This is a photo from one of the oil painting classes I taught earlier this year and I LOVE painting birds. Before we hit the canvas with oil paint, I had students "block in" the darkest parts of the bird with acrylic gesso (or acrylic paint). The acrylic dries fast so we can finish the painting in one sitting and it locks in the main parts of the bird. That way we could paint our background without losing our lines. Remember you can go over an acrylic with an oil, but you CANNOT go over an oil with an acrylic. I try to stress to my students to make your tools work for YOU! We had great results with this little painting as you can see from the work in progress shot here.
Here's our reference. I have amazing students!!!!