Streak-backed Orioles
Icterus pustulatus

As an NSF postdoctoral fellow at Queen’s University, I am investigating adaptive explanations for the maintenance and loss of elaborate female characters in orioles.

There is a striking geographic pattern in oriole coloration: most tropical groups are sexually monochromatic elaborate (both sexes elaborate), and all temperate groups are sexually dichromatic (only males elaborate). The same relationship between breeding latitude and dimorphism is found in a many avian taxa, including warblers and blackbirds, yet the evolutionary reasons behind the pattern remain obscure.

There is strong support for the hypothesis that monochromatism represents the ancestral condition in the orioles genus (Fig. 2). Thus, sexually dichromatic taxa are likely to have evolved from monochromatic elaborate ancestors due to the loss of female coloration (Fig 1b).


Our study focuses on the Streak-backed Oriole (Icterus pustulatus) (circled in fig. 2), which is ideally suited for studying the causes underlying the evolutionary maintenance and loss of female plumage characters because two of its subspecies vary geographically in their level of dimorphism.

The subspecies in the south is sexually monochromatic, whereas the subspecies in the north is strongly sexually dichromatic.

The most notable ecological/behavioral difference between the two subspecies is that the monochromatic southern subspecies is resident throughout the year, whereas the dichromatic northern subspecies is migratory.

Consequentially, we suspect that there is a significant difference in territorial behavior of the two subspecies. Males and females of the southern subspecies are likely to defend a year-round territory, whereas only males of the dichromatic northern subspecies are likely to defend a territory during the short breeding season.

We hypothesize that differences in migration and territorial behavior between the two subspecies have selected for different strategies in status signaling among females, and that this difference is a driving force behind changes in female coloration.

I am currently working with the southern subspecies in the Mexican state of Morelos. The habitat in the Sierra de Huautla is deciduous thorn-scrub, and the forests have been heavily grazed by cattle and donkeys (shown!). Streak-backed Orioles favor these disturbed forests, and this preference makes the species easy to study because netting and behavioral observations are far easier in these open habitats (Seven Streak-backed Orioles were captured at the same time in one net!).

Figure 2. Ancestral state reconstruction of female coloration and sexual dichromatism across all orioles. * denotes each migratory species. Note the repeated independent losses of elaborate female coloration: seven of the eight dichromatic orioles are migratory, and none of the migratory orioles are monochromatic (concentrated changes test, p < 0.01; K. Omland unpublished).

Figure 1. Possible historical pathways to the gain of sexual dichromatism. Male and female characters are shown then reconstructed using parsimony on a hypothetical four species phylogeny. +D indicates where dichromatism is gained. Grey represents a dull or cryptic character, and black represents an elaborate character. Path (A) shows the pathway often implicitly assumed in studies of sexual selection. However, loss of female characters can also lead to the gain of dichromatism (B).


© 2006 Troy Murphy