There ain't nothin' wrong with your lens! Excuse the double negative in my faulty grammar but reads TIMMERS post again and learn something.

This "bokeh" thing has become a misunderstood buzzword. So let's break it down to easily understood mechanics. So...you photographed a bird with a long lens at a fairly wide aperture- the bird is sharp and the background and FOREGROUND are out of focus. The out of focus background is comprised of little round blobs of light usually caused by light passing through things like tree branches and transilluminated foliage. The blobs are usually round and that is because they take on the shape of the aperture or the lens barrel- even if the diaphragm is wide open. Anything else that is placed in the light path, usually in front of the lens and remains out of focus will alter the shape of the blobs. Think of it as changing the shape of the aperture, especially if it is relatively close to the lens. Remember the objects in the foreground, when there is a minimal depth of field are ALSO out of focus so they are not defined in your image but they influence the shape of the blobs of light in the background.

So...the so-called bokeh is not magically produced by a special lens or one that is said to have especially good bokeh- and it won't always be uniform unless there is no foreground interference. Some lenses may have a more appealing effect to some photographers because they have more diaphragm blades, a unique aperture shape or are very fast and have an especially low f/stop such as f/1.0 or f/.95 where the depth of field is extremely shallow and the "bokeh" readily appears even at a shorter distance between the subject and the background.
Do a simple experiment to prove my point. Focus on an object, set a longer focal length, and a wide aperture, and make certain the background is well out of focus and illuminate as I have described. Get a thin dark object like a black pencil, a thin twig or a thin black cable tie and place in front of the lens in a diagonal fashion. Observe the blobs carefully and you will notice that there is a small diagonal line passing through the diameter of the "circle.". If you were to place a number of thin objects in front of the lens- odd shapes and direction, all kinds of different shape and pattern would occur in the blobs and some of them would become "misshapen"!

When you shot that bird, my money's on the probability that you shot through branches, twigs and other tree parts and that is a perfectly natural and pleasing effect. I purposely do that all the time when I am shooting portraits and other subjects where I want to get away for the usual effect, create a romantic or ethereal mood or a natural or colorful. vignette. Oftentimes I purposely shoot through the foliage, hedges, brush, high grass and I even have an old skylight filter with a few strips of black pinstripe tape just in cases there is nothing to shoot through.

There have been numerous "arguments" on this forum where folks are trying to define "bokeh"! It is simply a product of depth of field usage, selective focus techniques, and the photographs taste and visions. It can be smooth, blobs, or different lines or patterns. It can be out of focus foreground framing.

My general advice to many folks here on the HOG is to stop reading all the advertising and marketing propaganda and worrying about expensive lenses trademarked as "Art". The art is in knowing how to manipulate the optics. Google Mr. Fibonacci instead of Mr. Sigma or Mr. Zeiss! When you are out shooting, observe the effect at various focal lengths, apertures, and relative subject/background/foreground distances.

Oh, sorry for the omission of all the scholarly technical/optical terminology- I am sure "blobs" is not in the glossary and besides, I always found the term "circles of confusion" kinda CONFUSING!