Effects of supplementation with preferred foods on the reproductive axis of American Goldfinches (Spinus tristis)

Numerous field studies indicate that food supplementation of birds can advance laying date. In addition, laboratory studies, though less common, suggest that altering the amount of food, the predictability of food, or food type can affect reproductive physiology. American Goldfinches ( Spinus tristis , formerly Carduelis tristis (L., 1758)) breed in late summer when thistle (Cynareae) seeds become abundant, suggesting that specific food types may affect their reproductive physiology. We tested whether supplementation with preferred seeds would affect reproductive physiology of male and female American Goldfinches. Birds were photostimulated and fed a standard pellet diet, or supplemented with a variety of preferred seeds. Supplemented females developed larger ovarian follicles. Supplemented males had higher levels of plasma testosterone, but only if they were also housed with a female. These results suggest that preferred seed types can act as a supplementary cue in this species, and fine-tune photoperiod-driven changes in reproductive physiology.

Photorefractoriness and seasonal changes in the brain in response to changes in day length in American goldfinches (Carduelis tristis)

We examined neural and gonadal responses to photoperiod in a late-summer-breeding finch, the American goldfinch (Carduelis tristis). First we measured seasonal changes in the gonads of free-living goldfinches. Next we determined whether the gonads of goldfinches held on constant long days would eventually regress spontaneously. Finally, we compared the hypothalamic gonadotropin-releasing hormone (GnRH) system and song-control system of breeding and postbreeding birds. The results confirm that the gonads of wild goldfinches regress in late summer and show that the gonads of goldfinches held on constant long days regress spontaneously. Thus, according to at least one criterion, goldfinches become photorefractory, like other seasonally breeding songbirds. As well, goldfinches exhibited similar seasonal changes in the brain to spring-breeding birds. There was a decrease in GnRH immunoreactivity in autumn and a strong trend towards a reduction in the size of song-control regions (although this was statistically significant for only one area). Thus, although goldfinches breed late in summer as day length declines, some of their physiological responses to changes in photoperiod are similar to those that occur in spring-breeding songbirds.

Seasonal Acclimatization in the American Goldfinch Revisited: To What Extent do Metabolic Rates Vary Seasonally?

We evaluated seasonal changes in cold tolerance, basal metabolic rate (BMR), and summit metabolic rate (Msum) for American Goldfinches (Carduelis tristis) from southeastern South Dakota to determine if goldfinches differ in pattern of metabolic acclimatization from other species of small birds. Goldfinches were captured in winter (January–February), spring (April), and summer (June–August) and tested on the day of capture. Cold exposure tests involved subjecting individual birds to a decreasing series of temperatures in an atmosphere of 79% helium to 21% oxygen (helox) concurrent with open-circuit respirometry. The helox temperature eliciting hypothermia was designated the cold limit (Tcl). Whole-animal metabolic rates were analyzed. Winter goldfinches demonstrated significantly higher BMR (46%) and Msum (31%) and significantly lower Tcl (−9.5°C vs. 1.3°C) than their summer counterparts. Spring goldfinches also showed significantly higher Msum (21%) and significantly lower Tcl (−5.3°C) than summer birds. Winter birds had higher BMR (23%) and Msum (8%) than spring birds. In winter birds, Tcl was also significantly lower than in spring birds. These data support the view that prominent winter increases in Msum and BMR are components of winter acclimatization in American Goldfinches from South Dakota and that seasonal changes in metabolism in goldfinches are similar to those for other small temperate-wintering birds. La Aclimatación Estacional en Carduelis tristis Revisitada: ¿En qué Grado Varían Estacionalmente las Tasas Metabólicas? Resumen. Evaluamos los cambios estacionales en la tolerancia al frío, la tasa metabólica basal (TMB) y la tasa metabólica pico (Mpico) en individuos de Carduelis tristis del sudeste de South Dakota para determinar si esta especie difiere de otras aves pequeñas en el patrón de aclimatación metabólica. Las aves fueron capturadas en invierno (enero–febrero), primavera (abril) y verano (junio–agosto) y sometidas a exámenes el día de captura. Los exámenes de tolerancia al frío consistieron en someter a las aves a una serie decreciente de temperaturas en una atmósfera de 79% helio y 21% oxígeno (helox) al mismo tiempo que se practicaba respirometría de circuito abierto. La temperatura que provocó hipotermia se designó como el límite de tolerancia al frío (Tfr). Se analizaron tasas metabólicas de animales completos. Las aves capturadas en el invierno presentaron TMB y Mpico significativamente mayores (46% y 31%, respectivamente) y Tfr significativamente menor (−9.5°C vs. 1.3°C) que las capturadas en el verano. Las aves de primavera también presentaron Mpico significativamente mayor (21%) y Tfr significativamente menor (−5.3°C) que las aves de verano. Las aves de invierno tuvieron mayores TMB (23%) y Mpico (8%) que las de primavera. En aves de invierno, Tfr también fue significativamente menor que en las aves de primavera. Estos datos apoyan la idea de que los incrementos invernales prominentes en Mpico y TMB son componentes de la aclimatación de invierno de C. tristis de South Dakota y que los cambios estacionales del metabolismo en esta especie son similares a los de otras aves que inviernan en la zona templada.

Using a Commercially Available Radioimmunoassay to Quantify Corticosterone in Avian Plasma

Using a commercially available corticosterone I125 double-antibody radioimmunoassay, we developed and validated an assay procedure for determining corticosterone levels in small-volume (≤30 μL) avian plasma samples. We evaluated this procedure's utility by measuring plasma corticosterone levels in Indigo Buntings (Passerina cyanea), American Goldfinches (Carduelis tristis), Red-eyed Vireos (Vireo olivaceus), and Mourning Doves (Zenaida macroura). Standard biochemical validations (e.g., parallelism, recovery of exogenous corticosterone) demonstrated that the assay accurately and precisely measured corticosterone in avian plasma. We used a stress capture protocol to physiologically validate the assay's ability to determine biologically important changes in corticosterone levels. Males and females from four bird species exhibited a significant increase in plasma corticosterone in response to capture, handling, and restraint. Baseline and stress-induced corticosterone levels in our study were similar to reported values for other passerine species using other radioimmunoassay procedures. Our results suggest that this radioimmunoassay procedure is very effective for determining corticosterone levels in small-volume avian plasma samples and is sensitive enough to detect biologically important changes in the adrenocortical activity of birds. Thus, this assay has considerable utility for measuring stress levels and stress responses in small birds (<15 g), from which only small volumes of plasma (≤30 μL) can be collected.Utilización de un Radioinmunoensayo Disponible Comercialmente para la Cuantificación de Corticosterona en el Plasma de AvesResumen. Desarrollamos y validamos un proceso de ensayo para determinar los niveles de corticosterona en muestras de pequeño volúmen (≤30 μL) de plasma de aves utilizando un radioinmunoensayo para corticosterona I125 de doble anticuerpo disponible comercialmente. Evaluamos este procedimiento midiendo los niveles de corticosterona en Passerina cyanea, Carduelis tristis, Vireo olivaceus y Zenaida macroura. Validaciones bioquímicas estándares (e.g., paralelismo, recuperación de corticoesteroide exógeno) demostraron que el ensayo midió de modo exacto y preciso la corticosterona en el plasma de las aves. Utilizamos un protocolo de captura que producía estrés para validar fisiológicamente la habilidad del ensayo de detectar cambios biológicamente importantes en los niveles de corticosterona. Hembras y machos de las cuatro especies de aves mostraron un incremento significativo en los niveles de corticosterona en el plasma en respuesta a la captura, manipulación y retención. Los niveles basales e inducidos por el estrés de nuestro estudio fueron similares a valores reportados para otras especies paserinas que utilizaron otros procedimientos de inmunoensayo. Nuestros resultados sugieren que este procedimiento de radioinmunoensayo es muy efectivo para determinar los niveles de corticosterona en muestras de pequeño volúmen de plasma de aves y que es suficientemente sensible como para detectar cambios biológicamente importantes en la actividad adenocortical de las aves. De esta manera, este ensayo presenta considerable utilidad para medir los niveles y respuesta al estrés en aves pequeñas (<15 g) de las cuales sólo es posible colectar pequeños volúmenes de plasma (≤30 μL).