(click on image to enlarge) Read here. Full map source here. Approximately 700 to 1,000 years before current times, bird populations swelled on an island off the coast of Greenland. Using a 3.5 meter sediment core from a lake, researchers determined from bird "leavings" that natural warming attracted more birds.
"Key to the study were biogeochemical data that, in the words of the authors, reflect "variations in seabird breeding colonies in the catchment which influence nutrient and cadmium supply to the lake." This work revealed sharp increases in the values of the parameters they measured between about 1100 and 700 years before present, indicative of the summer presence of significant numbers of seabirds during what they described as a "medieval warm period," which had been preceded by a several-hundred-year period of little to no (inferred) bird presence. Thereafter, their data pointed to another absence of birds during "a subsequent Little Ice Age," which they say was "the coldest period since the early Holocene in East Greenland.""