Northern Cardinal Cardinalis cardinalis
Version: 2.0 — Published February 12, 2021
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Recent studies of Northern Cardinal have examined some physiological and behavioral impacts of the following hormones: testosterone (T), corticosterone (CORT), dehydroepiandrosterone (DHEA), thyroxine (T4) and triiodothyronine (T3). This species exhibits less sexually dimorphic behavior (e.g., both sexes sing, defend territories, and provide parental care) than do many other songbirds, making it an interesting species for studies of hormone-behavior relationships.
Testosterone. Testosterone (T) is a steroid hormone produced by both sexes (see Jawor  for Northern Cardinal annual profiles), whose production is regulated by the hypothalamic-pituitary-gonadal axis. Production is initiated via secretion of hypothalamic gonadotropin-releasing hormone (GnRH), which stimulates the anterior pituitary to produce luteinizing hormone which then induces the gonads to produce and secrete T. Testosterone circulates at measurable levels in both sexes year-round (31).
In non-captive cardinals, male T increases only slightly during the early breeding season and females have little fluctuation in levels through the year (31). This corresponds with data from captive cardinals, in which an increase in T and luteinizing hormone occurred in males, but not females, in their first spring (21). Both sexes in a Mississippi population are capable of significantly increasing T in response to standardized GnRH injections (bioassays that assess reproductive axis activity) during the months leading up to breeding (January to March; 32); however, the ability to increase T in response to GnRH injection was significantly dampened in both sexes when they were caring for offspring (30). Some individuals can begin to significantly elevate T following GnRH injections as early as the month of December (32), but the environmental cues responsible for enabling this response during the early winter are unknown (e.g. temperature alone was not a primary cue; 389). Future research should address whether maintaining relatively constant T levels year-round is important for nearly year-round song production in this species, and if it has negative physiological consequences, such as dampened immune function (390).
Studies from a Mississippi population have demonstrated that significant elevations in T do not appear necessary to support territorial aggression in males (33) or nest defense in incubating females (34). There also appears to be no correlation (either positive or negative) between naturally circulating levels of T and level of parental care provided by either sex (assessed via nestling feeding rates; 30).
Corticosterone. Corticosterone (CORT) is produced by the adrenal glands and is regulated by the hypothalamic-pituitary-adrenal axis; the hypothalamus produces corticotropin-releasing hormone, which stimulates the release of adrenocorticotropic hormone (ACTH) from the anterior pituitary which in turn induces the production and release of CORT from the adrenals. Corticosterone production is often elevated in birds following threatening or stressful situations to mobilize energy stores (e.g., glucose) to help individuals cope with stressful encounters. Prolonged (chronic) CORT elevation can lead to a re-directing of energy from physiological processes considered non-essential for immediate survival (e.g. reproduction, immune function). In birds, CORT levels are often elevated during the breeding season, although not to physiologically damaging levels, as individuals must cope with the stress of reproduction. However, definitive conclusions concerning links between CORT and avian physiology and behavior have yet to be drawn, as studies examining these relationships have yielded contradictory findings.
Males and females in a Mississippi population had similar annual profiles of circulating CORT (37). Further, cardinals had significant elevations in CORT following handling restraint (hereafter, restraint stress CORT) (males and females analyzed separately in a Mississippi study ; data combined from males and females in a Connecticut study ; males only in a Florida study [392, 35]). Elevations of CORT can also be induced in males via intraperitoneal injections of exogenous ACTH (35). Variation appears to exist in levels of initial circulating CORT and restraint stress CORT across life history stages (breeding and non-breeding) or seasons experienced in a single year (spring, summer, fall) (35, 391, 37). However, overall patterns of variation in initial CORT and restraint stress CORT levels differ in populations examined thus far. In a Connecticut population (sexes combined), levels of initial CORT and restraint stress CORT were highest in the spring (nest initiation and egg laying) and lowest in the summer (chick-rearing) (391). Fokidis (35) reported similar findings in males of a Florida population, with initial and restraint stress CORT levels gradually increasing from late winter (non-breeding) to a peak in early summer (breeding), then abruptly declining in late summer–fall (July–October). Conversely, Duckworth and Jawor (37) reported the opposite in a Mississippi population, with levels of initial CORT (males) and restraint stress CORT (males and females) being highest during the non-breeding season (September–February) as opposed to the breeding months (May–July).
Relationships between social context, environmental conditions, and CORT levels in cardinals are beginning to be elucidated. Higher levels of CORT were not correlated with aggressive behavior in males during simulated territorial intrusions (33, 36); however, males that witnessed their mates being captured had higher initial and restraint stress CORT than males that were captured before their mates (36). Male cardinals living in urban habitats had lower circulating CORT and restraint stress CORT than rural birds (36), but no differences in CORT measures were found between males residing in high versus low activity areas on a military base (with activity defined as soldiers in the area training on foot, in wheeled and tracked vehicles, or at occupied encampments) (392). Levels of CORT in territorial males also did not change in response to food supplementation, indicating that lower CORT in urban birds might not be explained by bird feeders providing additional food provisions (36).
Dehydroepiandrosterone (DHEA). In addition to CORT, the adrenal gland also produces the relatively weak androgenic hormone, DHEA. Little is known about its biological role and it presumably impacts physiology and/or behavior through conversion into other androgens (e.g., testosterone) and estrogens (e.g., estradiol). In cardinals, levels of DHEA in males vary throughout the year and are generally higher in fall and winter before declining during summer (35). Unlike CORT, DHEA levels appear to generally decrease following restraint (35, 36). Restraint stress DHEA concentrations are lower than initial circulating levels during most months of the year, but a significant increase following restraint has been documented during the month of July (35). Intraperitoneal injection of ACTH failed to increase DHEA levels in males, indicating that ACTH is not a primary regulator for DHEA secretion (35).
Initial measures of relationships between social context and DHEA indicate that hearing conspecific song playback does not impact initial levels of DHEA of male territory owners, but decreases in DHEA typically seen following restraint stress were negated if males had been exposed to 1 hour of song playback (36). As with CORT, males that witnessed their mates being captured had greater initial and restraint stress DHEA than did males that were captured before their mates (36). Initial and restraint stress DHEA of male territory owners were not affected by food supplementation and, unlike CORT, did not vary between urban and rural habitats (36). Findings imply that DHEA and CORT have different regulators at the physiological, environmental, and even social levels.
Thyroxine (T4) and Triiodothyronine (T3). Thyroid hormones, T4 and T3, are produced by the thyroid gland and have a large role in regulating metabolic processes. Male cardinals have higher levels of thyroid hormones (TH) in the late afternoon than early morning (393). Plasma TH varies with latitude, T4 was higher in birds in higher latitudes (Iowa, Michigan) and in lower latitudes (Alabama, Louisiana) than in intermediate locations (Indiana, Missouri) (393). For TH influences on metabolic parameters, see Metabolism and Temperature Regulation.
Resemble those of 7 other oscines, with a tripartite undulating membrane, which is unique to oscines (394).
Three of 33 hindlimb muscles varied among individuals in a series of cardinals (n = 23) from western Pennsylvania, eastern Ohio, and western Maryland. Both unilateral and bilateral variants observed. Variations not correlated with sex; no tendency for multiple variants to occur in the same individual. This study suggests that muscles vary little among individuals (395).
Trachea consists of 52 rings, with “about 12” bronchial half rings (Audubon 1840–1844). Lungs and air sacs (396): Lungs have 4 ventro-bronchi and 6 dorsobronchi, and vary among individuals in number of laterobronchi. Single cervical air sac and paired posterior thoracic and abdominal air sacs. Interclavicular and anterior thoracic air sacs are fused into a single unpaired sac.
Diploid number of “approximately 84,” consisting of series of graded sizes of macro-and microchromosomes with polymorphism of one chromosome pair among individuals collected from same site (397). Three sets of CHD1 primers are reliable for sex determination (398).
Cone sensitivity peaks at wavelengths of 370, 450, 480, and 570 nm were detected in physiological experiments on isolated eyecups (399); behavioral tests also reveal near-UV sensitivity (down to at least 350 nm; 400). From Fernandez-Juricic et al. (unpublished data): Average visual coverage around the head at the elevation of the bill is 333°, while average blind area at the rear of the head is 27°. Average binocular field at the elevation of the bill is 33° and average degree of eye movement per eye is 36°. Visual acuity is 9.92 cycles per degree and average eye axial length is 8.06 mm. Pecten (which generates a blind spot) occupies 5% of the visual area above the horizontal plane. There is one fovea per eye where vision is sharpest.
Some cardinals can remove plastic and even aluminum leg-bands (see 401), and leg-bands can be hard to see on cardinals. Successful marking techniques include: (1) use of spiral plastic bands wrapped twice around the leg and sealed with acetone (SLH; many lasted for years); (2) stainless steel bands (JMJ, MSD); (3) colored plastic "streamers" (20-mm long) attached to colored leg-bands (401); (4) black lines drawn on breast with a commercial felt marking pen (191; faded after 4–6 weeks); (5) colored tape sewn to proximal portions of adjacent rectrices (402; technique modified from 401; lasted until next rectrix molt); and (6) a double set of butt-end plastic leg bands (not spiral), the inner band with an internal diameter appropriate for cardinal legs, and the outer band with an internal diameter that allows it to slide and fit closely over the inner band; the inner band is placed on the leg, a bead of acetone, super glue, or modeling cement is placed on its outer surface, and then the outer band slides over the inner band; neither band's seam is sealed, but the seams are offset (Band size XCS over 1A; JMJ, DPS, rarely removed). During handling it is useful to allow cardinals to bite a soft plastic eyedropper, a cotton swab, raisins, or small twigs.