Polyethylene as in HDPE.
From what looks like a related article (though it may be talking about a different reservoir):
The reason LADWP is using the bird balls is because they are the ONLY National Sanitation Foundation (NSF) drinking-water-approved product of its kind on the market. When we discuss NSF approval, it means that this product has been tested precisely to ensure that it does not create the kinds of problems that some are insisting they create. The balls are made of High Density Polyethylene – a long-time water industry product also used for pipelines, and a black version of the same clear product you buy your one gallon container of milk in (look on the bottom for the HDPE label).
