The cards with the shaded corners are from the Wingspan Swift Start Pack. Page 3 of the current version of the core set rulebook covers these cards (see image below).
The earliest copies of the game released between January and August of 2019 didn't include the Wingspan Swift Start Pack. However, copies of the game sold after that do include these cards and an updated rule book.
You can confirm what version you have by looking at the serial number on the back of the box. Early versions of Wingspan will have the serial number STM900 and do not include this pack. Newer versions which include the Wingspan Swift Start Pack, will have the serial number STM910.
If your copy of the game did not include the Wingspan Swift Start Pack and you would like to have it, you can purchase it directly from Stonemaier Games through their website: Stonemaier Games Wingspan Swift Start Pack