The trees are silhouetted because the camera is adjusting the exposure to the bright sky. Sometimes you have to adjust the exposure manually to darken the picture to blacken the silhouette.
I cropped out the top third to cutout the sky with not much action so that the picture looks more balanced. (Nikon D90, Nikkor 18-200mm f/3.5-5.6 at 62mm f/4.8, 1/160, handheld, ISO = 800)