our collage may constitute copyright infringement but probably the more important questions are whether the rights holders in the sculptures will see your work, (and if they see it, whether they will care enough to do anything about it). Here's some background to help with your decision.
Copyright permission needed. As much as we like your theory about property releases, it's not accurate and it's not going to help. Sculptures are protected under copyright law. Assuming a sculpture you want to photograph and reproduce is protected under copyright law, you would need permission from the sculptor (or whoever the sculptor transferred copyright ownership to) in order to reproduce it. (The property owner is unlikely to be the copyright owner unless the sculpture was commissioned.) Mass-produced lawn ornaments may be protected under copyright but it's unlikely the manufacturer will see or care about your collage. Those folks are generally more interested in stopping competing manufacturers. Also, for copyright law purposes, it doesn't matter whether the sculpture is on public or private property; what matters is whether the animal sculpture is protected by copyright law and whether your reproduction is an infringement.
When do you need permission from the property owner? You mentioned property releases and the main reasons you would need one is that you're photographing a copyright-protected building (unlikely) from private property, or you're photographing a property location as the basis for an advertisement or a movie. Photographs of copyrighted buildings taken from publicly accessible areas don't require permission. (We offer a free property release if you ever need one.)
BTW Dept. May we inquire: Does that triple question mark in your last sentence signify something ??? and why is it inserted in mid-sentence? Hopefully that's not a trending grammatical quirk.