$2 < z < 10$ and metallicities $-0.33 <
[\mathrm{Fe}/\mathrm{H}] < -2.25$ at the redshift of each cluster. The bright magnitude limit corresponds to a mass of $10^7~M_{\odot}$ and the faint limit is the 50% completeness limiting magnitude for the cluster field determined using artificial star tests (see Appendix \[sec:artstartests\]). The limiting magnitudes are well above the magnitudes of the most massive GCs, with GCs expected to have apparent magnitudes $m_{814} \gtrsim
31$ mag. The number of sources that reside within the selection box is listed for each field in Figure \[fig:cmd\]. An excess of UCD candidates in the cluster core field is observed for all clusters, with the exception of MACS 1149.

In the Abell 370 cluster core field CMD, the sequence of sources bluer than the UCD selection box at $F606W-F814W \sim 0.5$ are likely GCs associated with the foreground elliptical galaxy PGC 175370 at a distance of ${\sim}200$ Mpc. This is the galaxy on the northern edge of the ACS field in Figure \[fig:locations\]. Photometric scatter at the faint end is a source of foreground contamination not captured by the parallel field. Restricting the UCD analysis to the region with WFC3 coverage removes the most likely contaminents 