Northern Cardinal Cardinalis cardinalis
Version: 2.0 — Published February 12, 2021
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Priorities for Future Research
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Priorities for Future Research
A revision of systematics of this species is greatly needed, with reconsideration and re-description of subspecies. Subspecies descriptions based on color should account for variation among individuals and fading or other changes in color that occur over the course of the year. Subspecies boundaries need to be mapped and interactions between subspecies should be studied where ranges come into contact. Behavioral differences between subspecies should also be investigated (for example, which subspecies flock during nonbreeding periods? What is the nature of song differences between subspecies, and do songs differ more between subspecies, than among individuals of the same subspecies?
In studies of cardinal ornamentation, colors should be described using a standardized modern system of color notation, and efforts to convert Munsell and Methuen color scores to usable avian color space data are needed for all species. Sensitivities of cone populations (see Other: Physiology: Vision) may be species-specific and in at least one case even subspecies-specific (403). Future work should assess a broader number of populations using easily comparable methodologies for plumage color measures and measures of quality and standardized conditions under which color data are collected. When composite measures of color are used, it would be useful to also separately report the standard color space measures of hue, saturation, and brightness (as a measure of gray scale value/luminance). This will help to determine whether these measures vary between populations.
In studies of cardinals engaged in prolonged territorial disputes endocrine mediators of aggressive behavior in different life history stages and different contexts (e.g., intrasexual versus intersexual conflict) should also be examined. Aggression during the nonbreeding period needs to be more strongly addressed in multiple populations to determine the extent of this behavior and underlying physiological factors influencing it. Such studies may also help address questions about why both sexes of cardinals have substantial levels of testosterone outside of the breeding period (31). Finally, studies of hormone receptor distribution and seasonal variation in expression or activation of hormone receptors are needed to determine how prolonged testosterone production does not interfere with some aspects of steroid-sensitive behaviors.
Research examining stress hormones and stress response in cardinals has yielded conflicting results. Studies comparing multiple populations throughout the species' range could help to determine whether stress responses reflect local selective pressures. Further, investigations examining relationships between stress hormones and aspects of immunity could provide more information about how cardinals allocate energy resources between energetically costly, and often simultaneously occurring, physiological processes.
Extra-pair copulations also need more study in cardinals: Does mate-guarding vary with plumage color of the guarding or guarded individual? With whom do females copulate, and how does the coloration and “quality” of these males compare to that of their social mates? Are extra-pair copulations timed to lead to maximal probability of fertilization? Do females engage in compensatory copulations with their social mate to avoid suspicion, and how are copulations with the social mate timed? What are the conditions that lead to females laying eggs in the nests of other females? Do these females also have nests of their own, with the same or a different male? Males captured during the breeding period should be checked for bare, edematous areas on the belly, to determine whether males are capable of assisting in incubation.
Fledgling survival rates need to be determined, as do the movements of fledged young after they leave the territories of their parents.
There is much to be learned about cardinal vocalizations. Sound spectrograms have not yet been published for some calls, and meaningful variation may exist within the category of chip calls. Do details of song repertoire use modify the messages being conveyed? For example, what is the significance of variations in successive renditions of the same song type (e.g., variation in total song length, and in numbers of repetitions of the component syllable types)? Do low-amplitude song elements serve a particular communicative function?
Cardinals are promising subjects for studies of a bird's ability to sing songs that appear to be physically challenging to produce. Are such songs used selectively in contexts in which a display of physical fitness would be adaptive? To what extent do individuals vary in their ability to produce physically challenging songs? Is the ability to produce physically challenging songs favored by sexual selection? Do the abilities to produce different kinds of physically challenging sounds covary (e.g., does the ability to produce long chirr syllables correlate with the ability to produce smooth transitions between frequencies produced by the left and right sides of the syrinx)? Is syllable repetition rate within a song limited by the frequency bandwidth of the repeated syllables? When syllable repetition rate increases as a song progresses, does syllable frequency bandwidth narrow, or is filtering of harmonic frequencies less effective?
Do responses of female cardinals to song playbacks reflect their mate preferences? Do female cardinals prefer mates whose song repertoires they share, and/or discriminate against males from different geographic areas? Do song repertoires change over decades in the same continuously-occupied location, when human land use has remained constant?
Despite the apparent potential of cardinal song to display an individual's fitness, some predicted correlations have not been found. Males with shorter and simpler songs had territories of higher quality and fledged more offspring in a rural east Texas population (50); males with shorter and slower songs were larger and lived in territories with greater shrub density, but had no greater reproductive success, in rural Ohio, while song was not correlated with male size or territory quality of urban cardinals in Ohio (20). Suggested explanations remain to be tested, as does the generality of these correlations.
Cardinals may respond more strongly to very local dialects than to songs from more distant locations (250, 212, 107; JMJ, MSD, and R. Anderson, personal communication), but current data are inadequate to evaluate this claim. Because cardinals sing differently in different contexts, it is challenging to construct and to interpret playback experiments, and significant care should be taken in these experiments to present song sequences typical of those sung in the context to be tested. To test responses to local vs. distant songs, songs should be recorded from local birds with which the focal bird has not interacted regularly, rather than from neighbors or the focal bird itself. In locations with denser cardinal populations, it is more likely that multiple cardinals will respond to playbacks, and that playbacks at randomly chosen locations will fall within a cardinal territory, increasing response levels over those in less dense populations. The strongest comparisons can be made with reciprocal tests using the same playback recordings. When multiple response measures are combined, it is difficult to interpret the biological significance of tests in which responses to playbacks vary qualitatively, e.g., with closer approach to the playback speaker in one population, but more song in another population. If responses do not differ significantly, they cannot be assumed to be the same without power analyses to quantify the probability of detecting differences; high variability in responses of small sample sizes of tested individuals can obscure actual differences.
Last, how are changes in climate affecting cardinal distribution and the rates of northward and westward range expansion? How is urbanization affecting movements, reproductive success, singing behavior, and plumage coloration? Cardinals adapt readily to urban environments, but more research is needed to better understand how individuals might change once established in urban environments, and/or if urban environments select for certain behavioral, physical, or physiological phenotypes. Which behavioral or physiological attributes allow the species to adapt so readily to urban environments?