0066486
American Oystercatchers in Maryland and Virginia, USA: Status and Distribution
ALEXANDRA L. WILKE1, DAVID F. BRINKER2, BRYAN D. WATTS3, ASHLEY H. TRAUT2, RUTH BOETTCHER4, JAMES M. MCCANN2, BARRY R. TRUITT1 AND PAMELA P. DENMON5
1The Nature Conservancy in Virginia, Virginia Coast Reserve, Nassawadox, VA 23413 Internet: [email protected]
2Maryland Department of Natural Resources, Annapolis, MD 21401
3Center for Conservation Biology, College of William and Mary, Williamsburg, VA 23187
4Virginia Department of Game and Inland Fisheries, Painter, VA 23420
5United States Fish and Wildlife Service, Eastern Shore of Virginia National Wildlife Refuge, Cape Charles, VA 23310
Abstract.—The conservation status of the American Oystercatcher (Haematopus palliatus palliatus) along the Chesapeake Bay, coastal bays, and barrier island shorelines of Maryland and Virginia has been investigated in detail in recent years. The region supports approximately 700 breeding pairs with more than 80% occurring on the east coast of the Delmarva Peninsula and less than 20% occurring along the shorelines of the Chesapeake Bay. The num- ber of breeding pairs in Maryland appears to have been stable or to have increased slightly during the past 20 years. The overall trend of the breeding population in Virginia is less clear, but recent evidence suggests that numbers on the barrier islands are increasing after more than two decades of a declining trend. The coastal bays and barrier is- lands typically support between 1,500 and 2,000 wintering birds with most occurring on the east coast of the Virginia portion of the Delmarva Peninsula. The shorelines of both states together play an important role in supporting core breeding and wintering populations of the American Oystercatcher in the eastern United States. Throughout the region, oystercatchers are facing threats common to all coastal waterbird and shorebird species such as predation and overwash events. The threat of habitat loss to development, however, is not as alarming as in other areas of the species’s breeding range due to a significant amount of habitat being in protective conservation ownership or being unfit for development and recreation purposes. Habitat loss attributed to sea level rise, barrier island dynamics, and the indirect effects of development, such as pollution and contaminants, may play more important roles in the sta- bility of breeding and wintering habitat for the American Oystercatcher in Maryland and Virginia. Key words.—American Oystercatcher, conservation, distribution, Haematopus palliatus, status, Maryland, Virginia. Waterbirds 30 (Special Publication 1): 152-162, 2007
The eastern race of the American Oyster- Humphrey 1994). Much of these habitats are catcher (Haematopus palliatus palliatus) oc- under intense pressure from humans for rec- curs along the east coast of North America reation and development. Coastal regions from Nova Scotia south to the Yucatan Penin- comprise only 17% of the contiguous land sula (Nol and Humphrey 1994; Mawhinney area of the United States, but 53% of the et al. 1999). It is one of only a few shorebird nation’s human population inhabits these ar- species that breeds and winters in the tem- eas, with population densities reaching their perate coastal regions of the Chesapeake Bay highest on the east coast (Culliton 1998). and the coastal bays and barrier islands of The American Oystercatcher is inherently Maryland and Virginia. The species is found intolerant to excessive levels of disturbance, in highly restricted coastal habitats through- has a small population size and a very restrict- out its range during both breeding and win- ed year-round range. These characteristics tering seasons. Oystercatchers are territorial, make the species particularly vulnerable to solitary nesters and typically breed on sand issues affecting all coastal shorebird species, and/or shell beaches, salt marshes and such as habitat loss and alteration due to in- dredge spoil islands (Nol and Humphrey creasing development and recreation, pollu- 1994). Wintering birds are found in the same tion, human disturbance and expanding coastal areas, close to abundant food re- predator populations (Erwin 1980; Nol and sources and suitable roosting areas (Nol and Humphrey 1994; Davis et al. 2001).
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OYSTERCATCHER STATUS IN MARYLAND AND VIRGINIA 153
The conservation status of the American ers et al. 2004; Brown et al. 2005). Many of Oystercatcher has received much attention these studies have focused on the southeast- in recent years. The United States Shorebird ern portion of the species’ range. A signifi- Conservation Plan lists the species as one of cant number of research and monitoring ef- “high conservation concern” at a national forts have focused on American Oystercatch- level because of its small population size and ers in Maryland and Virginia in recent years restricted range (Brown et al. 2001). Al- as well. The purpose of this paper is to sum- though the American Oystercatcher is not marize the existing information concerning listed as threatened or endangered by the the breeding and wintering status of Ameri- United States Fish and Wildlife Service En- can Oystercatchers along the Chesapeake dangered Species Act, several state govern- Bay, coastal bays and barrier island shore- ments include oystercatchers on their lists of lines of Maryland and Virginia and to ad- “species of concern,” including Florida and dress current threats and research needs for Georgia. The American Oystercatcher is not the species in these states. listed by Maryland or Virginia. Prior to the early 1980s, there was rela- FOCAL AREA tively little information in the literature re- garding American Oystercatchers. Earlier The geographic scope of information published reports focused on oystercatcher presented here includes three general re- distribution along the east coast of the Unit- gions within Maryland and Virginia: (1) bar- ed States and brief notes and observations rier islands, (2) extensive marsh/lagoon sys- about the species’ behavior (e.g., Frohling tems between the barrier islands and main- 1965; Cadman 1979; Kilham 1979). In the land portions of the Delmarva Peninsula, mid-1980s, detailed studies conducted on hereafter referred to as coastal bays, and (3) three northern barrier islands of Virginia western and eastern shorelines of the Chesa- provided important fundamental informa- peake Bay, not including tributaries (Fig. 1). tion about the breeding biology of American Detailed descriptions of these areas are Oystercatchers (Nol et al. 1984; Nol 1985, found in Wilke et al. (2005) for Virginia and 1989). In the years following this work, re- Traut et al. (2006) for Maryland. searchers in other parts of the species’s range began to expand on these baseline BREEDING POPULATION studies. Evidence of population declines (Williams et al. 2000; Davis et al. 2001; B. Wil- Historical Distribution liams, College of William and Mary, unpubl. data) and low productivity (Novick 1996; The details of changes in the range of Davis 1999), along with the aforementioned oystercatchers along the east coast of the shorebird conservation plan assessment and United States during the past century have a recognition of the American Oystercatch- been summarized in the literature. The spe- er’s potential as a bioindicator of the health cies is thought to have occurred in the past as of our coastal ecosystems, continued to em- far north as Labrador (see discussion in phasize the need for a better understanding Humphrey 1990); however, Virginia was con- of the population status and general ecology sidered the northern limit of the species’ of the species. Recent studies have focused breeding range at the beginning of the on documenting site specific productivity Twentieth century (American Ornitholo- and factors affecting reproductive success gists’ Union 1910). At that time, populations (Davis et al. 2001; George 2002; McGowan north of Virginia were believed to have been 2004; McGowan et al. 2005; Sabine et al. decimated by egg collecting, market hunting 2006), specific effects of human disturbance and human disturbance (Bent 1929; Hum- on oystercatcher productivity (George 2002; phrey 1990). Subsequent breeding records McGowan and Simons 2006), and status of throughout the early to mid-Twentieth cen- wintering populations (Nol et al. 2000; Sand- tury document a northward expansion of 0066488
154 WATERBIRDS
formal survey of breeding pairs during the state’s Breeding Bird Atlas Survey of 1983- 1987 (Brinker 1996). The Atlas Survey was the most comprehensive at that time and es- timated Maryland’s breeding population of American Oystercatchers to be 50 to 75 pairs (Brinker 1996). Bailey stated that the American Oyster- catcher “. . . is the next bird to become ex- tinct on our Virginia coast, for it is truly a scarce bird now” (Bailey 1913). Bailey’s re- marks and corresponding maps of nesting bird locations suggest that the population of breeding oystercatchers in Virginia was close to being extirpated at the beginning of the 20th century, as it was believed to be north of the state at that time. The next large scale survey of oystercatchers in the state was con- ducted along the barrier islands in 1979 and it documented over 1,150 adults (Williams et al. 1990). A comparison of the 1979 survey Figure 1. Three focal areas surveyed for American Oys- results for oystercatchers on Cobb Island tercatcher breeding pairs from April-July 2003 in Mary- (one of Virginia’s barrier islands) with notes land and Virginia: (1) barrier islands, (2) coastal bays, and (3) shorelines of the Chesapeake Bay, not including from 1931, suggests an increase from eight tributaries. to 157 adults (Austin 1932; Williams et al. 1990). Similar dramatic increases likely oc- curred on all of Virginia’s barrier islands the breeding population and a gradual re- during the 20th century, probably due to the colonization of most of the species’ former passage of the Migratory Bird Treaty Act in range. The first breeding record in Mary- 1918 and a subsequent decline in egg col- land was documented in 1939 (Stewart and lecting and market hunting. Robbins 1958) and records after 1940 docu- mented continued northward expansion of Current Status breeding into New Jersey (Kramer 1948), New York (Post and Raynor 1964) and Mas- Prompted by recent concerns about the sachusetts (Humphrey 1990). More recently, conservation status of oystercatchers along breeding pairs have been documented as far the southeast coast of the United States, two north as Maine and Nova Scotia (Mawhinney studies examined the distribution of breed- et al. 1999). Humphrey (1990) and Davis ing American Oystercatchers in Maryland et al. (2001) provide overviews of this range and Virginia in 2003 (Wilke et al. 2005; Traut expansion or recolonization. et al. 2006). Both studies attempted to en- Records of breeding American Oyster- compass all suitable oystercatcher nesting catchers in Maryland and Virginia after the habitat along the coast and consisted of foot turn of the Twentieth century are sporadic and boat based ground surveys for breeding until formal surveys were conducted in the American Oystercatchers. Together, the two early 1980s and late 1970s in each state, re- studies provide a comprehensive overview of spectively. Records of breeding pairs in the current breeding season status of the Maryland include the aforementioned re- species and offer insight into interpretation port from 1939, the first record from the of historical trends in both states. The Mary- Maryland portion of the Chesapeake Bay in land study documented 108 pairs of Ameri- 1972 (Robbins 1972), followed by a more can Oystercatchers, with approximately 53% 0066489
OYSTERCATCHER STATUS IN MARYLAND AND VIRGINIA 155 along the Chesapeake Bay shorelines, 36% single birds or birds in flocks. These data in coastal bays, and 11% on barrier islands suggest that the total number of adults with- (Traut et al. 2006). These results suggest that in the system may have increased slightly or there may have been an increase in the num- at least remained relatively stable. Even so, ber of breeding pairs since the early 1980s, the number of oystercatchers recorded dur- although the extent of the species’ breeding ing surveys of the coastal bays would not be range within the state has remained essen- enough to offset the documented declines tially unchanged (Traut et al. 2006). These on the barrier islands during that same time data also suggest that the Maryland popula- period. The decline of oystercatchers on the tion of oystercatchers has experienced a con- islands warrants concern; however without tinual increase since being considered extir- comprehensive, long-term data encompass- pated in the beginning of the 20th century. ing the entire distribution of oystercatchers Furthermore, biologists in the state do not in the state, the exact magnitude of the spe- consider suitable habitat for the species to be cies’ decline in Virginia can not be deter- saturated, thus allowing for potential popu- mined. More recently, intensive annual sur- lation expansion in the state to continue veys of the barrier islands between 2000 and (DFB, pers. obs.). 2007 documented a 41% increase in the The 2003 survey in Virginia documented number of breeding pairs (K. Terwilliger and 588 pairs of American Oystercatchers with R. Cross, The Nature Conservancy [TNC] 51% on barrier islands, 38% in coastal bays, and Virginia Department of Game and In- and 11% along the Chesapeake Bay shore- land Fisheries [VDGIF], unpubl. data). An- lines (Wilke et al. 2005). The results of this nual barrier island surveys and repeated survey are particularly important for assess- comprehensive statewide surveys every five ing the statewide status of oystercatchers be- years will contribute to a better understand- cause prior breeding population assessments ing of the future overall population trends focused only on the barrier islands. Barrier for breeding oystercatchers in Virginia. island surveys conducted from 1979 through In recent years, breeding season surveys 2002 documented a 66% decline in the num- of American Oystercatchers have revealed ber of oystercatcher adults (Williams et al. important information about the species’s 2000; B. Williams, unpubl. data). The 2003 current status and distribution in Maryland statewide survey revealed that biologists and Virginia (Table 1). Overall, breeding must consider the limited geographic scope pairs of oystercatchers are evenly distributed of the long-term barrier island surveys when between barrier islands and coastal bays. interpreting those data. For example, oppor- Fewer than 20% of the pairs are located tunistic breeding season surveys of oyster- along shorelines of the Chesapeake Bay; catchers within Virginia’s coastal bays in however, in Maryland, this represents more 1983 documented 490 birds, both breeding than half of the breeding pairs documented pairs and non-breeding individuals (M. Byrd, in the state. Together, Maryland and Virginia College of William and Mary, and K. Terwil- support approximately 27% of the estimated liger, Terwilliger Consulting, unpubl. data). number of breeding pairs on the east coast of Wilke et al. (2005) documented 550 individ- the United States (estimated using other uals in the coastal bays, 223 pairs plus 104 published or reported breeding estimates for
Table 1. Number (percentage) of American Oystercatcher breeding pairs counted in three general regions of Mary- land and Virginia, 2003. Data were summarized from Traut et al. 2006 and Wilke et al. 2005.
Region Maryland Virginia Total
Barrier Islands 12 (11) 302 (51) 314 (45) Coastal Bays 39 (36) 223 (38) 262 (38) Chesapeake Bay 57 (53) 63 (11) 120 (17) Total 108 588 696 0066490
156 WATERBIRDS east coast states from Massachusetts south to projects conducted in the 1980s (Nol et al. Florida). Researchers and managers should 1984; Anderson 1988; Nol 1985, 1989) and continue to make regular statewide surveys a projects between 2002 and 2006. Earlier priority to detect long-term changes in num- studies focused on relatively small propor- bers and distribution of the species and to tions of the local breeding population while assess and develop management and conser- later studies encompassed a broader geo- vation strategies based on those changes. graphic scope. Nol (1989) investigated reproductive Reproductive Rates rates on Chincoteague, Wallops, and Assa- woman Islands in Virginia and estimated an Several detailed studies of the breeding average of 0.24 young fledged per pair from ecology of American Oystercatchers in Mary- 44 pairs during three years. High tides were land and Virginia exist with varying spatial identified as the primary cause of nest loss and temporal coverage. In Maryland, infor- (Nol 1989). Anderson (1988) investigated mation on reproductive rates is limited to a reproductive success on Fisherman Island in comprehensive study conducted in conjunc- 1981 and 1982 and documented only one tion with the 2003 statewide survey. Traut fledged young in 1981 (0.02 young per pair) et al. (2006) monitored 108 pairs of oyster- and ten in 1982 (0.20 young per pair). Fish catchers during the 2003 breeding season, Crows (Corvus ossifragus) were identified as and documented 72 confirmed nesting pairs the primary cause of egg loss (Anderson and 38 fledged young. A reproductive rate 1988). Studies of reproductive success of oys- defined as the number of young fledged di- tercatchers on Virginia’s barrier islands be- vided by the total number of pairs moni- tween 2002 and 2006 encompassed between tored was not reported but may be calculat- two to nine sites and 102 to 284 pairs annual- ed as 0.35 young per pair. The study found ly (ALW et al., unpubl. data). The maximum that birds breeding in Maryland’s Chesa- number of pairs monitored annually repre- peake Bay region had significantly higher re- sented approximately 76% of the total num- productive success than birds breeding in ber of pairs estimated to be breeding along the coastal bays (Traut et al. 2006). The rea- the Virginia barrier islands. Reproductive sons for the difference remain unclear, but rates at these sites ranged between zero to are not thought to be associated with human 1.26 young per pair per year (ALW et al., un- disturbance or development (Traut et al. publ. data). 2006), which is the case in many other stud- The Maryland study and earlier Virginia ies of oystercatcher breeding success on the studies documented oystercatcher reproduc- east coast (Davis et al. 2001; McGowan et al. tive rates similar to those typically reported 2005). Flooding events and an unknown de- for the species (Novick 1996; Davis et al. gree of mammalian and avian predation 2001; George 2002; McCowan 2005; Sabine pressure are likely the predominant causes et al. 2006). Preliminary analysis of the 2002- of nest failure; however, more detailed stud- 2006 Virginia data suggests that certain pro- ies are needed to further clarify the factors portions of Virginia’s breeding oystercatch- limiting reproductive success (Traut et al. ers have higher reproductive rates than those 2006; DFB, pers. obs.). In addition, repro- typically reported. These oystercatchers may ductive rates for oystercatchers are highly be serving as significant sources of individu- variable both spatially and temporally (Nol als for the regional population. Reasons for and Humphrey 1994; Davis et al. 2001). In- higher reproductive success likely include formation on reproductive rates of oyster- the unique conservation status of much of catchers breeding in Maryland is limited to the region, predator management programs, one year of study, and further studies are and food supply. Further analysis of these da- warranted to determine long-term trends. ta will offer insight into how breeding oyster- Detailed studies of the breeding ecology catchers in coastal Virginia are contributing of oystercatchers in Virginia include several to the population as a whole. 0066491
OYSTERCATCHER STATUS IN MARYLAND AND VIRGINIA 157
WINTERING POPULATION short Maryland coastline and scarcity of suit- able habitat for high-tide roost sites in the Similar to other shorebird species, Amer- coastal bays region probably explain the ab- ican Oystercatchers gather in communal, sence of large wintering flocks. Further study high-tide roosting flocks during the non- is warranted to investigate the numbers and breeding season (Nol and Humphrey 1994). distribution of wintering oystercatchers in Surveys during the non-breeding season and the coastal bays region. resighting records of banded oystercatchers Extensive data exist on locations of win- have revealed that some roost site fidelity is tering high-tide oystercatcher roosts and on exhibited by wintering flocks and individual wintering population size estimates for Vir- birds (F. Sanders et al., South Carolina DNR, ginia. Comprehensive, boat-based surveys of unpubl. data; RB and ALW, unpubl. data). wintering oystercatchers in the coastal bays This behavior facilitates the task of obtaining have been conducted annually since 1999, wintering population size estimates because with the exception of 2001 (Nol et al. 2000; birds are concentrated on known roost sites TNC and VDGIF, unpublished data). Sur- during high tide. Wintering population sur- veys have encompassed all known high-tide veys throughout the range of the species are roost sites in Virginia’s coastal bays, and to- important for establishing overall population tals have ranged from 1,084 to 2,263 individ- size estimates since breeding season surveys uals (Nol et al. 2000; TNC and VDGIF, un- are logistically difficult (solitary nesters published data). The low estimate of 1,084 spread out over a large geographic area) and was documented in 2004 when aerial surveys do not necessarily account for non-breeding were conducted and was probably an under- individuals within the population (Sanders estimate (TNC, unpubl. data) because roost- et al. 2004; Brown et al. 2005). The impor- ing oystercatchers in Virginia tend to flush tance of obtaining population size estimates upon approach of an aircraft making aerial for all shorebird species has been empha- flock estimates difficult (Brown et al. 2005; sized by the United States Shorebird Conser- ALW, pers. obs.). Notwithstanding the 2004 vation Plan (Brown et al. 2001). Several stud- survey data, the wintering population counts ies of American Oystercatchers have focused for oystercatchers in Virginia between 1999 on this task by conducting winter surveys and 2005 ranged from 1,516 to 2,263 individ- (Nol et al. 2000; Sanders et al. 2004; Brown uals. Based on the results of the 2002-2003 et al. 2005) and have resulted in important re- range-wide winter survey, Virginia supported visions of the population estimate for Ameri- the third highest number of wintering oys- can Oystercatchers in the United States. The tercatchers, approximately 16% of the total most comprehensive of these surveys was estimated U.S. east coast population (Brown conducted by air and covered the wintering et al. 2005). range of the species in the United States from Oystercatcher roost sites in Virginia in- New Jersey south and west to the Mexican clude oyster shell rakes formed along salt border (Brown et al. 2005). The estimated marsh edges or in open water, fringing sand overall wintering population of American beaches along marsh edges, inlet beaches, Oystercatchers was 10,971 ± 298 individuals exposed sand or mud flats and topographic during the survey period November 2002 high spots within marshes. Sixty-nine active through February 2003 (Brown et al. 2005). roost sites have been identified throughout No systematic surveys of wintering oyster- the extensive and remote marshes of the Vir- catchers in Maryland exist and Brown et al. ginia coastal bays region (TNC and VDGIF, (2005) did not present numbers for the unpubl. data). The availability of roost sites state. Maryland hosts a small number of win- does not appear to be limited as many inac- tering oystercatchers within the coastal bays tive sites are available throughout the region region, including one small flock of at most that share habitat characteristics of highly 50 individuals on Skimmer Island in Worces- used sites (ALW, pers. obs.). In addition, not ter County (DFB, pers. obs.). The relatively all active roosts are occupied within any giv- 0066492 158 WATERBIRDS en high tide period and flocks appear to ex- ident birds, while others are birds that breed hibit preferential selection of roosts, likely in states north of Virginia or dispersed sub- depending on factors such as roost site habi- adult birds from states south of Virginia tat characteristics, distance from food sourc- (TNC and VDGIF, unpubl. data; American es, tidal height, wind direction and speed Oystercatcher Working Group [AOWG)], and presence of avian predators. The dy- unpubl. reports). In addition, winter resight- namics of roost site selection and fidelity in ing efforts in other states have documented Virginia’s coastal bays are currently being in- the presence of juvenile birds banded as vestigated in greater detail. chicks in Virginia (AOWG, unpubl. reports). Less is known about the availability of po- The relationship between local breeding tential roost sites and the presence of winter- and wintering populations is still being inves- ing oystercatchers along the Chesapeake Bay tigated, but preliminary data suggest that shorelines of both states. An aerial survey of adult inter-seasonal movement patterns are the eastern and western Chesapeake Bay more predictable than those of juveniles. shorelines in Virginia, including isolated bay islands, in November 2002 did not detect any CURRENT THREATS wintering flocks of American Oystercatchers (TNC and VDGIF, unpubl. data). This flight Predators was conducted in conjunction with the Brown et al. (2005) survey to confirm anec- Mammalian predators are a leading dotal evidence suggesting the absence of oys- cause of reproductive failure in American tercatchers in that region (RB, pers. obs.). Oystercatchers throughout much of the spe- Suitable habitat for roosting birds may be cies’ range (Nol and Humphrey 1994; Davis limited along these shorelines, especially et al. 2001). The degree of this threat to the along the western shore of the bay, thus the breeding population described here is spa- region probably does not support significant tially variable. In Maryland, most breeding numbers of wintering oystercatchers. Lyn- oystercatchers are found on salt marsh is- haven Inlet in Virginia Beach, however, sup- lands in the state’s coastal bays or in the ports a small flock of approximately 30 to 40 Chesapeake Bay and may not face severe birds and may be one of only a few roost sites threats from mammalian depredation. Salt along the Chesapeake Bay shorelines in Vir- marsh islands within the coastal bays that ginia (BDW, pers. obs.). Two oystercatchers, support about 36% of the total number of one banded as a chick and one as an adult on oystercatcher pairs, are not suitable for sup- islands in the upper Chesapeake Bay in Vir- porting mesocarnivore nest predators (Traut ginia, were subsequently resighted during et al. 2006; DFB, pers. obs.). Furthermore, the non-breeding season on a Virginia barri- Traut et al. (2006) reported that oystercatch- er island, suggesting that at least a portion of ers nesting on islands within the Chesapeake the birds using the bay shorelines during the Bay (53% of the total number of pairs) expe- breeding season disperse to the seaside dur- rienced relatively high productivity in 2003 ing the wintering season (TNC and VDGIF, in spite of there being some evidence of me- unpubl. data). Additional aerial surveys are sopredator presence. The only region in the needed to confirm the details of wintering state where mammalian predation was iden- oystercatcher flocks along the Chesapeake tified as a significant source of oystercatcher Bay shorelines in Maryland and Virginia. reproductive failure was Assateague Island, Ongoing banding and resighting studies which supported 11% of the state’s pairs in throughout the range of American Oyster- 2003 (Traut et al. 2006; J. Kumar, Assateague catchers are revealing how breeding popula- National Seashore, pers. comm.). These data tions and wintering populations within given suggest that mammalian predation pressure areas are linked. These efforts in Virginia on oystercatchers breeding in Maryland may and in other states have revealed that a pro- not be as great as in other parts of the spe- portion of wintering birds in Virginia are res- cies’ range because of the species’ distribu- 0066493 OYSTERCATCHER STATUS IN MARYLAND AND VIRGINIA 159 tion within state. It should be noted, howev- adjacent to the mainland shoreline and that er, that data are still lacking on the impacts of the ducks experience relatively high repro- mesocarnivore predators on oystercatchers ductive success (G. Costanzo, Virginia Dept. nesting on the Chesapeake Bay islands with- of Game and Inland Fisheries, pers. comm.). in Maryland and the relatively high nesting However, if a predator such as a Red Fox success rate documented in 2003 may not be (Vulpes vulpes) or Raccoon (Procyon lotor) ac- reflective of typical predation pressure. cesses one of these isolated islands early in The distribution of oystercatchers in Vir- the nesting season, production is almost en- ginia results in a different threat profile from tirely eliminated for that year (G. Costanzo, mammalian predators. More than half of the pers. comm.). Mammalian predators are estimated number of breeding pairs of oys- likely more of a significant and frequent tercatchers in Virginia occurs on barrier is- threat to birds breeding on the marshes at- lands (Wilke et al. 2005). Researchers have tached to this mainland shoreline as op- documented declines in numbers of colonial posed to on small, adjacent marsh islands. birds and colonies and an increase in num- Additional studies are needed to better un- bers of mammalian predators on the islands derstand all factors limiting oystercatcher re- over the past 20 years (Williams et al. 2000; productive success in this region. Erwin et al. 2001; B. Williams et al., unpubl. The impacts of avian predators on oyster- data; R. Dueser, Utah State University, unpubl. catchers have not been well studied through- data). Additional research on the islands has out the area, and most available information shown that those with high numbers of mam- is limited to anecdotal evidence. The suite of malian predators have very low numbers of, potential avian predators to both breeding if any, colonially nesting waterbirds (Keiss and wintering oystercatchers includes but is 2001). Mammalian predation is a primary not limited to Great Black-backed Gulls threat to breeding oystercatchers on the is- (Larus marinus), Herring Gulls (L. argenta- lands and has likely played an important role tus), Laughing Gulls (L. atricilla), Peregrine in the long-term decline of oystercatchers on Falcons (Falco peregrinus), Great Horned the islands between 1979 and 2002. It should Owls (Bubo virginianus), and Fish Crows. be noted, however, that recent local manage- Great Black-backed, Herring and Laughing ment efforts have focused on addressing the gulls target oystercatcher eggs and young threat of mammalian predation to all nesting during the breeding season, and at least one birds on the islands and may have reduced Great Black-backed adult was observed suc- the degree of this threat. These management cessfully depredating an adult oystercatcher techniques and resulting impacts to nesting (PPD, pers. obs.). Despite these predation birds are currently being investigated in fur- threats, monitoring efforts in Virginia and ther detail (ALW et al., unpubl. data). Maryland have documented high reproduc- As in Maryland, most of the habitats used tive rates of some oystercatchers nesting by breeding oystercatchers in the coastal within or adjacent to mixed species gull col- bays of Virginia do not support populations onies (ALW and AHT, pers. obs.). A two-year of mammalian predators (ALW and BRT, diet study of breeding Peregrine Falcons on pers. obs.). This area supports almost 40% of the Eastern Shore of Virginia documented the state’s breeding pairs. Little is known only three observations of American Oyster- about the degree of threat from mammalian catchers as prey at nesting platforms (E. predators to oystercatchers breeding along Long, College of William and Mary, unpubl. shorelines of the Chesapeake Bay in Virgin- data). Peregrine Falcons target adult oyster- ia, which support approximately 11% of the catchers as prey during the nonbreeding sea- state’s breeding birds. Monitoring of Ameri- son; however, the overall impact of pere- can Black Duck (Anas rubripes) reproductive grine depredation on the wintering popula- success along the eastern shore of the bay tion of oystercatchers in Virginia is un- has revealed that mammalian predators are known. Evidence of Great Horned Owl often not present on the small marsh islands depredation of oystercatcher young has 0066494 160 WATERBIRDS been found on several barrier islands in Vir- an exceptionally high level of protection ginia (ALW, pers. obs.), but the potential im- from habitat loss to development and exces- pact of this species to breeding or wintering sive human disturbance. oystercatchers has not been investigated. The threat of habitat loss to sea-level rise Several studies on Fisherman Island Nation- is less predictable and a significant future al Wildlife Refuge in Virginia have docu- threat to all coastal waterbird and shorebird mented Fish Crows as a primary predator of species. Several studies within Virginia’s oystercatcher eggs (Anderson 1988; BDW coastal bays have estimated annual rates of and PPD, unpubl. data). In addition, obser- marsh loss attributed to sea-level rise to be vations during nest monitoring on Cobb Is- from 0.15% to 0.67% (see discussion and ref- land suggest that Fish Crows may be target- erences in Erwin et al. 2004). Within the ing oystercatcher eggs as soon as they are coastal bays region and in the Chesapeake laid (ALW, pers. obs.) Overall, the threat Bay, salt marsh islands may become sub- from these avian predators is not well under- merged as sea levels rise (Titus and Richman stood and is difficult to document without 2001). Several large tern breeding sites on real-time observations. Innovative research islands have already been lost in the Mary- and monitoring techniques, such as camera land portion of the Chesapeake Bay (DFB, systems (Sabine et al. 2005) are needed to as- pers. obs.). In addition to actual habitat loss, sess the impact that avian predators are hav- chronic reproductive failure of beach-nest- ing on local oystercatcher populations. ing birds may result as the frequency of over- wash events on barrier islands and marsh Habitat Loss and Human Disturbance habitats increases as sea level rises. Overwash is one of the most important processes in The habitats currently occupied by barrier island dynamics as it results in large breeding and wintering American Oyster- quantities of sediment being deposited over catchers in Maryland and Virginia are rela- the interior of a barrier island, resulting in a tively protected from loss to development change in island shape, position and land- and excessive human disturbance. The barri- scape features (Dolan et al. 1980). These er islands extending from the Maryland-Del- same events are responsible for creating the aware border south to the Chesapeake Bay open, sparsely vegetated nesting habitat pre- are almost entirely in protective conserva- ferred by nesting species such as the Ameri- tion ownership with the exception of Ocean can Oystercatcher. Without such disturbanc- City, Maryland. Most oystercatcher pairs es, island vegetation would succeed to the breeding on the barrier island of Assateague point of making the habitat unsuitable for in Maryland are located on the northern many species of beach-nesting birds. At the portion of the island that is closed to visitors same time however, overwash events are doc- during the breeding season (J. Kumar, pers. umented as one of the primary causes of nest comm.). The Virginia barrier islands are loss for American Oystercatchers (e.g., Nol mostly accessible only by boat and receive 1989; Nol and Humphrey 1994; Davis et al. limited visitor use year-round. Salt marsh is- 2001). An increase in the frequency of these lands within the coastal bays of both states events could lead to low rates of reproduc- are unfit for development because of fre- tive success, which would be insufficient to quent tidal inundation, they receive little hu- maintain a stable population. man recreational use and, in Virginia, the Overall, the shorelines of Maryland and vast majority of these marshes are also in pro- Virginia play an important role in support- tective ownership (Wilke et al. 2005; DFB, ing core breeding and wintering popula- pers. obs.). Overall, 83% of breeding oyster- tions of American Oystercatchers. The catcher pairs in both states occurs on barrier unique conservation status of the coastal islands and within coastal bays and almost bays and barrier islands, which support most 100% of the wintering population occurs of the species’ numbers year-round, will af- within the coastal bays. These regions offer ford a level of protection for breeding and 0066495 OYSTERCATCHER STATUS IN MARYLAND AND VIRGINIA 161 wintering habitat into the future that is un- thesis, The College of William and Mary, Williams- paralleled along the U.S. east coast. Biolo- burg, Virginia. Austin, O. L., Jr. 1932. Cobb Island. Bird-Banding 8: 12- gists and managers are actively addressing 25. the threat of mammalian predators to breed- Bailey, H. H. 1913. The Birds of Virginia. J. P. Bell Com- ing oystercatchers on the barrier islands with pany, Inc., Lynchburg, Virginia. Bent, A. C. 1929. Life Histories of North American promising results. Sea-level rise will continue Shorebirds, Part 2. United States National Museum to affect these habitats and may be the most Bulletin 146, Washington, D.C. significant threat that the species will face Brinker, D. F. 1996. American Oystercatcher. Pages 142- 143 in Atlas of the Breeding Birds of Maryland and over the next century (Erwin et al. 2006). the District of Columbia (C. S. Robbins and E. A. T. Biologists have made significant progress Blom, Eds.). University of Pittsburgh Press, Pitts- towards understanding the oystercatcher’s burgh, Pennsylvania. Brown, S., C. Hickey, B. Harrington and R. Gill, Eds. breeding and wintering ecology in both 2001. The U.S. Shorebird Conservation Plan, 2nd Maryland and Virginia. However, fundamen- edition. Manomet Center for Conservation Scienc- tal information is still lacking. Additional at- es, Manomet, Massachusetts. Brown, S. C., S. Schulte, B. Harrington, B. Winn, J. Bart tention should be focused on birds occupy- and M. Howe. 2005. Population size and winter dis- ing Chesapeake Bay shorelines to better un- tribution of eastern American Oystercatchers. Jour- derstand the status of the wintering popula- nal of Wildlife Management 69: 1538-1545. Cadman, M. 1979. Territorial behaviour in American Oys- tion and factors affecting reproductive tercatchers. Wader Study Group Bulletin 27: 40-41. success in that region. More information is Culliton, T. J. 1998. Population distribution, density, also needed on year-round diet and foraging and growth. National Oceanic and Atmospheric Ad- ministration State of the Coast Report [Online, habitat requirements for the species URL:
162 WATERBIRDS
Keiss, O. 2001. Mammalian predator distribution and Nol, E., B. Truitt, D. Allen, B. Winn and T. Murphy. abundance on the Virginia barrier islands in rela- 2000. A survey of wintering American Oystercatch- tion to breeding habitats of colonial birds. M.S. the- ers from Georgia to Virginia, U.S.A., 1999. Wader sis, Utah State University, Logan. Study Group Bulletin 93: 46-50. Kilham, L. J. 1979. Location and fate of oystercatcher Novick, J. S. 1996. An analysis of human recreational im- nests on Sapelo and Cabretta Island (Georgia). Ori- pacts on the reproductive success of American Oys- ole 45: 45-46. tercatchers (Haematopus palliatus): Cape Lookout Kramer, E. V. 1948. Oystercatcher breeding in New Jer- National Seashore, North Carolina. M.Sc. thesis, sey. Auk 65: 460. Duke University, Durham, North Carolina. Mawhinney, K., B. Allen and B. Benedict. 1999. Status of Post, P. W. and G. S. Raynor. 1964. Recent range expan- the American Oystercatcher (Haematopus. palliatus) sion of the American Oystercatcher in New York. on the Atlantic Coast. Northeastern Naturalist 6: Wilson Bulletin 76: 339-346. 177-182. Robbins, C. S. 1972. The season: April, May, June 1972. McGowan, C. P. 2004. Factors affecting nesting success Maryland Birdlife 28: 105-177. of American Oystercatchers (Haematopus palliatus) Sabine, J. B., S. H. Schweitzer and J. M. Meyers. 2006. in North Carolina. M.S. thesis, North Carolina State Nest fate and productivity of American Oystercatch- University, Raleigh. ers, Cumberland Island National Seashore, Georgia. McGowan, C. P. and T. R. Simons. 2006. Effects of hu- Waterbirds 29: 308-314. man recreation on the incubation behavior of Amer- Sanders, F. J., T. M. Murphy and M. D. Spinks. 2004. ican Oystercatchers. Wilson Journal of Ornithology Winter abundance of the American Oystercatcher in 118: 485-493. South Carolina. Waterbirds 27: 83-88. McGowan, C. P., T. R. Simons, W. Golder and J. Cordes. Stewart, R. E. and C. S. Robbins. 1958. Birds of Mary- 2005. A comparison of American Oystercatcher re- land and the District of Columbia. North American productive success on barrier beach and river island Fauna Number 62, Washington, D.C. habitats in coastal North Carolina. Waterbirds 28: Titus, J. G. and C. Richman. 2001. Maps of lands vulnera- 150-155. ble to sea rise: modeled elevations along the U.S. At- Nol, E. 1985. Sex roles in the American Oystercatcher. lantic and Gulf Coasts. Climate Research 18: 205-228. Behaviour 95: 232-260. Traut, A. H., J. M. McCann and D. F. Brinker. 2006. Nol, E. 1989. Food supply and reproductive perfor- Breeding status and distribution of American Oys- mance of the American Oystercatcher in Virginia. tercatchers in Maryland. Waterbirds 29: 302-307. Condor 91: 429-435. Wilke, A. L., B. D. Watts, B. R. Truitt and R. Boettcher. Nol, E. and R. C. Humphrey. 1994. American Oyster- 2005. Breeding season status of the American Oys- catcher (Haematopus palliatus). Number 82 in The tercatcher in Virginia, USA. Waterbirds 28: 308-315. Birds of North America (A. Poole and F. B. Gill, Williams, B., R. A. Beck, B. Akers and J. W. Via. 1990. Eds.). The Academy of Natural Sciences, Philadel- Longitudinal surveys of the beach-nesting and colo- phia, Pennsylvania, and The American Ornitholo- nial waterbirds of the Virginia Barrier Islands. Vir- gists’ Union, Washington, D.C. ginia Journal of Science 41: 380-388. Nol, E., A. J. Baker and M. D. Cadman. 1984. Clutch ini- Williams, B., B. Akers, M. Beck, R. Beck and J. Via. 2000. tiation dates, clutch size and egg size of the Ameri- The 1998 colonial and beach-nesting waterbird sur- can Oystercatcher in Virginia. Auk 101: 855-867. vey on the Virginia Barrier islands. Raven 71: 42-45.