Depredation Patterns and Nest Success for Bobwhite and Songbird Populations on Field Borders in North Carolina

Abstract

Many bird species have been experiencing declines across their ranges due to habitat loss. Northern bobwhites (Colinus virginianus) and early succession songbirds have lost habitat from fragmentation, urbanization, fire suppression, and large-scale farming practices (Vance 1976, Brennan 1991, Askins 1993, Brawn et al. 2001, Hunter et al. 2001, Brennan and Kuvlesky 2005, Sauer et al. 2011). Field borders have been implemented in many states as a way of providing early succession habitat for these birds and other wildlife. A field border is a strip of planted or volunteer vegetation around a crop field that is maintained in early succession. Bobwhite and early succession songbird populations increase with the establishment of field borders (Puckett et al. 1995, Marcus et al. 2000, Bromley et al. 2002, Palmer et al. 2005, Riddle et al. 2008, Conover et al. 2009). However, additional work is needed to determine if field borders are beneficial to birds by providing quality nesting habitat, thereby increasing nest success. Quality nesting habitat can depend on surrounding landscape features such as edges. Predators could be using edges more intensely than other landscape features for foraging or travelling (Gates and Gysel 1978, Durner and Gates 1993, Marini et al. 1995, Dijak and Thompson 2000), thereby resulting in lower nest success. Field borders may reduce nest success because they juxtapose many different edge types in addition to row crops including woods, ditches, and roads. A variety of predators use edges and field borders. One of these, the black rat snake (Elaphe obsolete), was suspected of being the main nest predator of two songbirds, blue grosbeaks (Passerina caerulea) and indigo buntings (Passerina cyanea), on some farms (Riddle and Moorman 2010). However this was never confirmed as cameras were not placed on nests to accurately identify predators. Few studies have examined predator communities in field borders (but see Gillis 2000). My study had two primary objectives. My first objective was to determine whether northern bobwhite, blue grosbeak, and indigo bunting nests in field borders were more likely to fail if within close proximity to edge types such as woods, row crops, ditches, and roads. My second objective was to determine if snakes, such as the black rat snake, were the main nest predator for each of our focal species. My study sites included 178 field borders (totaling 77 ha) along the edges of row crop fields on four industrial hog farms owned and operated by Murphy-Brown, LLC. Three of the farms totaled approximately 312 ha in size and were located in a forest-dominated landscape. The fourth farm was approximately 1619 ha in size and was in an agriculture-dominated landscape. I searched each field border for nests of my three focal species at least twice during summer 2010 and four times during summer 2011. I recorded nest location, the number of eggs, nest stage, and other relevant information such as condition of the nest and its contents and whether the male or female was present. I monitored each nest every 3 to 4 days until it either failed or successfully fledged. I set up cameras on half of each of my focal species’ nests. The camera setup included three main components: a small bullet camera, a digital video recorder (DVR), and two 12-V 33- A hour batteries. I changed batteries and 16 gigabyte secure digital (SD) cards every 3 to 4 days during nest checks. I used Program MARK to estimate daily nest survival rates for each species. Specifically, I built models to estimate the effect of 7 different covariates: distance to closest woody, row crop, ditch, and road edges, field border width, year, and a camera effect. I placed predators caught on camera into three groups: snake, avian, and mammalian. I then used a Chi-square goodness of fit test (α > 0.05) to determine the relative importance of these three nest predator groups. I found a total of 26 northern bobwhite nests, 29 blue grosbeak nests, and 12 indigo bunting nests in two field seasons. The top three models in Program MARK for northern bobwhites were constant nest survival, year effect, and camera effect (AICc weights = 0.23, 0.18, and 0.12, respectively). The top three models for blue grosbeaks were constant nest survival, field border width, and distance to the nearest ditch (AICc weights = 0.22, 0.17, and 0.15, respectively). The top three models for indigo buntings were distance to nearest crop, distance to nearest road, and constant nest survival (AICc weights = 0.52, 0.41, and 0.02, respectively). All of the covariates had slope betas with 95% confidence intervals that overlapped zero indicating that none of the covariates exerted a strong influence on the nesting success of any of our focal species. In other words, I found no edge effect in relation to nest success for any bird species because no variables were significantly related to nest success. I set up cameras at 14 northern bobwhite nests, 14 blue grosbeak nests, and 5 indigo bunting nests. I recorded 4 snake (3 king snakes (Lampropelitis getula getula) and one unidentifiable snake) and 2 Virginia opossum (Didelphis virginiana) depredation events at northern bobwhite nests (P = 0.1353). I recorded 2 unidentifiable mammalian, 3 unidentifiable snake, and one avian depredation events (P = 0.3620) for blue grosbeaks. I recorded 3 black rat snake depredations (P = 0.0498) for indigo buntings. Lack of edge effects on my focal species nest success may have been influenced by landscape-level patterns. All northern bobwhite nests and all but 2 blue grosbeak nests and 3 indigo bunting nests were found on field borders in an agriculture-dominated landscape rather than a forest-dominated landscape. This resulted in the average distance from the nest to the closest woody edge being relatively large for northern bobwhites (403.6 m ± 271.3 SE), blue grosbeaks (363.5 m ± 239.4 SE), and indigo buntings (152.1 m ± 194.0 SE). In addition, distance to the closest road was relatively large while distances to the closest row crop and ditch were small because all field borders were next to a row crop on one side and most had a ditch on the other side. Snakes were the most frequent nest predator for all of my species, but were only statistically significant for indigo buntings. Specifically, black rat snakes were the main nest predator for indigo buntings. This may be related to the closer proximity of indigo bunting nests to woody edges compared to the other two species. Landowners and farm managers appear to have little restrictions on where they can place field borders with respect to the four edges studied (woody, row crop, ditch, road) and nest success of my three focal species. Therefore, in agriculture-dominated landscapes, landowners and farm managers should focus more on adding field border acres and less on where these acres are placed on a farm. Future studies should focus on gaining a larger sample size for each focal species in field borders in each landscape. Indigo bunting and blue grosbeak nests were found in ditches in both landscapes, but were not included in my analysis because I did not consider ditches as part of a field border. Therefore, studies also should focus on songbird nest success and prevalence of nests in ditches compared to those in field borders. In a forest-dominated landscape, a comparative study should examine whether songbirds prefer to nest on the forest edge, field borders, or ditches.