SACRAMENTO  When a company called Ausra filed plans for a big solar power plant in California, it was deluged with demands from a union group that it study the effect on creatures like the short-nosed kangaroo rat and the ferruginous hawk.

By contrast, when a competitor, BrightSource Energy, filed plans for an even bigger solar plant that would affect the imperiled desert tortoise, the same union group, California Unions for Reliable Energy, raised no complaint. Instead, it urged regulators to approve the project as quickly as possible.

One big difference between the projects? Ausra had rejected demands that it use only union workers to build its solar farm, while BrightSource pledged to hire labor-friendly contractors.

As California moves to license dozens of huge solar power plants to meet the state’s renewable energy goals, some developers contend they are being pressured to sign agreements pledging to use union labor. If they refuse, they say, they can count on the union group to demand costly environmental studies and deliver hostile testimony at public hearings.