Population declines of migratory shorebirds have been of increasing concern to researchers and conservation agencies (Warnock and Gill 1996, Brown et al. 2001, Gratto-Trevor et al. 2011, Andres et al. 2012). The probable causative factors are environmental deterioration and wetland loss in addition to hunting on wintering grounds (Morrison et al. 1994). Ydenberg et al. (2004) found that length of stay at one stopover site in British Columbia declined as falcon populations increased, and postulated that perceived census declines of shorebirds at traditional staging sites may be linked to disturbances by Peregrine Falcons (Falco peregrinus), which have made a strong population recovery after the 1970s when the use of toxic chemicals (DDT) in agriculture was banned (Cade and Burnham 2003).

The Pacific coast from southern British Columbia to Mexico serves as both an important migration route and a wintering range for the pacifica subspecies of Dunlin (Calidris alpina; Warnock and Gill 1996). The Fraser River delta in British Columbia, Canada, supports a wintering population of approximately 30,000 Dunlin, which appears to have been stable over the last 50 yr despite industrial expansion along the waterfront of metropolitan Vancouver (Butler and Vermeer 1994, Shepherd 2001, Drever et al. 2014). A review of Christmas Bird Count data from across North America found that although neither the western nor the eastern populations of migratory Dunlin have declined over the long term, they show large and irregular fluctuations; however, the role of predation in these fluctuations is unknown (Xu et al. 2015).

An observational field study of peregrine predation on Dunlin wintering at Boundary Bay in the Fraser River Delta was begun in 1994 and continued periodically until 2015. The objective of the study was exploratory, to gain insight into the dynamic interaction of peregrines and Dunlin. Here we review a large sample of empirical data collected on peregrine abundance and hunting success rates within individual winters and seasonally from October to February (Dekker 1998, Dekker et al. 2012, Dekker and Drever 2015).

When the intertidal zone floods, Dunlin collect on the waterline along the salt marsh, but the birds often flush and shift to other points. False alarms are frequent (Beauchamp 2010). If attacked by a peregrine, roosting Dunlin fly out over the bay in an apparent antipredator strategy termed over-ocean flocking, later renamed high-tide flight (Dekker 1998, 2013). The strategy is explained by the premise that predation risk is lower for individual Dunlin that take part in the aerial maneuvers of a dense flock than the predation risk for Dunlin roosting or foraging on the ground or in shallow water. The hunting success rate of peregrines attacking Dunlin over the open tide flats or ocean was 9.1%, compared to 23.6%, in surprise attacks near the salt marsh (Dekker 2003).

The timing and altitude of high-tide flight depend on the height of the tide and wind speed. To reduce energy expenditure, the mode of flight includes moments of sailing on lateral airflows (Hentze 2012), and the duration of flight is asymmetrical around high-tide time, i.e., longer before the high-tide time than after it (Dekker 2013).

The purpose of this report is to consolidate two decades of information on the interactions of Peregrine Falcons and Dunlin wintering at Boundary Bay, British Columbia. We also augment the results with comparative data for December 2015.

Methods

Results

Between 1994 and 2015, 1297 peregrines were sighted in 1811 hr of observation (0.71/hr; Table 1). The sighting frequency declined seasonally from 1.34/hr in October to 0.56/hr in February (β_month = −0.18, SE = 0.026, t = −7.20, P < 0.001). Peregrine sightings increased over time from 0.36/hr in 1994 to 1.24/hr in 2015 (β_winter = 0.026, SE = 0.005, t = 5.19, P < 0.001; Fig 1).

Table 1. 

Hours of observation, sightings of Peregrine Falcons, sightings/hr, hunts, hunts/hr, captures of Dunlin, and percent hunting success at Boundary Bay, British Columbia, 1994–2015. Rows in bold represent summaries for each month.

Figure 1. 

(a) Abundance and (b) hunting success of Peregrine Falcons (Falco peregrinus) in October–February at Boundary Bay, British Columbia. Lines indicate predicted values from General Linear Models representing peregrine sightings and hunting success as a function of month. (c) Abundance and (d) hunting success of Peregrine Falcons (Falco peregrinus), 1994 to 2015. Lines indicate predicted values from General Linear Models representing peregrine sightings and hunting success as a function of year (predicted values calculated at median values for parameters not shown in each panel). Different symbols indicate month of observations, as shown in the upper panels.

Over the duration of the study, we observed peregrines launch 1369 hunts aimed at Dunlin, with a capture rate of 15.0% (Table 1). The hunting success rate increased from 10.8% in October to 15.0% in February, although this increase was not statistically significant (β_month = −0.051, SE = 0.071, t = 0.71, P = 0.46; Fig.1).

The Dunlin reacted to the flooding of the intertidal zone with different strategies (Table 2). These strategies included: (1) high-tide flight over the water at various altitudes; (2) roosting in dense aggregations on the foreshore; (3) alighting on driftwood logs or floating debris well offshore; and (4) flying inland to roost or forage on muddy fields and flooded meadows. For weather-dependent variability and duration of each strategy, see Dekker (2013).

Table 2. 

High-tide behavior of Dunlin, 6–19 December 2015.

In December 2015, high-tide flight was observed everyday, but only a portion of the population took part and the length of time the birds stayed airborne varied greatly. High-tide flight of large flocks (>100 birds) was routine on days when wind speeds were 1–5 m/s). When wind speeds were >10 m/s, the flocks flew low (1–5 m) over the waves, often >1 km from shore, where they could easily escape detection by observers because the flock only occasionally rose high enough above the waves to make them visible through binoculars. Light rain or showers did not stop the flights, but heavy rain did. The longest time that some flocks remained airborne over the water was recorded on 18 December 2015 and included 183 min before the high-tide time and 132 min after, for a total time of 315 min.

Discussion

Given the low chance and high amount of time involved in actually witnessing the split-second action of a falcon seizing a sandpiper in flight, the sample size of most visual studies of peregrine predation on shorebirds is very small (Buchanan et al. 1986, Cresswell 1996). The results presented herein constitute the largest number of shorebird kills observed directly in the field.

The seasonal decline in peregrine sightings from 1.34/hr in October to 0.60/hr in January was to be expected because some or all first-year immature peregrines migrate away for the winter. The fall departure of the inexperienced immature birds and the proportional increase in adult falcons would explain the observed rise in hunting success rate from 10.8% in October to 16.3% in December (Table 1). The capture rate of adult peregrines hunting Dunlin at Boundary Bay was 26.8%; the comparative rate for first-year immature birds was 9.0% (Dekker 2003).

Dekker and Drever (2015) linked a 2006–2014 decline in peregrine sightings/hr at Boundary Bay to kleptoparasitic pressure from Bald Eagles, which increased from circa 200 in 1994 to 1800 in 2014 (Elliott et al. 2006, Dekker et al. 2012). Eagles were also common in December 2015. In 11 instances, we observed one or more eagles interfering with hunting peregrines, resulting in the loss of two Dunlin prey. High eagle presence near the high-tide line may well explain why the majority (80%) of peregrine hunts observed in December 2015 took place far out over the ocean, often >1 km from shore. Due to distance and poor visibility, we could not determine the outcome of 36 of 76 (46%) peregrine attacks on Dunlin recorded in December 2015. Nevertheless, the observed capture rate of 35% in 40 hunting sequences was more than double the 15% overall rate. To help determine whether a distant hunt was successful or not successful, we watched the reaction of eagles. After failed hunts, falcons flew away and were not chased by eagles. Conversely, if a Dunlin had been captured, the successful peregrine was pursued by one or more eagles for varying distances.

Because of their limited skill at capturing Dunlin, usually involving multiple swoops at fleeing prey, immature peregrines were particularly vulnerable to pursuing kleptoparasitic Bald Eagles. By comparison, adult peregrines often caught a Dunlin at the first attack, which allowed the falcon sufficient time to get away from eagles (Dekker and Drever 2015). Frequent harassment and prey loss to eagles presumably drove immature peregrines away from the coast, resulting in a proportional increase in adult falcons, which would explain the observed rise in overall hunting success rate in December 2015 (Fig. 1).

At inland locations where eagles were less common than along the coast, wintering and migrating peregrines routinely hunted ducks such as wigeon and pintails (Dekker 1987, 1995). The rarity of duck kills reported after 2000 at Boundary Bay further supports the contention, first posed in 2003, that prey losses to Bald Eagles forced peregrines to stop hunting ducks because they were too heavy to be carried away at the approach of eagles. Thus, kleptoparasitic pressure from Bald Eagles tended to force peregrines to concentrate on lightweight prey such as Dunlin at Boundary Bay.

Peregrine sightings in December 2015 were relatively high at 1.24/hr, which may have been associated with weather conditions. The ambient temperature during December 2015 did not drop below 0° C. By contrast, in winters when the bay became temporarily covered with ice, Dunlin left the area, followed by the departure of some or perhaps most of the peregrines (Out 2010, Dekker 2013).

A noteworthy behavior of the local Dunlin was their different reaction to high tide. While a portion of the population engaged in prolonged high-tide flight, other flocks roosted on the foreshore of the salt marsh or departed for inland fields (Table 2). This variation in high-tide behavior might be related to dissimilar age or physical condition, such as body mass or fat reserves, of the birds in question. Another possible differentiating factor might be the birds' geographic origin. Dunlin wintering at Boundary Bay most likely originated from populations that breed at different latitudes and each group may have its own method of coping with intertidal flooding. Such questions were outside the scope of this study and might be a subject for future research.

The results of this long-term study suggest that the interactive dynamics of Peregrine Falcons and Dunlin wintering at Boundary Bay did not change in a significant way over 22 yr. Peregrine attacks on sandpipers, whether lethal or not, constitute a powerful and sustained disturbance to the foraging needs of sandpipers, perhaps forcing them to shorten their stay on migration stopovers and jeopardizing their chances of accumulating enough fat to complete their migrations (Lank et al. 2003). The information presented herein suggests that Dunlin in winter expend much time and energy in predator avoidance. Yet, despite this energetic cost and the reported increase in the number of peregrines at Boundary Bay, the local Dunlin population does not appear to have declined in size over a 22-yr period, suggesting that the anti-predator strategies adopted by these Dunlin may allow population maintenance despite increasing predator numbers.