Mark has a Ph.D. in Ecology and Evolution from SUNY Stony Brook and taught in the Environmental Studies Program at Prescott College from 1988 until his retirement in January 2023. His courses offered a strong foundation in the biological sciences and combined theory with an experiential emphasis. This was especially the case in his field courses in Mexico, Costa Rica, and East Africa, but his on-campus courses also included numerous field trips and, in many instances, hands-on laboratory exercises. In all his teaching, he strived to weave together an understanding of ecological principles with evolutionary theory so that students can cultivate an appreciation for the origin and maintenance of biological diversity. Mark also encouraged his students to think beyond conventional views regarding the nature of organic evolution by integrating a dynamic developmental perspective.

Mark's continuing research focuses on questions concerning the diversity of avian plumage patterns. Why do similar plumage patterns repeatedly evolve in distantly related lineages while strikingly different plumage configurations may be exhibited in closely related taxa? Why do some species have streaks while others have bars? Is there a relationship between plumage pattern and morphology? Accordingly, he has identified an empirically derived plumage pattern trajectory that covaries with body size and morphology across the Class Aves (see Riegner, 2008; figure below). Furthermore, the trajectory is recursive at various taxonomic levels, such as within genera and within families, and thus serves as an indicator of parallel evolution. The recursive nature of the trajectory, in which morphological and chromatic elements recur in modified configurations, has perhaps contributed to an underestimate of parallel evolution in birds.

In addition, Mark is interested in exploring dynamic ways of thinking about understanding the origin, diversity, constraints, and evolution of biological form. This has led him to research key figures in the history and philosophy of the biological sciences (see Riegner, 2013). He has also been engaged in a long-term collaborative study of the occurrence, abundance, breeding phenology, and foraging ecology of herons and other wading birds in Kino Bay, Sonora, Mexico (where Prescott College has a field station; see Clark, Fleishman and Riegner, 2015), as well as research on color pattern in mammals and on avian morphology. Regarding the latter, Mark and co-author Ron Bassar have recently completed a long-term study on morphological trade-offs between cranial dimensions and leg lengths in herons (ardeids), and have identified a relationship between onset of foot prehensility in nestlings and later expression in foraging behavior of adult herons (Riegner and Bassar, 2023). Based on a literature review and field experience in the Brazilian Pantanal and elsewhere, Mark also studied the "morphodynamic convergence" between parrots and people, and has described some previously reported and some new insights in behavior, development, phylogeny, and even morphology between the two groups (Riegner 2023).

  (Hyacinth Macaw. Photo by Mark Riegner; Porto Jofre, Brazil; 2017)