Cannot source file using find, fzf, and xargs

I am trying to use fzf to select and activate different virtual environments. However, I am receiving the follow error:
xargs: source: No such file or directory

My command comprises three parts: find | fzf | xargs
(1) find locates my virtual environments,
(2) a virtual environment is selected using fzf, and
(3) xargs is used to source the selected file.

Below is the full command:
find ~/.virtualenvs/ -maxdepth 1 -type d | fzf | xargs -I {} source {}/bin/activate
xargs: source: No such file or directory

I can directly source the file (i.e., command prompt shows entry into fzf_delete virtual environment):
brian:~$ source /home/brian/.virtualenvs/fzf_delete/bin/activate
(fzf_delete) brian:~$

I can select the virtual environment using fzf:
find ~/.virtualenvs/ -maxdepth 1 -type d | fzf
/home/brian/.virtualenvs/fzf_delete

I have also isolated the issue to xargs by removing the dependency on find and fzf:
echo "/home/brian/.virtualenvs/fzf_delete" | xargs -I {} source {}/bin/activate
xargs: source: No such file or directory

I have tried every variation of quoting, use of variables, string concatenation, command substitution, using bash -c, etc. that I can think of to no avail.

**How can I get xargs to source the selected file?**
PS - I am aware that null termination should be used with find, fzf, and xargs in case file names contain spaces, but that does not appear to be causing the immediate issue.