ground. The mean height was 19.3 cm ± 11.8. All nests located in exotic grasses were built directly on the ground. Statistically, there was no difference in concealment of nests built in native grasslands (94% ± 4%) compared to those built in exotic grasslands (96% ± 5%; P = 0.32, α = 0.05); however, average concealment percentages tended to be lower for nests in native grasses (n = 17) than for those in exotic grasses (n = 33). Nest-site microhabitat In addition to the 37 nests found during an active nesting stage, we found 13 Texas Botteri’s Sparrows nests that were no longer active, but were of use to us for nest-site microhabitat analyses. Texas Botteri’s Sparrows nested in both exotic Table 2. Mayfield probabilities of nest survival for Texas Botteri’s Sparrows on the King Ranch in southern Texas for 2004 and 2005. This shows the probability (P) of nest survival for each nesting phase (I = incubation, H = hatching, and N = nestling), and overall nest survival from incubation to fledging (I to F). Data from both years were combined for overal l Mayfield probabilities. Year n P I P H P N P I to F 2004A Native 4 0.67 ± 0.01 0.58 ± 0.14 0.34 ± 0.06 0.34 ± 0.05 Exotic 4 1.00 ± 0.00 1.00 ± 0.00 0.39 ± 0.17 0.39 ± 0.17 Total 8 0.86 ± 0.01 0.82 ± 0.06 0.46 ± 0.09 0.45 ± 0.09 2005 Native 11 0.45 ± 0.01 0.36 ± 0.00 0.29 ± 0.04 0.29 ± 0.04 Exotic 16 0.65 ± 0.04 0.64 ± 0.06 0.52 ± 0.06 0.36 ± 0.12 Total 27 0.57 ± 0.03 0.54 ± 0.04 0.43 ± 0.05 0.35 ± 0.09 Overall Native 15 0.50 ± 0.01 0.41 ± 0.03 0.30 ± 0.04 0.30 ± 0.04 Exotic 20 0.72 ± 0.05 0.70 ± 0.08 0.52 ± 0.07 0.42 ± 0.16 Total 35 0.61 ± 0.49 0.57 ± 0.03 0.42 ± 0.06 0.36 ± 0.10 ADue to the small sample size in 2004, we combined the data with 2005 for statistical analyses. Table 3. Scientific and common names of grass species used as nest support by Texas Botteri’s Sparrows on the King Ranch, southern Texas, 2004–2005. Binomial name, common name Exotic or native Number of nests Cynodon dactylon, Bermudagrass Exotic 27 Spartina spartinae, Gulf Cordgrass Native 8 Dichanthium annulatum, Kleberg Bluestem Exotic 5 Paspalum denticulatum, Longtom Paspalum Native 4 Pennisetum ciliare, Buffelgrass Exotic 3 Paspalum monostachyum, Gulfdune Paspalum Native 2 Setaria parviflora, Knotroot Bristlegrass Native 1 2013 K.S. Miller, E.M. McCarthy, M.C. Woodin, and K. Withers 393 (n = 33) and native (n = 17) grass species (Table 3). Exotic grasses in which nests were built included Bermudagrass (Fig. 1A), Buffelgrass, and Dichanthium annulatum (Forssk.) Stapf (Kleberg Bluestem). Nests were also constructed in the following native grasses: Gulf Cordgrass (Fig. 1B), Paspalum denticulatum Trin. Figure 1. Digital image of Texas Botteri’s Sparrow nests, indicated by rectangle, on the King Ranch in southern Texas. Nests were built on the ground in: A. non-native grass (Bermudagrass), and B. native grass (Gulf Cordgrass). 394 Southeastern Naturalist Vol. 12, No. 2 (Longtom Paspalum), P. monostachyum Vasey (Gulfdune Paspalum), and Setaria parviflora (Poir.) Kerguélan (Knotroot Bristlegrass). In addition to grass species, a wide range of herbaceous and woody plants were found in areas surrounding Texas Botteri’s Sparrow nests. The most common plant species were Mimosa latidens (Small) B.L .Turner (Karnes Sensitive Briar), Clematis drummondii Torr. and A. Gray (Texas Virgin’s Bower), and Honey Mesquite. There was no significant difference (P = 0.16) between plant species richness in a 5-m radius of nests built in native (4.2 ± 1.4) and exotic grasses (3.6 ± 1.5). However, there was a tendency for more species to occur in native grasslands than in exotic grasslands. There was a significant difference (P < 0.001) between the height of vegetation surrounding nests built in exotic vegetation and those constructed in native vegetation. The average height of vegetation surrounding nests built in native grass was 43.2 cm ± 12.8, whereas the average height of vegetation surrounding nests built in exotics was 28.7 ± 10.9. The density of vegetation in a 5-m radius around the primary nesting vegetation was higher in native grasses (5.4 hits/10 m2 ± 5.2) than in exotic grasses (1.8 hits/10 m2 ± 4.1; P = 0.011). Discussion Overall reproductive effort Average clutch size was lower for Texas Botteri’s Sparrows than for the Arizona subspecies (3.41 ± 0.05 SE, n = 101, Jones 2003; 3.3 ± 0.70 SE, n = 23, Webb 1985). However, our clutch size is not unreasonable for a grassland ground-nesting sparrow. We expect that with a larger sample size our clutch size would likely be more comparable to the Arizona populations. Apparent nesting success was also lower for Texas Botteri’s Sparrows than for Arizona Botteri’s Sparrows (58.3%, n = 24 nests; Webb 1985). We acknowledge that apparent nest success is a biased estimate; observers are more likely to find successful nests. Webb (1985) reported apparent nest success for Arizona Botteri’s Sparrows, and we are reporting apparent nest success for the purposes of comparison to her research and to other grassland bird studies. Webb also included in her thesis the information necessary to calculate Mayfield estimates, allowing us to make comparisons between the populations. The probability of a nest surviving from incubation to fledging was higher in Arizona Botteri’s Sparrow populations (0.513). In both Botteri’s Sparrow populations, there was no significant difference in survival between habitat types. Research on the Arizona Botteri’s Sparrow population also indicated that the number of young fledged/adult male and the number of young fledged/plot did not differ significantly between three habitats (exotic, native, and sacaton habitats; Jones 2003). Nest characteristics Within the scope of our study, Texas Botteri’s Sparrows used exotic grasses more than native grasses as their primary nesting vegetation. We did not measure availability of exotic and native grass species; however, the majority of nests 2013 K.S. Miller, E.M. McCarthy, M.C. Woodin, and K. Withers 395 were found in an exotic grassland monoculture of Bermudagrass. It is possible that the Texas Botteri’s Sparrows are following the same expansion trend as the Arizona subspecies. In Arizona, the exotic Eragrostis sp. (African lovegrass) resembles sacaton, and the Botteri’s Sparrows have expanded their range with the expansion of the lovegrass (Bock et al. 1986, Jones and Bock 2005, Webb and Bock 1996). This change in nesting habitat has also been seen in several other North American sparrows: Melospiza melodia Wilson (Song Sparrows; Arcese et al. 2002), Spizella wortheni Ridgway (Worthen’s Sparrows; Behrstock et al. 1994), Peucaea cassinii Woodhouse (Cassin’s Sparrows; Dunning et al. 1999), and Ammodramus bairdi Audubon (Baird’s Sparrows; Green et al. 2002), which are increasing their acceptance of cultivated lands that now structurally resemble prairie (Green et al. 2002). One reason for the Texas Botteri’s Sparrows’ usage of exotic grasses may be the structural resemblance of some (e.g., Bermudagrass) to native grasses (Gulf Cordgrass). On the King Ranch, exotic pastures most heavily populated by nesting Botteri’s Sparrows had been root-plowed, leaving high and uneven soil mounds, which have been colonized by Bermudagrass and resembled fields of tall, dense Gulf Cordgrass. This may have attracted the Botteri’s Sparrows to exotic grasses; however, long-term effects of nesting in exotic grasses are unknown. We also acknowledge that nests in exotic grasslands and inland native grasslands were more visible than those in Gulf Cordgrass. Nest-cup structure, depth, and diameter were unaffected by primary nesting vegetation. Texas Botteri’s Sparrows built nests inside or under the bunchgrass clump, with a tightly woven inner cup and outer nest cup loosely woven into the host plant (also observed by Webb 1985). Bermudagrass is not normally a bunchgrass, but it resembled bunchgrass in this study due to root-plowing. These data support nest-site selection based on vegetative structure rather than plant species (Jones and Bock 2005). Nearly 40% of nests built in Bermudagrass were woven around the base of shrub stems. Reasons for building around shrub stems could vary. It may be the product of a learned behavior, a response to shrub invasion of the grassland, or it may result from the structure of the grass cover. Bermudagrass is a much softer, less erect grass than Gulf Cordgrass, and incorporating a shrub base into the outer nest cup may increase nest stability or concealment, as well as provide perching substrate for adults and fledglings. Texas Botteri’s Sparrows build nests on or off the ground in response to the cover type; in Gulf Cordgrass, nests were built off the ground, with thinner nest cups, despite the availability of shrubs. Elevated nests may have improved thermoregulation and concealment, and reduced danger from flood events in wet years (Wolff 1977). On a larger scale than our sample size, neststructure alterations in response to a structural component of exotic grasses could influence nesting success. Nest-site microhabitat Structural components of grass, such as height and density, are used in nestsite selection by grassland birds (Flanders et al. 2006, Vickery et al. 1995). 396 Southeastern Naturalist Vol. 12, No. 2 Greater mean vegetation height and density in native grass on the King Ranch suggest that although Texas Botteri’s Sparrows seem to be nesting successfully in exotic grasses, long-term effects may be negative. Texas Botteri’s Sparrows are cryptic, and increased vegetative density and height generally afford better concealment for nests, fledglings, and adults (Jones and Bock 2005). Both Jones and Bock (2005) and Webb (1985) describe Botteri’s Sparrows using taller and denser grass clumps for nesting. Conclusions As in the case of Arizona Botteri’s Sparrows (Jones and Bock 2005), the spread of exotic grasses may be providing Texas Botteri’s Sparrows with regained habitat and increased abundances, thereby contributing to potential range expansion and population increase. Provided that “grass is sufficiently tall and dense” (Jones and Bock 2005), Botteri’s Sparrows have been considered adaptable to “pasturing or diversified agriculture” (Marshall and Clapp 1985). Increasing “improved” pasture area and disturbed grassland sites may actually create more available nesting habitat. However, the use of the exotics for nesting could be detrimental to the King Ranch Botteri’s Sparrow population in the longterm. 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