Sigma do make some pretty average lenses, the 100-300mm f/4 is the best thing they have going for general birding photography, the Sigma 300-800mm f/5.6 is too slow( but optically excellent) and the 200-500mm f/2.8 is even heavier than my Pentax 67 M*800mm f/6.7 ED - and any 35mm lens that is heavier than that lens is just isn't worth the trouble in my books.
here is an image from my 100-300mm f/4 on My pentax K7 - f/5.6 ISO 800 1/125th -focal length around 200mm, and I used a AF540 Flash with a 1/2 CTO gel