Piping Plover predation attempt?

Piping Plovers eyeballing Red-winged Blackbirds sitting on their predator exclosure

Today I witnessed what seemed like an (unsuccessful) predation attempt on a Piping Plover nest by three Red-winged Blackbirds at Sandy Hook's North Beach. As it was happening in front of my eyes, I tried to grab some video of it.

Just prior to this, I had been observing shorebirds feeding on the sand spit at the 'False Hook'. Nearby, I noticed three adult male Red-winged Blackbirds scavenging the flood line for sand flies, but didn't pay them any further attention.

That is, until I saw them land on one of the Piping Plover predator exclosures and even go inside! The nest owners started calling and tried to scare off the blackbirds with threatening postures and wing spreading. Some of this can be seen in the video (although, regrettably, very little of it remains visible after down-sizing it for online posting). The plovers did not feign injury, as they will do when faced with bigger predators, but simply stood their ground and defended their nest. The blackbirds quickly gave up and left. One of the plovers resumed incubation while the other stood nearby.

Piping Plover incubating inside predator exclosure

The Piping Plover is a federally endangered beach-nesting shorebird, increasingly dependent on management. Conservation efforts toward Piping Plover populations include predator exclosures, sometimes electrified as the ones used on Sandy Hook. A recent paper in Avian Conservation and Ecology reports that predator exclosures enhance reproductive success but increase adult mortality of Piping Plovers (Barber et al. 2010). The exclosure is meant to keep out foxes, gulls and other predators.

The Red-winged Blackbird, one of North America's best studied birds, is primarily granivorous, although animal matter (insects) take up a major percentage of its food intake during the breeding season. It is not known to predate on eggs of other birds, and perhaps what I witnessed was nothing more than three blackbirds being curious about a Piping Plover predator exclosure. I was astounded, however, to see one of them go inside, as I'm sure the plovers were too!

Cited literature:
Barber, C., A. Nowak, K. Tulk, and L. Thomas (2010) Predator exclosures enhance reproductive success but increase adult mortality of Piping Plovers (Charadrius melodus). Avian Conservation and Ecology 5 (2): 6.