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Quantifying predator type and abundance adjacent to Common Tern nesting platforms at Tommy Thompson Park

Quantifying predator type and abundance adjacent to Common Tern nesting platforms at Tommy Thompson Park

by Jesse Hughes

Jesse Hughes

Common terns are a long-lived migratory waterbird whose population is declining on Lake Ontario. The Toronto and Region Conservation Authority (TRCA) at Tommy Thompson Park (TTP) – designated an Important Bird Area – have used floating platforms for common terns as nesting sites for three decades. In my EUCURA project, I quantified the presence of mammalian predators and aerial predators near the platforms. Three methods were used: 1) camera traps; 2) aerial surveys; and 3) monitoring e-bird for any aerial predators seen throughout the park. Four mammalian species were considered predators that may be able to access the nesting platforms: raccoons, mink, otter, and Virginia opossum. Three camera traps were deployed at two stations on the Embayment D shoreline: trail 16 (station 1) and on the South shoreline just west of the rafts (station 2); and one camera was on the southeast shore by the raft in Cell 3 (station 3). Three times per week, 1 hour aerial surveys were conducted to detect any bird predation activity (i.e., raptors, herring gulls, black-crowned night-herons, etc.) around the radius (200-300m) of the tern rafts. Any observations of aerial predators, the predators’ response, the terns’ responses were recorded. Lastly, e-bird was monitored from daily to calculate how many aerial predators were seen by the public to determine the number and species of aerial predators over time observed.

Fig 1. Common Terns nesting on artificial platforms at Tommy Thompson Park (Photo: G. S. Fraser). Note the metal siding is to prevent mammals from accessing the platform.

Camera traps resulted in a total of 1,651 photos, and four mammalian predators (raccoons, coyotes, Virginia Opossum, striped skunk) were caught on camera, only two of which were deemed a risk to nesting terns (Figure 2). Of the photos taken of these mammals from 25 May-June 9, 93.5% of trap events involved raccoons. From 10 June - 5 July, raccoon detection declined to 52.6%.

Only two raptors were observed in the aerial predator surveys and terns responded differently: one juvenile Bald Eagle at Cell 3 - no response from the terns, and one Osprey at Embayment D, which caused the birds to flush and chase the Osprey. I also observed 3 Herring Gulls at Cell 3 with no disturbances.

Fig 2. Images of four predatory mammals from trail camera sampling at Tommy Thompson Park.

I also observed 15 non-predator events during the surveys which caused the terns to “panic” – where the whole colony silently flies up (14/15 panics happened at Embayment D).
eBird data captured 20 raptors and 4 Corvids reported on lists that could be a potential threat to the nesting terns.
Mammalian predators, while in the area, did not access the nesting platforms indicating that the platform design is effectively deterring mammalian predators. The terns in Embayment D had more panics compared to the platform in Cell 3, which may be a result of their night-time nest desertions because of nightly visits by an owl (likely a Great Horned Owl Bubo virginianus; A. Matthews & G. S. Fraser personal communication). Interestingly, neither the park naturalists (pers. Comm.), nor the eBird reports for TTP detected the presence of an owl (I. Sturdee pers. comm.). I concluded that this was the major disruption (i.e., complete failure of all nests) to terns in 2023.

Common terns are long-lived birds (oldest known bird is 29 years old) that can afford to have reproductive failure in any one year, but repeated annual predation pressures could result in terns abandoning the site and reducing overall biodiversity to TTP.


Jesse Hughes is a recipient of the EUC Undergraduate Research Award (EUCURA) and worked with Gail Fraser on the project ‘Quantifying predator type and abundance of nesting common terns at Tommy Thompson Park.