With the increasing severity of climate stressors, the elevational ranges of plant and animal species are pushing skyward. The channeling and redistribution of species is expected to increase in frequency and magnitude globally. However, any elevational gradient will be associated with decreasing air pressure, which will impose a degree of hypoxia on colonizing organisms. Insect species developing under depleted oxygen (O₂) concentrations in controlled laboratory conditions are known to exhibit smaller adult sizes, reduced reproductive capacity, and lower survival rates, all of which represent major fitness consequences for insects. This begs the question as to whether altitudinal hypoxia might produce the same types of consequences as artificially reduced O₂ in laboratory experiments. With this question in mind, we observed the development of a native bee species, Osmia lignaria across an elevational gradient in the Rocky Mountains, USA. Known commonly as the blue orchard bee, O. lignaria is a univoltine (single generation per year) solitary (non-social) bee that nests within hollow stems and reeds. Fully provisioned reeds were collected in the foothills of the Rocky Mountains in Salt Lake City, UT, and were distributed among three sites representing an elevational gradient: 1300 m, 1900 m, and 2500 m. The fully provisioned reeds were placed within unsealed plastic containers, in climate-controlled indoor spaces at each of the three elevations. Here we report a series of observations that reveal a surprising pattern of uneaten pollen-provisions, increased mortality, and markedly reduced sizes among the bees at higher elevations. The incidence of uneaten pollen increased with rising altitude, while pupal (or adult) weights declined with altitude, suggesting that with increasing altitude, the bee larvae ceased feeding prematurely, becoming smaller pupae/adults. As pollinator populations emigrate skyward, these communities will likely exhibit a diversity of responses to altitudinal hypoxia, with each species potentially experiencing its own ‘hypoxia ceiling.’ Given that bees are key players in the foundational mutualisms between microbes and flowering plants and that declining pollinator diversity has been linked to reduced plant diversity, the basic functioning of high-elevation ecosystems may be constrained by the ceilings imposed upon pollinator populations.