Andreas Jonsson's Weblog

Mourning Warbler (top) and MacGillivray's Warbler (bottom). Male specimens collected in Ontario in June 1936 and in British Columbia in June 1931, respectively. Royal Ontario Museum Ornithological Collection.

Last fall I wrote about a side by side comparison of Mourning Warblers and Connecticut Warblers. Today I’ll compare the Mourning Warbler with another species of the same genus (Oporonis), the MacGillvray’s Warbler. The MacGillvray’s Warbler is in fact the Mourning Warbler’s closest relative. The two forms originally used to be considered as a single species. Now they are believed to form an east-west species pair complex, with MacGillvray’s in the west and Mourning in the east, and with minimal overlap of the breeding ranges.

The Sibley “Guide to Birds” states that the MacGillvray’s “averages slightly longer-tailed and rounded-headed than Mourning”. It is “smaller and shorter-winged” and has “white eye-arcs in all plumages”. For the two male specimens I examine here, from the Royal Ontario Museum (ROM), the MacGillvray’s is indeed longer-tailed than on the Mourning, however, the most striking distinguishing character is the extent and structure of the black on the throat and chest: note that on the MacGillvrays’s this area is uniformly dark slaty gray while on the Mourning the area is gradually darker toward the lower chest.

This character is not mentioned either in Sibley or in the National Geographic “Field Guide to Birds of North America”, although the illustrations in both books suggest that it could be a diagnostic character. The Peterson “A Field Guide to Warblers of North America” confirms that the character is useful for identification of the two species. It is somewhat surprising that you have to go the specialist literature for such an obvious character. The Peterson guide also brought my attention to another important distinction between the two species, which is readily visible in the title image: spring adult male MacGillvray’s has dark lores while the lores on spring adult male Mourning generally has the same shade as the rest of the face.

Same two specimens as shown above: Mourning Warbler (left) and MacGillivray's Warbler (right). Royal Ontario Museum Ornithological Collection.