The Duke and Duchess of Cambridge are making hearts flutter with a sweet new photo of their daughter Princess Charlotte. Prince William and Kate Middleton shared a photo on Instagram Saturday of Charlotte, 6, in a periwinkle shirt and pink floral shorts gazing at a butterfly while she gently held the insect in her hands. The photo of the young royal was to spread awareness for the U.K.'s Big Butterfly Count, a campaign to encourage the public to count the number of butterflies they see in nature to survey the health of the environment. Charlotte Elizabeth Diana is the middle child and only daughter of Prince William and Duchess Kate. Her younger brother, Prince Louis, turned 3 on April 23, and her older brother, Prince George, turned 8 on July 22. Tradition has dictated the royals release new photos to commemorate family members' birthdays each year: Will and his wife, Duchess Kate, have previously shared special shots of their three children on each of their birthdays, a custom they started on George's first birthday.