Status of Caspian Tern in the South Lake Simcoe Watershed
P.J Harpley and R.J. Milne

1. Introduction

Increasing urbanization, forest loss and development of service corridors all threaten the success of breeding birds in southern Ontario (Friesen et al., 1995, Cheskey ). Much of the past emphasis of concern has focused on the breeding success and location of threatened and indicator species (Austen et al., 1994). The South Lake Simcoe watershed has been identified as an important nesting site for Caspian Tern (Sterna caspia). Nests have been located on the small gravel bars characterisitically called the Sand Islands near Georgina Island, similar to other breeding sites found throughout the Great Lakes. Caspian Tern populations were first documented at the Sand Islands in the mid-1930's (Austen et al., 1994). However, the importance of this region for the staging and foraging of terns has not been extensively documented. Througout the summer, birds can be found foraging along the southern shore of Lake Simcoe and on small lakes in the sourrounding landscape.

Although the importance of landscape pattern and human disturbance has been studied extensively for forest interior species (Freemark and Merriam, 1986; Burke and Nol, 1998; Wyatt et al., 1998), there has been little work on the importance of the pattern of wetlands and indicator species in southern Ontario. There has been little work on the relationship between open water and foraging for species such as Caspian Tern (see Gibbs et al., 1991). Land development, both current and proposed, has added to the natural stresses that are potentially threatening the size of some of the colonies in this region ofthe Greater Toronto Area. Negative impacts include the human disturbance of nesting sites and less obvious stresses that degrade or acually eliminate food sources.

2. Critical Nesting Site

The southern shore of Lake Simcoe is located in the Town of georgina in the Region of York. Georgina Island is situated in Lake Simcoe, approximately 1.5 km northwest of the southern shoreline at Duclos Point and 2.5 km east of Sibbald Point near the community of Sutton?jackson’s Point (Figure 1). The island is administered by the Georgina Island Chiippewa Native Band. The southern shore is characterized by low undulating sand and clay plains formed in glacial lake Algonquin during and following the Wisconsin glaciation. The island is part of a drumlinized till plain (Chapman and Putnam, 1966). The soils are predominantly loam-textured with some clays. Much of Georgina Island was cultivated until 20 to 30 years ago and is now being allowed to regenerate with a secondary forest cover and overgrown orchards (Ecologistics, 1982).

The present lakeshore is characterized by shallow sand beaches, gravel poiints and important offshore shoals considered critical as spawning beds for cold water fish species (Ecologistics, 1982). An important nearshore shoal exists offshore around virtually all of Georgina Island. The width of the shoal is approsximately 0.3 to 0.5 km and water depths range from 1 to 6 m. Much of this shallow water zone is underlain by rock 15-30 cm in diameter, but the south shore is more sandy.

At the southwest end of Georgina Island, gravel and sand deeposits on the shoal form several small islands known as the Sand Islands. The Sand Islands themselves are predominantly sand with some rounded gravels and wooded bluffs to the west (Figure 1). The west exposures of the islands face theopen lake and autumn, winter and spring swells have created wavecut bluffs in some locations. The islands are elongate, oriented roughtly north to south with particularly prominent sand spits to the south end of each island consistent with the direction of predominantly west to esast longshore drift expectecd in the lake at this point (mcGillivary, 1990). East and west sides of the islands are low sand expanses with graminoid grasses predominating. These areas are generally sheltered by the island and except in occassional major storm events, are not subject to disturbance. All parts of the island shores are subject to fluctuating water levels seasonally in the lake and dependent on federal water level control and regulation of Lake Simcoe water levels. The southern outlet beach sites, predominantly sand, are of special note as this is where most time is spent by Caspian Terns and the area of their nest locations (Figure 2, photo).

Georgina Island has been identified as an Environmentally Significant Area (ESA) within the Lake Simcoe Region by the Lake Simcoe Regional Conservation Authority. This classification was based ont eh presence of rare and endangered plants and the importance of this area for the completion of life cycles for Smallmouth Bass (Micropterus dolomieu), Northern Pike (Esox lucius) an dGreat Blue Hewron (Ardea herodias). The reported nesting of Caspian Terns on the Sand Islands in the 1920's and 1930's was only incidentally noted (Ecologistics, 1982).

3. Landscape Ecology

The South Lake Simcoe Watershed consists of a number of significant wetland areas inland, forming an extensive wetland complex. The study sites where Caspian Terns have been observed include the Black River, Mud lake and Wagner Lake/Uxbridge Brook. These sites are in the Lake Simcoe basin on the north slope of the Oak Ridges moraine. They contain sufficient areas of open water to provide habitat for feeding spots for Caspian Tern. The average distance between adjacent open water areas within the complex these three wetland sites form is approximately 9 km. Proximity from Lake Simcoe is a short distance and similar for all three locations.

The area of open water of Wagner Lake is approximately 0.33 km2. A tributary river from the Pefferlaw Brook, Uxbridge Brook, connects Wagner Lake to Lake Simcoe. The proximity of the open water to the lake is approximately 17 km. This wetland site is located in a rural area that is experiencing increasing disturbance from residential/cottage development around the lake and the establishment of a subdivision on the southeast uplands. The majority of the upland forest cover has been selectively removed for cottage lots.

The Black River wetland area is similar to the Wagner Lake/Uxbridge Brook site, it is a riparian system draining northward to Lake Simcoe. A ponding is formed where the Black River meets the Zephyr Creek. The open water area of this site is slightly smaller than Wagner Lake, approximately 0.24 km2 located close to 11 km from the lake. The site is a relatively undisturbed wetland/wildland area.

Also within this complex is Mud Lake which drains north to Lake Simcoe, connected by the Pefferlaw Brook. It si located a distance of approximately 16 km from the lake in a rural agricultural setting. Mud Lake has the smallest area of open water, compared to the former sites and measures about 0.11 km2. The adjacent lands to Mud Lake are undeveloped with a secondary growth forest cover, except for the north end of the lake where it is bordered by a farmer’s field.

The open water within the wetland complex of the Black River, Mud Lake and Wagner lake/Uxbridge Brook are located at a relatively close distance from the Sand Island nesting colony site and the Lake Simcoe shore foraging areas. The pondings have sufficient areas of open water which provide critical foraging habitat for Caspian Terns as evidenced by the late summer observations.

Methodology

The methodology had two segments which included historical records from individual sightings and long-term monitoring of selected sites within the South Lake Simcoe Region. Historical records have been collected and digitized by the local naturalist club for a number of years between 1980 to 1996. This includes location, date and numbers and in some cases the distribution between juvenile and adults. As well, nesting status on the known nesting site, the Sand Islands was recorded including recent years (Austen et al., 1994).

Long-term monitoring has been initiated for several sites in the South Lake Simcoe watershed to create baseline data on forest and wetland species (Milne and Harpley, 1996). The number of sites has grown from a single location in 1990 to 15 sites in 1996. These sites have included wildland complexes, combinations of wetlands, small lakes and upland forest. The monitoring included intensive observation throughout the breeding seasons. There was a minimum of 6 hours a season to a maximum of 30 hours over 10 separate days. The level of observation varied depending on the nature of the project for a given year.

Breeding Status

COSEWIC (1997) considers the Caspian Tern to be a vulnerable species in Ontario. Similary, this species is considered rare by the Ministry of Natural Resources and the Ontario Rare Bird Breeding Program (Austen et al., 1994). In southern Ontario, almost all reports of breeding are on very large bodies of inland water, including Lake Ontario and Lake Huron, especially on Georgian Bay and in the north channel (Austen et al. 1994). Elsewhere in eastern Ontario breeding has been documented at the Thousand Islands group and the St. Lawrence river (Godfrey 1966). In the Greater Toronto Area breeding has been recorded at the Leslie Street Spit (Steers 1979) and Toronto Outer Harbour (Haymes and Blokpoel 1978). The nesting colony on the Sand Islands at Lake Simcoe is one of only several sites found inland of the Great Lakes in southern Ontario (Cadman et al. 1987). The sand beaches found at this site are ideal for nesting larids as documented elsewhere (Harrison 1975; Diamond and Prys-Jones 1986).

The existence of an active Caspian Tern nesting colony at the Sand Islands has been known for some time. During the years between 1985 and 1996, the occurrence of Caspian Tern has been observed and documented in the South Lake Simcoe region (Harpley 1993a, 1993b). In the 1980's there were confirmed records of nests and eggs on the Sand Islands. The observation included one nest containing three eggs (Table 1). Ontario Ministry of Natural Resources staff have made observations of the colony over the years but no rigorous breeding sruveys have been undertaken since the early 1980's. Recent non-systematic observatons have indicated a decline in the colony and there has been some concern that the colony may be in trouble (Harpley 1993a, 1993b).

Staging and Foraging

Significant recent spring records of large numbers of Caspian Terns along the southern Lake Simcoe shoreline are noteworthy and indicate perhaps a strengthening of the local populations (Figure 2). Fifteen birds were observed in May, 1993 on De La Salle beach (Table 1). This indicates that this area is an important location for early spring migration staging before the birds move the Sand Islands nesting colonies.

Similarly, the population of the colongy maybe increased in mid-summer as birds from northern colonies on the Great Lakes move south. Birds in late spring and summer were observed congregating on the beaches of the Sand Islands and entering and leaving grassy areas inland from the beach. In the 1991 season, early in July a total of 36 adult birds and 1 immature were observed (Table 1). Tozer and Richards (1974) reported that there was an influx of terns on Lake Scugog, situated to the southeast of Lake Simcoe (Figure 3), beginning in the first half of July.

The presence of Caspian Terns on small water bodies inland, south of Lake Simcoe (Figure 3) has been documented for the summer and early fall. These inland records often represent mature and young birds leaving the nesting colonies and staging areas searching for alternative feeding sites, prior to migration. Small, riverine lake locations are preferred. One adult and an immature were observed at Black River in the summer of 1994 and 1995 (Table 1). Uxbridge Brook is also an important area for foraging for the Caspian Terns. During the summer, repeatedly between the years 1992 to 1996, Caspian Terns have been observed (Table 1). One observation of an individual tern was made at Mud Lake during the summer of 1996. The bird was using the area to forage for food. The birds use the small lakes to forage for food and it is assumed they returned to Lake Simcoe and possibly Lake Scugog later in the day (Milne and Harpley, 1996).

Discussion

It is noteworthy that the availability of alternate food sources in close proximity to the nesting grounds is an important factor in the feeding patterns of the terns. As populations increase during the summer by the addition of local young and individuals from other nesting sites this increases the demand for food supplies. Individuals extend their forage range to include th small lakes near the south shore.

However, recent land use proposals present both direct and far-reaching threats to the tern colony. Direct threats include disturbances to nest sites on the Sand Islands. Increased disturbance on the lake, nearby shoreline and on Georgina Island could all have a negative effect on the Caspian Tern populations. Disturbances arising from cottage development, increased development on the island and recreational activity on the lake (i.e. boating and fishing) have a negative effect on water quality and fish populations.

Less obvious effects can also impact on the tern populations. For example, the Interim Waste Authority (IWA) identified a dump site near Sutton o the short list for the York Region waste site in the early 1990's. Caspian Terns were observed within two kilometres of the proposed site along the Lake Simcoe Shoreline. The anticipated gull problem expected with a possible landfill heightened concern for the Tern populations (Harpley 1993a, 1993b).

The Caspian Terns share the beach with other colonial nesting species (Figure 4, photo). The Ring-billed Gull (Larus delawarensis) is particularly common here as in other southern Ontario locations today. Considerable aggressive behaviour is apparent between species even during casual observation. The Ring-billed Gull can out-compete Common Tern (Sterna hirundo) and Caspian Tern by arriving at the breeding grounds first to build their nests and simply outnumbering the tern populations (Steers 1979). There is increasing evidence that Ring-billed Gull is encroaching on the traditional nesting habitat of Caspian Tern (Cadman et al. 1987). Another possible threat is predation. Herring Gulls (Larus argentatus) and Ring-billed Gulls have been observed preying on the eggs and chicks of Common Tern (Quinn and Morris 1986; Morris et al. 1992). These factors negatively affect the breeding success of the vulnerable species Caspian Tern.

Far-reaching effects which threaten the Caspian Tern populations include the disturbance of small lakes used by terns for foraging. Land clearance and forest fragmentation on the periphery of the water sources and increased activity on the lakes can affect water quality and fish populations. Also fragmented wetlands formed by corridors from highways and hydro lines may disrupt feeding patterns.

In recent years some riverine pondings, water areas impounded by human constructed dams are being removed to encourage fisheries activities. This research demonstrates that riverine pondings have important ecological functions for wetland species including threatened Caspian Terns. Future management suggestions of removing riverine impoundments for fisheries should be critically challenged to consider other habitat function of these pondings.

Acknowledgements

The authors would like to thank all those who have documented Caspian Tern observations for this paper. Special thanks to Provincial and Federal Grant Funded research staff, Manisha Chauhan, Mike Hall, Stephanie Hawkins and particularly Jill Clyde who assisted in the field and was instrumental in bringing this paper together with the authors.

Table 1. Observations of Caspian Tern (Sterna caspia) in the South Lake Simcoe Region, 1985-1994.

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