Effects of prescribed fire on depredation rates of natural and artificial Seaside Sparrow nests.
Abstract: We compared depredation rates of natural and artificial nests of Seaside Sparrows (Ammodramus maritimus) within winter burned and unburned marsh breeding habitats. Natural nests on burned sites in 2002 were depredated at a higher rate (35.3%) during the incubation stage, compared to unburned sites (13.3%). Depredation rates of natural nests were similar between burn treatments during the nestling stage. Artificial nests exhibited significantly higher depredation rates during the incubation stage on burned compared to unburned sites in 2002. No artificial nest studies were conducted in 2003, but we examined natural nest depredation rates. Depredation rates on natural nests in 2003 were similar between burned and unburned sites during both incubation and nestling stages. Differences in nest depredation rates between 2002 and 2003 may be due to increased rainfall in 2003 leading to higher biological productivity, reduced burn effectiveness and coverage, as well as a change in nest placement by Seaside Sparrows on burned sites. Shrub-nesting species may not be as vulnerable to higher rates of nest depredation induced by prescribed burning because fire appears to only minimally impact woody shrubs, while greatly reducing biomass of herbaceous vegetation.
Article Type: Report
Subject: Prescribed burning (Environmental aspects)
Predation (Biology) (Control)
Environmental protection (Methods)
Birds (Breeding)
Birds (Observations)
Authors: Almario, Barbara S.
Marra, Peter P.
Gates, J. Edward
Mitchell, Laura
Pub Date: 12/01/2009
Publication: Name: The Wilson Journal of Ornithology Publisher: Wilson Ornithological Society Audience: Academic Format: Magazine/Journal Subject: Biological sciences Copyright: COPYRIGHT 2009 Wilson Ornithological Society ISSN: 1559-4491
Issue: Date: Dec, 2009 Source Volume: 121 Source Issue: 4
Topic: Event Code: 690 Goods & services distribution Advertising Code: 59 Channels of Distribution Computer Subject: Company distribution practices
Product: Product Code: 9006200 Conservation & Land Mgmt-Total Govt; 9913700 Environmental Management NAICS Code: 924 Administration of Environmental Quality Programs
Geographic: Geographic Scope: United States Geographic Code: 1USA United States
Accession Number: 216267541
Full Text: Historically, humans have manipulated coastal marshes to improve habitat for game species, reduce risk of wildfire, or increase marsh productivity (Chabreck 1988, Mitchell et al. 2006). Commonly used marsh manipulations include altering water levels, ditching, managing grazing animals, and prescribed burning. Prescribed fire is often used to promote or maintain certain plant or animal species, enhance species diversity, eliminate undesirable species, or protect against uncontrolled fires (Whelan 1995, Mitchell et al. 2006). Prescribed fires during the winter (Dec-Mar in brackish-saline marsh habitats) are often used specifically to promote waterfowl habitat by regulating species composition of saltmeadow cordgrass (Spartina patens), smooth cordgrass (S. alterniflora), and three-square bulrush (Schoenoplectus spp.) (Nyman and Chabreck 1995).

Little is known about the effects of winter prescribed fire on non-target species including Seaside Sparrow (Ammodramus maritimus maritimus), Coastal Plain Swamp Sparrow (Melospiza georgiana nigrescens), Saltmarsh Sharp-tailed Sparrow (Ammodramus caudacutus), Black Rail (Laterallus jamaicensis), and other secretive marsh birds that use tidal marshes as foraging and breeding areas. Most research to date has focused on measuring the impacts of fire on bird community composition and species abundance (Taylor 1983, Gabrey et al. 1999, Gabrey and Afton 2000, Walters et al. 2000). Few studies have examined the effects of fire on demographic parameters such as population age structure or reproductive success.

Nest depredation is the leading cause of reproductive failure for birds (Ricklefs 1969, Martin 1995). Thus, rates of nest depredation can provide a useful index of habitat suitability and are known to vary according to the amount of nest concealment (Gabrey et al. 2002, Weidinger 2002). Depredation of nests for open-habitat nesting bird species may be magnified by management practices designed to reduce vegetative cover such as prescribed fire, an action that may decrease important vegetative cover for nesting.

Seaside Sparrows are obligate tidal wetland specialists of management concern throughout most of their range (Post and Greenlaw 1994), and listed as a "Bird of Conservation Concern" for North American Bird Conservation Initiative (NABCI) Bird Conservation Regions 27, 30, 31, and 37, and for U.S. Fish and Wildlife Service (USFWS) Regions 2, 4, and 5 (USFWS 2002a). Changes to plant species composition, cover, and vertical structure caused by prescribed burning influences the relative abundance of Seaside Sparrows (Gabrey et al. 1999) and may also impact reproductive success. Populations of year-round resident Seaside Sparrows (A. m. fisheri) in Louisiana coastal marshes were markedly reduced during the first breeding season post-burn (Gabrey and Afton 2000). These results do not explain how prescribed fire specifically impacts factors underlying sparrow population dynamics, such as rates of nest depredation or overall nesting success.

Artificial nests are popular tools for examining rates of nest depredation because they allow users to control a number of confounding factors (Whelan et al. 1994, Buler and Hamilton 2000). Artificial nests also allow experiments at large spatial scales with increased sample sizes as well as lower costs and effort relative to finding and monitoring natural nests (Faaborg 2004). Most studies measuring depredation rates of artificial nests fail to provide comparative data for natural nests (Major and Kendal 1996). Several studies have tested the validity of using artificial nest data because of the negative influences of olfactory cues and human scent (Whelan et al. 1994, Donalty and Henke 2001), nest type (Martin 1987), or egg type and size (Roper 1992, Maier and DeGraaf 2000, Svagelj et al. 2003) on depredation rates and predator assemblages. The validity of artificial nest studies remains questionable without comparative data on natural nests.

The few studies comparing depredation rates between artificial and natural nests characterize the reproductive success of species assemblages rather than single focal species (Buler and Hamilton 2000, Mezquida and Marone 2003). Artificial nests should mimic the nest type of the focal species under examination for accurate inferences to be drawn (Villard and Part 2004). This is critical for establishing a consistent relationship between depredation rates of natural and artificial nests.

We investigated the effects of winter prescribed burning on nest depredation rates of Seaside Sparrows breeding in Chesapeake Bay tidal marshes. We predicted that prescribed burning, typically conducted in late winter (Jan-Apr), significantly reduces dead vegetative cover and vegetation height and density, and may also reduce nest concealment and increase the probability of nest depredation. We compared depredation rates of natural Seaside Sparrows nests and artificial nests between burned and unburned marshes to test this hypothesis.

METHODS

The study was conducted at Blackwater National Wildlife Refuge (38[degrees] 24' N, 76[degrees] 0' W), a subdivision of the Chesapeake Marshlands National Wildlife Refuge Complex (hereafter Blackwater) and the State of Maryland's Fishing Bay Wildlife Management Area (38[degrees] 23' N, 76[degrees] 59' W) (hereafter Fishing Bay), Dorchester County, near Cambridge, Maryland. The study area consists of ~17,800 ha of tidal marsh on the Chesapeake Bay. Dominant plant species include saltmeadow cordgrass and smooth cordgrass with three-square bulrush patchily distributed throughout. Dominant shrubs include Jesuit's bark (Iva frutescens) and eastern baccharis (Baccharis halimifolia).

Managers at Blackwater and Fishing Bay have used prescribed fire for >60 years (USFWS 2002b). Approximately 40% of the areas are burned annually between January and March, the dormant season for marsh vegetation, to reduce hazardous fuel-loads, increase efficiency of furbearer harvest, and promote target vegetation growth (USFWS 2002b). Blackwater fire management staff established experimental plots in 1995 with variable fire frequencies (annual, 3-5 year, 7-10 year, and fire-excluded) along the Transquaking River to evaluate effects of fire frequency on above-ground vegetation (Flores 2003).

The study on natural nest depredation rates was conducted in 2002 and 2003, and included three annual-burn (4A, 5A, and 6A) and three no-burn sites (4D, 5D, and 6D) each year ranging in size from 70.4 to 181.7 ha. Each annual-burn site was burned between January and March in both 2002 and 2003. No-burn sites were last burned in 1994, the year before the establishment of experimental plots. Sites 4A and 4D were considered inappropriate for the study following the 2002 population counts and vegetation measurements. Few Seaside Sparrows nested on these two sites. Sites 7A (annual-burn) and 5C (no-burn) were added in 2003 based on habitat and vegetative characteristics. A 5-ha systematic survey grid consisting of 10 transect lines at intervals of 25 m was established on each site to facilitate territory mapping. No artificial nest study was conducted in 2003 and we considered the data on natural nest depredation rates from that year to be a follow-up to the primary artificial-natural nest comparisons in 2002.

Seaside Sparrows are territorial and build covered or domed ground nests in marsh grasses. Most nests have a canopy of residual (dead) and green grass, some are built under tidal debris, and all nests are at least partially covered (Post and Greenlaw 1994). Finding nests involved observing cues from adults and systematically searching territories every 3 days. Nests were marked using flagging tape tied to tufts of grass 3 to 4 m from the nest with each flag denoting a direction and distance to the nest. Each nest was visited every 3 days until the nestlings fledged. The incubation period lasted ~12 days with a 9-day brooding period. Nests were considered depredated when all eggs were removed or disturbed in such a way that the female stopped incubating, when nestlings disappeared prior to brood completion date, or when the nest appeared highly disturbed.

We experimentally measured rates of nest depredation in early June 2002 on burned and unburned sites. Twenty artificial nests were used on each of the six sites for a total of 120 artificial nests. Artificial nests were placed at least 25 m from each 5-ha grid in an area that resembled vegetation characteristics within the grids. Nests were arranged every 25 m along transects, 25 m apart in suitable Seaside Sparrow nesting habitat. We avoided areas of unsuitable nesting habitat. Each nest was constructed by wrapping grass, usually saltmeadow cordgrass, into a ring and tying it into a knot to resemble a small cup (Gabrey et al. 2002). Nests were placed under clumped vegetation to simulate natural canopy cover and concealment of Seaside Sparrow nests. Each artificial nest contained two Japanese Quail (Coturnix japonica) eggs and one plasticine egg. The plasticine egg simulated the size of Seaside Sparrow eggs and was mixed with ground litter to simulate brown flecks and spotting found on natural Seaside Sparrow eggs. We placed flags a minimum of 3 m from the nest with a nest designation and compass bearing on each flag to facilitate nest relocation and reduce visual cues for predators.

Artificial nests were checked and removed after 12 days, the average incubation period for Seaside Sparrows on the sites. We attempted to limit human scent near or at artificial nests by visiting nests only at the end of the exposure period and by wearing rubber boots while placing artificial nests in the field at the beginning of the study. Nests were considered depredated if the quail or plasticine eggs were missing or had visible bite or beak marks. Plasticine eggs were evaluated for teeth or beak marks which were compared to representative skull specimens within the predator categories of small mammal, medium mammal, large mammal, reptile, or bird.

Artificial nests and eggs simulated natural nests during incubation only; thus, we analyzed depredation rates of natural nests that occurred only during the incubation stage and compared them to artificial nest depredation rates (Thompson and Burhans 2004). We used a Chi-square goodness of fit test to analyze pooled depredation rates of natural and artificial nests on each burn treatment. Bonferroni's correction was applied to groups of data to reduce Type 1 error. Natural and artificial nest data were compared to examine if depredation rates differed between burn treatments, and between artificial and natural nests. We present depredation rates for natural nests as a total of both incubation and nestling stages in each treatment to examine if artificial nests were a good indicator of total natural nest depredation rates. Nest failures due to factors other than depredation were not included in the analysis.

Vegetation Measurements.--We collected vegetation data at 100 points spaced 25 m apart in each study plot during June and early July (peak Seaside Sparrow breeding period) each year. Percent live vegetation cover (all live plant species combined) and percent dead vegetation cover (all dead plant species combined) were regarded as suitable nesting vegetation and were measured from total transect area using a modified point-intercept method described by Bonham (1989). A 1-m pole, marked at 0.1-m intervals, was placed horizontally on the ground under the nest, facing north. All plant species touching each 0. 1-m mark were recorded. Live and dead material was identified to species when possible. Bare ground was recorded when no plant species were touching a mark, and water was recorded when no plant species was touching the mark and the area appeared permanently flooded. Percent suitable nesting vegetation (live and dead vegetation combined) data were compared to examine differences between treatments and between years.

We analyzed vegetation data using Principal Components Analysis (SAS 1999) and found no correlations between vegetation characteristics. We used a Chi-square goodness of fit test to analyze pooled percent live and dead vegetation cover on each burn treatment.

[FIGURE 1 OMITTED]

RESULTS

Natural Nests.--We found 96 active Seaside Sparrow nests in 2002, 45 on unburned and 51 on burned sites. Significantly fewer natural nests were depredated during incubation on unburned (13.3%) compared to burned sites (35.3%; [chi square] = 6.15, df = 1, P = 0.02; Fig. 1A). Total depredation rates combining nestling and incubation stages did not differ between burn treatments with 26.6% of nests on unburned sites and 41.2% of nests on burned sites being depredated ([chi square] = 1.10, df = 1, P = 0.29; Fig. 1B).

[FIGURE 2 OMITTED]

We found 146 active Seaside Sparrow nests in 2003, 67 on unburned and 79 on burned sites. We did not detect a difference in rates of depredation during the incubation stage between unburned (25.4%) and burned treatments (24.1%; [chi square] = 0.02, df = 1, P = 0.34; Fig. 1A). Total depredation rates combining nestling and incubation stages in 2003 did not differ between unburned (43.3%) and burned (36.7%) sites ([chi square] = 0.28, df = 1, P = 0.59; Fig. 1B).

Artificial Nests.--Twenty-one (35.0%) of the 60 artificial nests placed on unburned sites in 2002 were depredated compared to 48 (81.3%) of 59 (1 nest could not be relocated) artificial nests on burned sites. Depredation rates were significantly lower on unburned compared to burned sites ([chi square] = 17.16, df = 1, P < 0.001; Fig. 2).

Bite marks on plasticine eggs suggest that several different predators prey upon nests, but no predator appeared specific to a particular treatment. Small mammals marked 14 eggs on unburned sites and 11 eggs on burned sites. Medium-sized mammals, most likely common raccoons (Procyon lotor), marked four plasticine eggs on unburned sites and seven eggs on burned sites. Snakes and avian predators were each responsible for markings on one egg in each treatment. Five of the eggs or nests were moved from the nest site, possibly by birds.

Vegetation.--Suitable nesting vegetation was greater in 2002 on unburned sites (87.7%) than on burned sites (63.9%; [chi square] = 376.2, df = 1, P [less than or equal to] 0.001). Vegetation characteristics in 2003 changed from the previous year. The percentage of suitable nesting vegetation remained greater on unburned sites (80.5%) compared to burned sites (76.0%; [chi square] = 16.6, df = 1, P [less than or equal to] 0.001), but the amount of suitable vegetation that was live vegetation on burned sites increased (16.5%) from the previous year ([chi square] = 83.87, df = 1, P [less than or equal to] 0.001).

DISCUSSION

Artificial and natural nests during the incubation stage in 2002 exhibited significantly lower nest depredation rates on unburned compared to burned sites. This difference was not evident when examining total depredation rates across nest stages in natural nests (artificial nests have no "nestling" stage and cannot reveal total depredation rates). The result is consistent with other studies as poorly concealed nests are found by nest predators early during the incubation stage; nests reaching the nestling stage usually experience lower depredation rates and higher nest success (Nice 1957, but see Davis 2003). Overall, artificial nests had higher rates of nest depredation compared to natural nests; this is similar to results of other studies (Major and Kendal 1996, Zanette 2O02).

We did not experimentally measure depredation rates using artificial nests in 2003 and we considered that year's natural nest depredation rates as follow-up data. Depredation rates of natural nests differed between 2002 and 2003. No differences were detected in depredation rates in 2003 during the incubation stage between unburned and burned sites. However, in 2003 the region where our sites were located received considerably more precipitation than in 2002 (a less-than-normal precipitation year), during May, June, and July (normal average: 9.4 [+ or -] 4.3 cm vs. 2002:4.0 [+ or -] 0.6 cm; and 2003:15.8 [+ or -] 1.9 cm) (Maryland State Climatologist Office 2006). We suspect that unusually high rainfall years may increase biological productivity, plant growth, and nest concealment, and possibly reduce potential negative effects of burning on marsh vegetation (Zimmerman 1992). Above-average rainfall (normal average: 109.2 cm vs. 2003: 160.4 cm) (Maryland State Climatologist Office 2006) during the first 10 months of the year may have made the 2003 winter burns less thorough, leading to increased dead residual biomass during the 2003 nesting season versus 2002. Dead vegetative cover is important for nest concealment and placement of nests above ground level. Dead vegetative cover left standing in 2003 may explain why depredation rates during incubation were less than in 2002. Sparrow nest placement in burned sites also differed between the 2 years with sparrows in 2003 choosing unburned patches for nest sites significantly more often than burned patches (BSA and PPM, unpubl, data). Disproportionate use of unburned patches by Seaside Sparrows in burn treatments may have further negated some of the negative effects of prescribed burning on nest success.

An alternative explanation for increased depredation rates during the incubation stage in 2002 may be a functional behavioral response related to prey-search in which local predators focus their attention on areas where they were likely to find nests. An increased density of artificial nests adjacent to the 5-ha natural nest grids may trigger certain search image effects (Crawford and Jennings 1989, Major and Kendal 1996). Predator density may also increase in response to an increase in prey density (Gibb 1960).

Use of quail eggs in artificial nests has raised concerns of bias (Whelan et al. 1994, Zanette 2002, Mezquida and Marone 2003, Faaborg 2004). For example, quail eggs are larger in size than sparrow eggs which could increase the detection of eggs in artificial nests by predators that search visually, and potentially overestimate nest depredation (Mezquida and Marone 2003). Lack of parental concealment or defense of artificial nests may also overestimate nest depredation because activities of parents defending nests can modify the probability of depredation (Major and Kendal 1996, Part and Wretenberg 2002). Artificial nest estimates of depredation were higher compared to data from natural nests. Larger eggs may overestimate survival because small mammals, potentially important nest predators of natural nests (Major and Kendal 1996), are unable to crack through the thick shells (Roper 1992, Maier and DeGraaf 2000, Part and Wretenberg 2002). A plasticine egg was placed in each nest to record any attempts by smaller predators to take eggs.

Plasticine eggs recorded 15 cases of likely small mammal depredation, nine of which showed no other evidence of depredation. Absence of nest defense by parents may also result in higher nest depredation rates of artificial nests (King et al. 1999). Human scent left during nest placement may have also increased the number of olfactory predators (Donalty and Henke 2001), although some studies have found no relationship between human scent and depredation risk (Part and Wretenberg 2002).

Nests used during the study were found throughout the breeding season; many were found before or during incubation, but a number were not found until the nestling stage. All nests were not discovered during the incubation stage, and our study may be biased against nests depredated early in the incubation stage but not found. It is difficult to find all nests during the incubation stage because of the secretive nature of Seaside Sparrows.

CONSERVATION IMPLICATIONS

Thousands of hectares of coastal marsh habitat are burned on refuges and private lands each year (Mitchell et al. 2006), and the impacts of prescribed burning on target and non-target organisms have rarely been studied (Rotenberry et al. 1995, Whelan 1995, Mitchell et al. 2006). Annual prescribed burning of coastal marshes in this study increased nest depredation rates of Seaside Sparrows in 1 year but only during incubation. The effects differed between years, possibly due to differences in annual rainfall patterns. Species nesting primarily above ground in woody shrubs may not be as vulnerable to dormant season prescribed burning because fire does not appear to impact woody shrubs (e.g., Iva frutescens, Baccharis halimifolia) as much as it does ground vegetation. Woody shrubs primarily lose their foliage during a burn but resprout early in the season, while residual, dead ground marsh graminoids are removed by fire and new spring growth may not provide sufficient cover early in the season. Many Seaside Sparrows in the burn treatments shifted to woody shrubs as nest sites, possibly lessening the potential detrimental effects of prescribed burning (BSA and PPM, unpubl, data).

Fire is a natural phenomenon within marsh habitats. It historically occurred on an irregular basis in the mid-Atlantic, and probably less frequently than in other regions where Seaside Sparrows nest such as along the Gulf Coast states or in Florida (Frost 1998), which experience higher daily lighting strikes than elsewhere in the United States (Purvis et. al. 1997). Frost (1998) estimated the pre-settlement fire frequency of the lower Chesapeake Bay drainage at 4-6 years, but noted that natural fire frequency at a particular site depends upon many factors, including topography and land surface form, fire compartment size (areas of continuous fuel and no firebreaks), and lightning frequency.

The extensive annual burns set by wildlife managers in the mid-Atlantic states may pose problems for ground-nesting saltmarsh species, which may be adapted to less frequent, large-scale losses of nesting cover. Additional data on adult survival and lifetime reproductive success across burn treatments over multiple years should better incorporate the vagaries of climate and provide additional insights into the long-term impacts of this management practice. We recommend managers use more precaution and increase the periodicity of prescribed burns in saltmarsh habitats of the Chesapeake Bay and in other regions of the United States.

ACKNOWLEDGMENTS

We thank Steven Gabrey for advice on artificial nests and Seaside Sparrow biology. Joe Smith, Jim Jenkins, Elizabeth Krone, Andrea Evans, Karin Roux, Jenny Pickar, and Nora Diggs helped with fieldwork and logistics. The Blackwater National Wildlife Refuge Fire Management Program, especially Bill Giese and Joe Krish, provided assistance and were helpful. The U. S. Geological Survey provided funding for this project.

Received 15 June 2007. Accepted 30 April 2009.

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BARBARA S. ALMARIO, (1,4) PETER P. MARRA, (2,5,6) J. EDWARD GATES, (1) AND LAURA MITCHELL (3)

(1) University of Maryland Center for Environmental Studies-Appalachian Laboratory, 301 Braddock Road, Frostburg, MD 21532, USA.

(2) Smithsonian Environmental Research Center, 647 Contees Wharf Road, Edgewater, MD 20137, USA.

(3) U.S. Fish and Wildlife Service, Chesapeake Bay Field Office, 177 Admiral Cochran Drive, Annapolis, MD 21401, USA.

(4) Current address: Florida Fish and Wildlife Conservation Commission, 11650 Munson Highway, Milton, FL 32570, USA.

(5) Current address: Smithsonian Migratory Bird Center, National Zoological Park, 3001 Connecticut Avenue, NW, Washington, D.C. 20008, USA.

(6) Corresponding author; e-mail: marrap@si.edu
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