All dogs retain some hunting instinct, no matter how much humans have messed with their breeding. They evolved as hunting animals, or would not have survived to the transition to human companions.
One example is our other dog, a male Shih-Tzu we adopted a decade ago. Willie's bigger than average, 22 pounds when lean and mean. Turned out he LOVED to hunt--not birds but cats. We have a fair-sized garden in the back yard, and local cats tend to climb the cedar-plank fence on the back of the garden, along the alley, to graze on stuff like raspberries, beans and peas--along with doing other damage.
We found early on that Willie liked to stalk along the downwind side of the taller plants, whether the rows of raspberries, or peas/beans, moving slowly along the rows until he smelled one of those darn cats. Then he'd rush up the row to flush the cat--which often ran over one of the woven-wire fences on the other side of the garden. Lena the Labrador soon learned to hang out around the outside of the woven wire, to "encourage" any cat that jumped the wire to leave our property....
