What should I do about sacrifical altars I find in the park?

A few months ago my girlfriend and I found a burning candle in the center of a pile of sand, in the bottom of a hollow tree. There were some other things with the candle, but I don’t remember exactly. This is in a forested section of a park directly across the street from where we live. We called the parks department, and they were unconcerned. They said it was a usual occurrence. This is in NYC, mind you. Today I found two altars, one with 2 dead chickens, 3 coconuts, an open can of beer, 3 styrofoam cups, and a burning candle in a pile of sand. The other had 2 dead chickens, 2 ears of corn, and what I think was a mango (I didn’t get too close because of all the flies). I put the candles out both times, but my concern is that somebody is leaving burning objects unattended in the forest. Does anyone know what these might be, and what can I do about this fire hazard?