Avian-habitat relationships in urban and suburban tidal marshes of Connecticut
by Kristin Schaumburg1, William M. Giuliano2, and Gail A. Langellotto3
1 Louis Calder Center—Biological Field Station, Department of Biological Sciences, Fordham University, 53 Whippoorwill Road, Armonk, NY 10504
2 Department of Wildlife Ecology and Conservation, University of Florida, PO Box 110430, Gainesville, FL 32611
3 Department of Horticulture, Oregon State University, 4017 Ag and Life Sciences Building. Corvallis, OR 97331
Tidal marshes support an array of avian species and provide essential habitat for tidal marsh specialists. Recent population declines of marsh specialists are largely attributed to habitat change, particularly in urban and suburban landscapes. The rapid geographic expansion of invasive common reed (Phragmites australis) throughout tidal marshes is of particular concern. Habitat structural changes of tidal marshes associated with the spread and dominance of common reed will likely have direct and indirect impacts on avian use of such sites. Our objective was to determine the relationship between habitat characteristics and avian abundance and species richness within urban and suburban Connecticut tidal marshes, with a focus on the effect of common reed on avian communities, particularly tidal marsh specialists. We collected data via point counts and vegetation and macroinvertebrate sampling from 39 tidal marshes in Connecticut, from 2004 to 2005. Relatively high use by avian tidal marsh specialists and dominance of habitat classes indicative of "native" marshes suggest that study marshes have retained some habitat for marsh birds and at least some of the function of unaltered tidal marshes. Similarly, use by at least 8 species of conservation concern suggests that these marshes serve as important habitats for many at-risk species. Avian abundance, including that of tidal marsh specialists, differed among habitat classes, with habitats dominated by native species often supporting greater numbers of birds than common reed habitats. Site-level avian-habitat relationships provided further support for the value to marsh birds of habitats that maintain native conditions. To prevent further declines of many of avian species at the local site level, marshes must be protected from habitat loss and alteration.
Keywords: avian, habitat, macroinvertebrate, marsh, Phragmites australis, suburban, tidal, urban
Urban and suburban Connecticut tidal marshes support a wide array of avian species, including obligate tidal marsh specialists (i.e., species that nest exclusively in tidal marshes), facultative breeders, foraging shorebirds, and wading birds (Benoit and Askins 1999; Shriver and Vickery 2001). Population declines of obligate tidal marsh species, particularly the seaside sparrow (Ammodramus maritimus), saltmarsh sharp-tailed sparrow (Ammodramus caudacutus), and clapper rail (Rallus longirostris), have been documented throughout northeastern North America (Eddleman et al. 1988; Sauer et al. 2001; Connecticut Department of Environmental Protection 2004). Numerous factors have been implicated in the population declines of these marsh birds, including habitat loss and alteration, tidal flow restrictions, and pesticide use (Eddleman et al. 1988; Warren et al. 2002).
The loss and alteration of avian habitat in tidal marshes, caused by real estate development (DeLuca et al. 2004), ditching (Clarke et al. 1984), and exotic plant invasion (Benoit and Askins 1999), may be the most critical factors affecting tidal marsh bird populations (Shisler and Schulze 1976; Benoit and Askins 2002; Parsons 2003) and communities (Burger et al. 1978; Erwin et al. 1991; Benoit and Askins 1999; Bertness et al. 2002; Guntspergen and Cully 2006). Human-induced alterations may severely compromise the ability of altered sites to support endemic avian communities. Of particular concern is the rapid expansion of the common reed (Phragmites australis) throughout marshes of the Atlantic coast (Burger et al. 1978; Benoit and Askins 1999; Warren et al. 2002). Common reed has historically inhabited North American freshwater marshes; however, a nonnative European genotype was introduced to the Atlantic coast in the early 1800s (Saltonstall 2002). The more salt-tolerant European genotype of P. australis (Vasquez et al. 2005) presently dominates marshes that historically did not contain the native genotype (Saltonstall 2002). By forming monocultures and changing edaphic, hydrological, and detrital conditions (Angradi et al. 2001), the nonnative genotype of common reed can potentially have detrimental effects on plant (Cross and Fleming 1989), invertebrate (Angradi et al. 2001; Robertson and Weis 2005), and vertebrate (Benoit and Askins 1999) communities.
Common reed often replaces endemic plant species such as Spartina patens, Distichlis spicata, and Spartina alterniflora, thereby radically altering tidal marsh vegetative structure (Bertness et al. 2002; Guntspergen and Cully 2006). Because vegetative structure is considered a primary determinant of site utilization and breeding success for many bird species (MacArthur et al. 1962; Gjerdrum et al. 2005; Winter et al. 2005), structural changes caused by common reed domination are likely to directly impact avian site utilization, and ultimately species richness and abundance of tidal marsh birds (Guntspergen and Cully 2006).
Macroinvertebrates are a critical food source for many nesting birds and their chicks (Whittingham and Robertson 1994), therefore the proliferation of common reed in Atlantic coast tidal marshes and its corresponding effects on macroinvertebrate populations may influence avian growth, survival, and nest success. In addition to the potential direct effects of common reed dominance on avian site utilization, the impacts to the invertebrate community may represent an important indirect factor influencing avian communities (Bertness et al. 2002; Fell et al. 2003; Gratton and Denno 2005; Guntspergen and Cully 2006; Schaumburg 2007).
Developing a more thorough knowledge of how vegetation composition directly and indirectly affects avian communities is critical for reestablishing healthy, productive marsh communities. A comprehensive approach, based on the evaluation of multiple habitat characteristics, is most likely to lead to effective management of these communities. Several studies have addressed avian-habitat relationships, but most have examined landscape-level relationships, and few have examined these patterns in the Northeast. Our objective was to determine the relationship between habitat (food and cover) characteristics and avian species richness and abundance within urban and suburban Connecticut tidal marshes, with a specific focus on the effect of common reed on avian communities, particularly tidal marsh specialists.
Data for this study were collected from 39 tidal marshes in Connecticut (Schaumburg 2007). These sites were selected to represent tidal marshes of various sizes and vegetative types spanning the entire Connecticut coast from New York to Rhode Island. Marshes varied in size (2 to 307 hectares) and vegetation composition, ranging from sites dominated by common reed to those of solely native species. The marshes were located within approximately 10 kilometers of the coast, where 43.4 % of the land-use was highly developed, 12.4 % was agricultural, turf, and grass, and the remainder had various intensities of development (Center for Land Use Education and Research 2008). Fourteen of the marshes used in this study had been managed to restore tidal flow and/or to reduce common reed populations (P. Capotosto, CT Department of Environmental Protection, personal communication; Schaumburg 2007).
To assess bird species richness and abundance, we conducted point-count and playback surveys during 2004 and 2005. Survey locations were determined within each marsh following a stratified random design, with the number of locations within a marsh based on marsh size, and locations randomly placed within an area of uniform vegetation composition (Schaumburg 2007). We established 97 survey locations throughout the 39 marshes and visited each location three times between April and September.
Marsh bird communities were quantified using 50-meter fixed-radius point counts (Hutto et al. 1986). Point counts consisted of a 20-minute listening and observation period, followed by 9 minutes of broadcasting tape-recorded calls of least bittern (Ixobrychus exilis), American bittern (Botaurus lentiginosus), Virginia rail (Rallus limicola), king rail (Rallus elegans), clapper rail (Rallus longirostris), sora (Porzana carolina), black rail (Laterallus jamaicensis), pied-billed grebe (Podilymbus podiceps), and willet (Catoptrophorus semipalmatus). Broadcasts consisted of 30 seconds of vocalizations followed by 30 seconds of silence for each species. Individuals detected within the 50-meter radius during the broadcast survey were not included in the abundance estimates if they were also detected during the point-count survey.
All birds seen or heard within the 50-meter point-count area were identified to species. Only species in direct contact with the marsh (i.e., no flyovers) during the point-count and broadcast surveys were recorded. Species richness was determined for each survey location by counting all unique species encountered during the three bird counts. Abundance was determined by adding up all individuals noted during the three counts, and relative abundance by counting all individuals noted for a particular species and dividing by the total number of individual birds encountered during the three counts.
We determined categorical habitat classification of plant communities from the percentage of cover of specific plant species along two perpendicular 50-meter transects originating from the center of point-count locations (Benoit 1997). Transect sampling was conducted twice at each bird count location between June and September of 2004 and 2005. Potential avian-habitat classes were defined as: common reed (> 50% P. australis cover); cattail (> 30% Typha angustifolia with no other species having > 30% coverage); short-grass meadow (> 50% S. patens, D. spicata, Juncus gerardii, and S. alterniflora combined, with S. alterniflora coverage < 50%); cordgrass meadow (> 50% S. alterniflora); and brackish mixture (containing short graminoids such as S. patens, J. gerardi, D. spicata surrounded by any combination of taller plants, including Scirpus spp., P. australis, T. angustifolia, where each taxon represented < 50% cover). We also determined coverage of mudflat, water (i.e., water other than that in anthropogenic ditches), ditch (anthropogenic), and flotsam along transects and estimated the proportion of each in each habitat class. We used the number of count points within each habitat class to estimate the proportion of marshes occupied by each marsh class.
To examine the effects of habitat characteristics on avian species richness and abundance, we examined vegetation and other site characteristics at all point-count locations. Specifically, we determined plant species present, plant species richness, percentage of horizontal ground cover, mean maximum vegetation height, and absolute maximum vegetation height. Ground cover was identified as vegetation (e.g., P. australis, S. patens, S. alterniflora, D. spicata, Scirpus spp., T. angustifolia, or other species), mudflat, water, ditch, or flotsam. Percentage of horizontal ground cover was estimated as the amount (distance) of cover intercepting two 50-meter transects divided by the total transect length (100 meters). We estimated mean and maximum vegetation height at each point-count location by measuring the tallest vegetation within each 5-meter segment of the transects. We determined plant species richness by summing the number of unique species encountered along the transects.
We conducted macroinvertebrate sampling from April to September 2004 to quantify macroinvertebrate species richness, abundance, and biomass at each point-count location in 20 randomly selected marshes. We collected two samples at each location within 24 hours of a point count. We culled the samples within randomly located 1 m2 plots, less than or equal to 25 meters from the point-count location (Schaumburg 2007). Macroinvertebrates were collected from vegetation by suctioning with a modified gas-powered garden/litter vacuum (Snapper, model sb2000m) for 1 minute. Macroinvertebrates were trapped within the intake of the vacuum in a mesh bag and immediately sealed in plastic bags. Macroinvertebrates were then hand sorted in plastic trays, placed in vials, and preserved in 70% ethanol until they were identified and counted in the lab. Following identification, macroinvertebrates were placed in a desiccating oven for 48 hours at 100°C and then weighed. Macroinvertebrates were identified to the taxonomic classification of family and, when possible, species level. We determined macroinvertebrate species richness, abundance, and biomass for each point-count location.
Analysis of variance (ANOVA), with marsh as a blocking factor, was used to assess the effect of habitat class (e.g., common reed and cattail) on avian species richness and abundance (all birds combined and all tidal marsh specialists combined; see Table 1 for a list of specialists), macroinvertebrate species richness, abundance, and biomass, and habitat characteristics (e.g., vegetation height; see Table 2). We considered tests significant if p ≤ 0.100 (Taylor and Garrodette 1993; Fidler et al. 2006). Model assumptions were tested for all analyses. Data that failed assumptions were natural log or log10 transformed prior to analysis to meet the assumptions of ANOVA (Sokal and Rohlf 1995).
Simple linear and multiple regression analyses were used to identify relationships between habitat characteristics (independent variables) and bird community characteristics (dependent variables). Independent variables included % P. australis; % S. patens; % S. alterniflora; % D. spicata; % Scirpus spp.; % mud; % water; % ditch; % flotsam; mean maximum vegetation height; maximum vegetation height; plant species richness; and macroinvertebrate species richness, abundance, and biomass. Dependent variables included avian species richness and abundance. Model assumptions were tested for all analyses. Arcsine transformations were required for the S. patens and S. alterniflora variables; the remaining independent variables were log transformed to satisfy assumptions.
To control type I error across simple linear regression analyses, we used a Bonferroni adjustment (p ≤ 0.007). Methods described by Cohen et al. (2003) were used to reduce multicolinearity problems and the number of independent variables considered in multiple regression analyses (i.e., when independent variables were correlated [r > 0.7], the one that explained the least variation in the dependent variable was excluded). Independent variables were entered into analyses in the order of greatest to lowest value of r2, using a forward stepwise procedure (F-to-enter = 0.10, F-to-remove = 0.10, and Tolerance = 0.10). Standardized multivariate regression coefficients (SMRC) were used to assess relative effects of independent variables in the final model. To control type I error across multiple regression analyses, we used a Bonferroni adjustment (p ≤ 0.051). All analyses were preformed using SYSTAT (2004) software.
Avian Community Characteristics
We recorded 985 individual birds, comprising 47 species, throughout all marshes. Twelve species were common (> 10 observations), with 4 of these being very common (> 100 observations). Of the 47 avian species we observed, 3 were tidal marsh specialists and 8 were considered endangered, threatened, or of special concern (Table 1).
Differences Among Habitat Classes
Our marshes differed in the amount of each habitat class they contained: Short-grass meadow was most prevalent (33%), followed by cordgrass (28.9%), brackish mixture (15.5%), common reed (15.5%), and cattail (7.2%). Total avian and tidal marsh specialist abundance, macroinvertebrate abundance and biomass, plant species richness, flotsam cover, and mean and maximum vegetation height differed among habitat classes (p ≤ 0.100; Table 2). However, no differences among habitat classes were found for avian and macroinvertebrate species richness or ditch, water, and mudflat cover (p > 0.100; Table 2).
Effects of Habitat Characteristics
Avian species richness was influenced by S. alterniflora cover (standardized regression coefficient [SRG] = -0.387, r2 = 0.090, p = 0.002) and maximum plant height (SRG = 0.291, r2 = 0.068, p = 0.006) but not by any other variables. However, the combination of S. alterniflora cover (SMRG = -0.316) and plant species richness (SMRG = 0.149) best explained variability in avian species richness (r2 = 0.111, p = 0.001).
Total avian abundance was influenced by S. alterniflora cover (SRG = -0.577, r2 = 0.086, p = 0.002) but not by any other variables. However, the combination of S. alterniflora cover (SMRG = -0.951), plant species richness (SMRG = 0.502), and macroinvertebrate species richness (SMRG = 0.060) best explained variability in total avian abundance (r2 = 0.412, p ≤ 0.001).
Avian tidal marsh specialist abundance was influenced by S. patens cover (SRG = 1.456, r2 = 0.266, p ≤ 0.001), maximum plant height (SRG = -0.586, r2 = 0.086, p = 0.002), and macroinvertebrate abundance (SRG = 0.457, r2 = 0.198, p = 0.002) and species richness (SRG = 0.094, r2 = 0.186, p = 0.002) but not by any other variables. However, the combination of S. patens cover (SMRG = 0.954), macroinvertebrate abundance (SMRG = 0.310) and species richness (SMRG = 0.063), and ditch cover (SMRG = -0.286) best explained variability in avian tidal marsh specialist abundance (r2 = 0.556, p ≤ 0.001).
The number of individuals and species of birds found in our urban and suburban study marshes is comparable to other studies of tidal marsh birds in northeastern North America (e.g., Burger et al. 1978; Benoit and Askins 1999; Shriver and Vickery 2001), many of which had sites with more native conditions and surrounded by less urbanization. The relatively high use (> 100 observations/species) by avian tidal marsh specialists, such as saltmarsh sharp-tailed and seaside sparrows, and the dominance of habitat classes indicative of native marshes, such as short-grass meadow, cordgrass, and brackish mixtures (a combined 77.4 % of the marsh area), suggest that these urban and suburban marshes have retained some habitat for tidal marsh birds and at least some of the function of unaltered, "native" tidal marshes. Similarly, use by endangered, threatened, and of-concern species, such as bobolinks (Dolichonyx oryzivorus), saltmarsh sharp-tailed sparrows, seaside sparrows, king rails, glossy ibises (Plegadis falcinellus), great egrets (Ardea alba), snowy egrets (Egretta thula), and yellow-crowned nigh herons (Nyctanassa violacea), suggests that these marshes serve as important habitats for many at-risk species (Connecticut Department of Environmental Protection 2004).
Total avian abundance and abundance of avian tidal marsh specialists differed among habitat classes, with habitats dominated by native species, such as brackish mixtures, cordgrass, and short-grass meadow, often supporting greater numbers of birds than common reed habitats (e.g., tidal marsh specialists were > 3 times more abundant in short-grass meadow than common reed). Our findings are consistent with Benoit and Askins (1999) and Shriver and Vickery (2001), who found negative associations between the use of marshes by tidal marsh specialists and the prevalence of common reed in marshes of northeastern North America. The greater numbers of birds in native habitats appears to be the result of: 1) greater numbers and biomass of invertebrates (i.e., as much as 8 times more individuals and 10 times more biomass of macroinvertebrates in native compared to common reed habitats), important foods for many avian species; and 2) generally shorter and often more diverse plant communities, which may provide better cover (foraging, nesting, and escape), plant food sources, and invertebrates than areas dominated by the exotic common reed. With vegetative structure being a primary determinant of avian habitat use (MacArthur et al. 1962; Gjerdrum et al. 2005; Winter et al. 2005), the structural difference between common reed habitats and more native ones (i.e., reduced stem count, larger stem diameters, greater plant height, and greater detritus accumulation in common reed; Able and Hagan 2000; Meyerson et al. 2000; Leonard et al. 2002; Rooth et al. 2003) likely led to differences in avian use among habitats. In addition, lower plant species diversity and poor litter quality (i.e., high lignin and cellulose content) associated with common reed may indirectly affect avian use through its effects on an avian food source, the macroinvertebrate community (Fell et al. 2003; Gratton and Denno 2005).
Taller vegetation in common reed habitats, which may provide more cover for birds, and comparable numbers of avian individuals and species in common reed habitats and some native habitats is probably not indicative of the value of common reed habitat to all marsh specialist birds, particularly obligate tidal marsh specialists. Use of and nesting in common reed habitats is typically dominated by generalist bird species such as red-winged blackbirds (Schaumburg 2007). Although some marsh birds, such as the little blue heron (Egretta caerulea), snowy egret, and black-crowned night-heron (Nycticorax nycticorax), have been found to utilize these habitats in proportion to their availability (Parsons 2003). The tall, dense growth form of common reed may allow more generalist species to use these areas, as such birds are not adapted to tidal fluctuations and other environmental conditions associated with tidal marshes in more native condition. Thus, while common reed may sometimes support many individuals, such generalist avian species may outcompete tidal specialists and other species in these habitats, reducing the overall value of the sites to many marsh birds.
Site-level avian-habitat relationships provide further support for the value of habitats in more native condition to marsh birds, particularly tidal marsh specialists. Both plant height and species richness were positively associated with greater numbers of bird species; taller vegetation may allow generalists to use the area, and a diverse plant community may provide more varied food sources and cover opportunities. Similarly, plant species richness, often higher in native habitats, and macroinvertebrate species richness were positively associated with avian abundance. Further, avian tidal marsh specialist abundance was negatively associated with plant height (with common reed habitats having the tallest plants) and ditch cover (an anthropogenic feature that may allow invasion of an area by common reed; Cross and Fleming 1989; Angradi et al. 2001; Saltonstall 2002), and positively associated with S. patens cover (a native plant and habitat), macroinvertebrate abundance (a common food source found more often in native habitats), and macroinvertebrate species richness.
Although tidal marshes in our study have retained a preponderance of native habitats and continue to support numerous avian species, including many at-risk species and tidal marsh specialists, populations of many of these species continue to decline and marsh habitats continue to be altered. To prevent further declines of many avian species at the local site level, marshes must be protected from habitat loss and alteration. The expansion of common reed into tidal marshes is one such alteration, reducing its value to many birds, particularly tidal marsh specialists. Programs designed to remove common reed or prevent its invasion in to tidal marshes, including water level manipulation, selective herbicide application, mowing, burning, and biological control (Ailstock et al. 2001; Chambers et al. 2003), have proven effective and should be undertaken.
Primary funding was provided by the Long Island Sound Program of the Connecticut Department of Environmental Protection. Additional funding and support was provided by Fordham University and the University of Florida. John Warzybok and Caroline Poli assisted with fieldwork. This research followed all guidelines set forth by the appropriate institutions for animal research and complied with the laws of the state and country in which it was performed.