High Quality Content by WIKIPEDIA articles! Four closely related North American bird forms?the eastern Myrtle Warbler (ssp coronata), its western counterpart, Audubon's Warbler (ssp group auduboni), the Northwest Mexican Black-fronted Warbler (ssp nigrifrons), and the Guatemalan Goldman's Warbler (ssp goldmani)?are periodically lumped as the Yellow-rumped Warbler (Dendroica coronata). Since 1973, American Ornithological Union has elected to merge these passerine birds as one species. The Myrtle and Audubon's forms were apparently separated by glaciation during the last ice age, and developed distinguishing physical characteristics.