It lacks critical sharpness. It looks almost like it was photographed through a window. Overall yeah, it looks sort of sharp, but it lacks critical sharpness in this, the 36-megapixel, Nikon D800 era. And....you missed the submission guidelines' all-important minimum SIZE qualification pretty easily. The site is most definitely interested in VERY high-quality, and somewhat bigger image files. Not that your photo is bad or awful, but it does lack that feeling, that sensation of really high, high acuity.
For example: if you take a really BIG image, like say a 36-megapixel image and down-size it and sharpen it carefully in software, even at smaller sizes it has a certain look to it, and it can look very sharp...there's plenty of detail to give that visual impression of acuity, or sharpness, of SNAP! This image does not convey that same sense of critical sharpness; look at the area on the closest wing...it looks like this was shot through a window to me. or, with a slightly sub-par lens.
A good example: last week I shot images with a 5-ED element Nikkor 80-200 AF-S zoom lens. And then I shot with the 200mm f/2 AF-S VR-G prime telephoto. MOST of the shots were done at 200mm with the zoom, but some were made of equipment (combines) much closer, with the zoom at shorter lengths. The difference? The zoom lens images were sharp; the prime lens images were ULTRA-sharp. Same aperture on most all, f/4.5. You can see a similar type of less-than-critical sharpness on a lot of zoom lenses at some range within their focal length range. Sharp, yes, but not critically sharp. people who run image banks and stock libraries these days want critically sharp images, and they ant them to be of a certain size.