Predation of adult piping plovers Charadrius melodus near nests protected by predator exclosure cages in the northern Great Plains of Alberta and Saskatchewan, Canada, and North Dakota and Montana, USA
Murphy R.K., Michaud I.M.G., Prescott D.R.C., Ivan J.S., Anderson B.J. & French-Pombier M.L. (2003) Predation on adult piping plovers at predator exclosure cages. Waterbirds, 26, 150-155
The piping plover Charadrius melodus is an endangered shorebird with breeding strongholds in the Northern Great Plains of northern USA and southern Canada. Poor breeding success is one reason for a declining population; the plovers favour beaches around prairie alkali lakes on which to breed, but suffer significant losses of eggs and chicks to predators that have increased in abundance in recent decades.
Studies have shown that reproductive success of shorebirds can be improved by predator exclosure cages placed over individual nests (with mesh diameter large enough to allow incubating bird access, but small enough to exclude most potential predators). Experiments were carried out on breeding piping plovers on a number of alkali lakes in the northern Great Plains region to examine the efficacy of such exclosure cages for the species.
Study sites: The studies were undertaken on alkali lake beaches of Alberta and Saskatchewan (Canada), and North Dakota and Montana (USA). These lakes have salt-encrusted, white beaches with sparse vegetation; beaches used by breeding plovers are generally 10-40 m wide.
The piping plover breeding season extends from late April until early August. A 4-egg clutch is laid in a shallow depression directly on the sand/gravel substrate. Incubation lasts about 28 days, the chicks are able to leave the nest scrape within hours of hatching.
Nest cages: During the breeding seasons of 1993-2002, a total of 1,355 nest cages were placed over piping plover nests. The nest cages were of several designs including small diameter (1-1.7 m) and large diameter (3-4 m) cages, some with wire mesh tops and some with soft netting tops. Nests were monitored and chick productivity and predation at these nests was compared with 420 nests that received no protection.
Adult nesting piping plovers were killed near cages at 68 (5%) of caged nests; from remains found, predation appeared attributable to raptors. In contrast, no losses of adult plovers were detected at the 420 uncaged nests. Predation was greatest (up to 48%) when small (1-1.7 m) diameter cages with wire mesh tops were used at sites with low (on average 4%) or moderate (15%) tree cover within 2 km of the nest. In areas with low tree cover, predation decreased to 0.7% of cage provisions/year when large (3-4 m) diameter cages with soft netting tops replaced other exclosure cage designs.
There may be considerable site variability upon the effectiveness of cage design, for example no predation was recorded at 393 plover nests along the relatively treeless North Dakota-Montana border that were covered by small cages.
Conclusions: Evidence from this study suggests that predator exclosure cages should be used with caution when attempting to protect eggs of piping plovers (and probably similar species of shorebirds) in order to boost chick productivity. In some situations, enhanced productivity from use of the cages is outweighed by increased mortality of adult birds. In an otherwise fairly featureless landscape, the attention of raptors was drawn to the nest cages. In areas with trees, these probably serve as vantage points on which raptors can perch, watch and locate piping plovers returning to their nests to incubate.
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