Fifty shades of brown: Macroevolution of plumage brightness in the Furnariida, a large clade of drab Neotropical passerines

Abstract

Both natural and sexual selection are thought to affect the evolution of bird color. Most studies of the topic have focused on sexually dichromatic taxa and showy plumages, which are expected to be more influenced by social selection and usually result in increased conspicuousness. However, many bird clades display dull brown or gray plumages that vary greatly in brightness (lightness), but little in hue (shade). Here, we examine the macroevolution of brightness in one such clade, the Furnariida. We make comparisons across light environments, body parts, monochromatic lineages, and each sex of dichromatic lineages. We found that support for models including light environments is greater for the dorsum than for the venter, and that brightness evolution is more constrained in the former than in the latter. Plumages in this clade have evolved to be darker in darker habitats, consistent with natural selection for increased crypsis. Finally, the features of brightness macroevolution are broadly similar across the sexes of the dichromatic clade, challenging the view that sexual dichromatism is driven by different evolutionary processes acting in each sex. We conclude that, in the Furnariida, light environments and dorsal–ventral variation are more important than sex as axes of color evolution.

Climate, crypsis and Gloger’s rule in a large family of tropical passerine birds (Furnariidae)

“…In contrast, if Gloger’s rule is mainly a result of birds adapting to be darker in darker (forest) habitats, bird species occupying forest habitats are predicted to be darker than their nonforest-based relatives, even if they inhabit similar climatic regimes. Marcondes and Brumfield (2019) previously demonstrated that furnariid species have evolved to be darker in darker habitats, consistent with Endler’s (1993) predictions for crypsis. Here, we sought to investigate the interacting roles of climate and habitat type in driving interspecific variation in plumage brightness in the Furnariidae.…”

“…We used the color dataset previously described in Marcondes and Brumfield (2019) and deposited on the Dryad digital repository under DOI 10.5061/dryad.s86434s (embargoed until July 16, 2021). Briefly, this dataset includes reflectance data for 250 (84%) furnariid species, with an average of 6.4 specimens per species (range: 1-8).…”

“…We calculated plumage brightness (percentage of reflected white light) and averaged it across all specimens of each species. Because the Furnariidae are sexually monochromatic with no evidence of cryptic sexual dichromatism (Remsen 2003; Tobias et al 2012; Diniz et al 2016; Marcondes and Brumfield 2019), we considered the sexes together in our analyses. Finally, we used principal component analysis (PCA) to reduce our dataset to one principal component for the venter and one for the dorsum.…”

“…Finally, we used principal component analysis (PCA) to reduce our dataset to one principal component for the venter and one for the dorsum. The first principal components of dorsal and ventral PCAs were both loaded in the same direction by brightness in all plumage patches, thus representing overall brightness of that body surface; subsequent principal components captured various aspects of contrasts between plumage patches within each body surface (Marcondes and Brumfield 2019). Our final color dataset thus consisted of the first principal component score of brightness for each body surface (hereafter, simply “dorsal brightness” and “ventral brightness”) for each species.…”

“…Endler (1993) predicted that, to enhance crypsis, animals inhabiting dark light environments ( e.g . the interior of dense forests) should be darker than those inhabiting open areas with bright light conditions ( e.g ., savannas), a prediction that has received wide support from comparative studies on birds (McNaught and Owens 2002, Gomez and Thery 2004, Dunn et al 2015, Maia et al 2016, Shultz and Burns 2017, Marcondes and Brumfield 2019). Because forests, particularly tropical rainforests, are more prevalent in rainier climates, this raises the possibility that the tendency for birds to be darker in more humid places (Gloger’s rule) is confounded by a need for crypsis in darker environments.…”