Biden's numbers have also come down about 8 points from their high right after he announced. Take a look at the average of polls and you'll see that Biden and Bernie have both been volatile recently while Warren is slowly trending upwards. Can't really say Warren's 1 point change in the last couple weeks is at Bernie's expense when Biden has lost 3 points, Bernie 2, and Buttigieg 1 over that same period.
(I can't figure out how to show their poll average graph as an image, so I'm just linking the webpage.)
https://www.realclearpolitics.com/epolls/2020/president/us/2020_democratic_presidential_nomination-6730.html