We present a statistical study of the environments of 63 high-redshift radio galaxies ( HzRGs ) between redshifts 1 \leq z \leq 5.2 , using the 24 \mu m waveband of the MIPS instrument aboard the Spitzer Space Telescope .
Using a counts-in-cell analysis , a statistically significant source overdensity is found in 1.75 \arcmin radius circular cells centred on the HzRGs when compared to reference fields .
We report an average overdensity of \delta ( = \bar { N } _ { targets } / \bar { N } _ { reference } ) = 2.2 \pm 1.2 at a flux density cut of f _ { 24 { \mu m } } = 0.3 mJy .
This result implies that HzRGs are likely to lie in protoclusters of active and star-forming galaxies at high redshift .
Over 95 % of our targeted HzRGs lie in higher than average density fields .
Further , 20 ( 32 % ) of our selected fields are found to be overdense to at least a 3 \sigma significance , of which 9 are newly identified protocluster candidates .
We observe a weak correlation between redshift and 24 \mu m source density , and discuss the populations being probed at different redshifts .
In our uniformly selected sample , which was designed to cover two orders of magnitude in radio luminosity throughout z = 1 - 4 , we find that the 24 \mu m source density does not depend on radio luminosity .
We also compare this result with recent work describing IRAC source overdensities around the same HzRGs and find correlations between the results .