More recently, she has been involved in large scale collaborations looking at patterns of global biodiversity, and has assessed the taxonomic status of South Asian vultures.

She was the main author of Birds of South Asia: The Ripley Guide, a landmark publication due to its greater geographical and species coverage compared to its predecessors.

[1][4] Rasmussen is a visiting assistant professor of zoology, and assistant museum curator of mammalogy and ornithology, at Michigan State University (MSU), having formerly been a research associate for the eminent American ornithologist S. Dillon Ripley at the Smithsonian Institution in Washington, D.C. She is a member of the American Ornithological Society (AOS) North American Classification Committee (NACC), a scientific associate with the bird group of the British Natural History Museum zoology section at Tring, and an associate editor of The Ibis, the scientific journal of the British Ornithologists' Union.

[4] She is a scientific affiliate for the Field Museum of Natural History and the founder and editor of AVoCet, MSU's avian vocalizations center.

[6] Rasmussen is married to Michael D. Gottfried, who is Curator of Paleontology, Associate Professor of Geology, and Director of the Center for Integrative Studies in General Science at MSU.

[7] Rasmussen's early work was largely focused on studies of the systematics, ecology and behaviour of Patagonian seabirds, notably cormorants.

[17] She rediscovered the forest owlet Athene blewitti, which had not been seen since 1884, in western India,[18][19] previous searches by S. Dillon Ripley, Salim Ali and others having failed because they relied on fake documentation from Richard Meinertzhagen.

[20][21] In November 1997, Rasmussen and Ben King of the American Museum of Natural History spent ten days unsuccessfully searching two east Indian locations before driving west to the site of another old specimen, where King spotted a small, chunky owl with short, heavily white-feathered legs and huge claws, which Rasmussen confirmed as the target species whilst the owl was videotaped and photographed.

Each criterion explained less than 24% of the variation in the other factors, suggesting that even within a single taxonomic class, different mechanisms are responsible for the origin and maintenance of various aspects of diversity.

[33] Other studies by Rasmussen and her international colleagues looked at the importance of energy availability,[34][35] and a 2007 paper showed that global patterns of spatial turnover are driven principally by widespread species rather than restricted ones.

This complements other work, and helps to establish a unified model of how terrestrial biodiversity varies both within and between the Earth's major land masses.

[36] A fossil site at a borrow pit in near Cheswold, Delaware created during highway construction unearthed 11 specimens of fragmentary and unassociated avian fossils, which were identified by Rasmussen as including a small loon, a small gull-like species and five specimens of a gannet-like seabird, probably Morus loxostylus, a common species in the Miocene.

[38] In 1992, Rasmussen took the position of assistant to S. Dillon Ripley, the former secretary of the Smithsonian, who was planning to produce a definitive guide to the birds of South Asia.

He also believes that the over-reliance on sometimes very old museum specimens and dismissal of the wealth of observational data filed by amateur travelling birders is a mistake, and states that many of the taxonomic decisions appear to be random choices, unsupported by published research.

[44] Apart from the Meinertzhagen fraud, which is discussed in the next section, and the death of S. Dillon Ripley, other problems in the production of Birds of South Asia included the loss of the main map database during a trip to Burma, and poorly prepared specimen skins.

There were also difficulties reconciling sources, delays in producing illustrations and maps, and in obtaining reliable data for "difficult" areas like Assam, Arunachal Pradesh, Bangladesh and Afghanistan.

[46] Rasmussen revealed the true extent of the major fraud perpetrated by the eminent British officer, ornithologist and expert on bird lice, Colonel Richard Meinertzhagen.