I ran across a similar issue and found this thread on search for an answer:
Signal 13 means something is written to a pipe where nothing is read from anymore (e.g. see http://people.cs.pitt.edu/~alanjawi/cs449/code/shell/UnixSignals.htm ).
The point here is that the ls command as executed by xargs is still writing output when the following head command already got all the input it wants and closed its input-pipe. Thus it’s safe to ignore, yet it’s ugly. See also the accepted answer in https://superuser.com/questions/554855/how-can-i-fix-a-broken-pipe-error