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Reports on nesting debris generated by great blue heron (Ardea herodias), arboreal nesting birds with 'semi-altricial' young, are limited. In this study, surface and subsurface sampling were conducted in 2012 and 2013 in a heronry near the Missouri River in Montana, including bi-weekly collections in 2013 documenting accumulations over the breeding season. Disarticulated juvenile heron bones with lesser numbers of microvertebrates (fish and small mammals) and invertebrates (crayfish and snails) dominated the assemblage beneath the nests. Eggshell on the surface was present but uncommon (0.85 eggshells/m2) with eggshell orientation varying both by location and fragment size. Eggshell surveys differed with respect to sampled trees and sampling time, but generally favoured concave-up orientation whereas the early season small eggshell survey further from sampled trees favoured concave down. Shallow (10 cm deep) excavation revealed a subsurface assemblage very similar to that on the surface but favouring more resistant elements. Hence, the subsurface assemblage was nearly devoid of eggshell, with a lower representation of invertebrates and fragile bone, but an excess of heron hindlimbs. Our study demonstrates that arboreal nesting sites have the potential for long-term preservation and provides information useful for recognition and reconstruction of arboreal nesting sites from the fossil record. [ABSTRACT FROM AUTHOR]