Caspian Tern Research

Caspian tern, Deer Lake (Ken Knowles photo)

Historical Nesting Colony at Lake Simcoe

Status of Caspian Tern in the South Lake Simcoe Watershed.
Paul J. Harpley and Rob J. Milne – 2012


Lake Simcoe is home to a number of colonial water birds that breed along coastal areas and islands within the lake. There is limited suitable habitat for these nesters and therefore it is critical to identify these sites and identify which areas have successful nesters. It is also important to track the continued success or changes to these sites both naturally and from human impact. Increasing urbanization, forest loss and development of service corridors all threaten the success of breeding birds in southern Ontario (Friesen et al., 1995). Much of the past emphasis of concern has focused on the breeding success and location of threatened and indicator species in forest habitats (Austen et al., 1994). Less is known of the nature of colonial nesters and therefore it is important to raise the awareness of these species. The ecology of these species can be more complex than forested birds as they require specific habitats that can support large numbers as well as complex foraging areas that can range over many kilometers and are dependent on the water quality of nearby lakes and rivers.

One species of particular concern in the Lake Simcoe watershed is the Caspian Tern (Sterna caspia). 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 the north channel (Austen et al. 1994). 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. 2007). Nests have been located on the small gravel bars characteristically 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). The sand beaches found at this site are ideal for nesting larids as documented elsewhere (Diamond and Prys-Jones 1986).

The tern population in southern Lake Simcoe has documented through a combination of 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 and Zephyr Society members for a number of years. The principal nesting site of Caspian Tern is at the southwest end of Georgina Island, gravel and sand deposits 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. 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.

The landscape of the south Lake Simcoe watershed is also a critical component of the Caspian Tern’s continued success. The region has a number of significant wetland areas inland, forming an extensive wetland complex. Caspian Terns have been observed foraging along the Black River, Mud Lake and Wagner Lake/Uxbridge Brook. These water features have sufficient areas of open water which provide critical foraging habitat for Caspian Terns as evidenced by the late summer observations (Milne and Harpley, 1994). The average distance between adjacent open water areas within the complex these three wetland sites form is approximately 200-300 km2 which is typical of this wide-ranging species (Cuthbert and Wires, 1999).

This species status has improved in Canada over the past several decades and it was removed from the list of vulnerable species by COSEWIC. However, recent collapses in the population related to avian botulism reveals the sensitivity of certain colonial waterbirds to environmental change and emphasizes the need to continue to monitor and maintain conservation efforts (Cadman et al., 2007). Locally, 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.

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. However, 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.


Austen, M.J.W., M.D. Cadman and R.D James. 1994. Ontario Birds at Risk:
Status and Conservation Need. Federation of Ontario Naturalists, Don
Mills, Ontario and Long Point Bird Observatory.

Cadman, M.D., D.A. Sutherland, G.G. Beck, D. Lepage and A.R. Couturier. 2007. Atlas of the
Breeding Birds of Ontario, 2001-2005. Bird Studies Canada, Environment Canada, Ontario Field Ornithologists, Ontario Ministry of Natural Resources and Ontario Nature.

Cuthbert, F.J. And L.R. Wires. 1999. Caspian Tern (Sterna caspia). In the Birds of North America. No. 403 (A. Poole and F. Gill, eds.). The Birds of North America Inc.

Diamond, A.W., and R.P. Prys-Jones. 1986. The biology of terns nesting at Aldabra Atoll, Indian Ocean, with particular reference to breeding seasonality. Zoo London: 527-534.

Friesen, L.E., P.F.J. Eagles, and R.J. MacKay, 1995. Effects of residential development on forest-dwelling neotropical migrant songbirds. Conservation Biology 9: 1408-1414.

Milne, R.J. and P.J. Harpley. 1994. Landscape ecology, avian information and the rehabilitation of wildland complexes in the Greater Toronto Area. In Proceedings of 1993 Canadian Land Reclamation Association Annual Meeting, Lindsay, Ontario, August 11th-13th, Canadian Land
Reclamation Association, Toronto, pp. 48-56.

Full Paper:

Harpley, P.J. and Milne, R.J. (2012) Status of Caspian Tern in the South Lake Simcoe Watershed. The Zephyr Society of Lake Simcoe, Pefferlaw, Ontario. Web Site Content 2012—2015-05.

Please contact the Zephyr Society of Lake Simcoe for full paper.