Bergmann's rule is followed at multiple stages of postembryonic development in a long-distance migratory songbird

Ecological factors drive the divergence of morphological, colour and behavioural traits in cactus wrens (Aves, Troglodytidae)

The study of ecological mechanisms influencing organisms' phenotypic variation is a central subject of evolutionary biology. In this study, we characterized morphological, plumage colour and acoustic variation in cactus wrens Campylorhynchus brunneicapillus throughout its distribution. We assessed whether Gloger's, Allen's and Bergmann's ecogeographical rules, and the acoustic adaptation hypothesis relate to geographical trait variation. We analysed specimen coloration in belly and crown plumage, beak shape and structural song characteristics. We tested whether the subspecific classification or the peninsular/mainland groups mirrored the geographical variation in phenotypes and whether ecological factors were associated with patterns of trait variation. Our results suggest that colour, beak shape and acoustic traits varied across the range, in agreement with two lineages described by genetics. The simple versions of Gloger's and Allen's rules are related to variations in colour traits and morphology. Conversely, patterns of phenotypic variation did not support Bergmann's rule. The acoustic adaptation hypothesis supported song divergence for frequency-related traits. Phenotypic variation supports the hypothesis of two taxa: C. affinis in the Baja California peninsula and C. brunneicapillus in the mainland. The ecological factors are associated with phenotypic trait adaptations, suggesting that divergence between lineages could result from ecological divergence.

Parental favoritism in a wild bird population

In most taxa with altricial young, offspring solicit food from their parents using a combination of visual and acoustic stimuli, but exactly what these young are communicating, and how selection shapes parental responses, remains unresolved. Theory posits that parents' interpretation and response to begging should vary with the likelihood of a return on their investment. We tested this in a wild population of prothonotary warblers (Protonotaria citrea), predicting that parents bias food non-randomly toward certain individuals within their broods depending on both the size and number of offspring. We observed parent-offspring interactions and detected strong dependence between brood size and nestling size in shaping parental responses to begging. Larger siblings were less likely to solicit food during feeding events than their smaller siblings, but they received a disproportionate share from parents in nests containing fewer-than-average young, whereas the smaller-than-average nestlings were disproportionately fed in broods containing a greater-than-average number of young. These findings suggest that parents respond to begging signals according to multiple social cues, favoring the stronger siblings with greater survival prospects when few copies of their genes are present, but overtly favoring runts to ensure whole-brood survival when capable of fledging more young. Future experimental studies may shed light on the contributions of parental decision-making and memory, how young nestlings learn in parent-offspring communication systems, and the adaptive significance of these behaviors.