Refer to the figure.
These results show the occurrence of nest predation by cowbirds on prothonotary warblers. After cowbirds laid an egg in the nest, the nests were divided into three treatments:
1) The cowbird egg was removed, and the nest opening left as is.
2) The cowbird egg and the nest entrance were left as is.
3) The cowbird egg was removed, and the nest opening was made smaller to prevent cowbirds from entering.
What hypothesis do these results support?