Corticosterone and the Stress Response in Young Western Screech-Owls: Effects of Captivity, Gender, and Activity Period

We used a standard handling protocol to examine the stress response of captive young western screech-owls during their active (nighttime) and inactive (daytime) periods and to compare the stress responses of captive and free-living owls. Circulating corticosterone levels were significantly higher during the inactive period than in the active period in this nocturnal species. This suggests that the daily pattern of corticosterone secretion is reversed in nocturnal birds and is correlated with activity period rather than with the light/dark cycle. Young (ca. 4-5 mo old) screech-owls of both sexes showed increases in plasma corticosterone up to 30 min after capture, followed by significant decreases at 60 min. This pattern is similar to those of other species of birds examined previously, except that decreases in corticosterone at 60 min rarely have been observed. Such decreases may be the result of physiological differences between adult and young birds, habituation to handling in captive birds, or the effects of body condition. Corticosterone levels and the response to capture and handling were comparable in captive and free-living owls, which suggests that the captive owls were not subjected to chronically high levels of stress

Body Condition and the Adrenal Stress Response in Captive American Kestrel Juveniles

We examined the adrenal response to handling stress of birds in different body conditions. In order to affect the birds’ body condition, young (73-d old) female American kestrels (Falco sparverius) were maintained for 6 wk on one of three diets: a control diet (fed ad lib.) and two calorically restricted diets. To invoke a stress response, we removed birds from their cages and took repeated blood samples over the course of an hour. All birds responded to handling stress with an increase in plasma corticosterone, but control birds (in good body condition) showed a more rapid increase to maximum corticoste rone levels, followed by a decrease. Both groups of food-restricted birds had a slower rate of increase to maximum corticosterone levels and then maintained high corticosterone levels through 60 min. These results suggest that birds in good physical condition respond more quickly to stressors and adapt physiologically to stressful situations more rapidly than do birds in poor physical condition. This difference may reflect the ability of birds in good condition to mobilize fat for energy, while birds in poor condition must mobilize protein (i.e., muscle)

Sex Determination of Red-Tailed Hawks (\u3cem\u3eButeo jamaicensis calurus\u3c/em\u3e) Using DNA Analysis and Morphometrics

Currently the sex of Red-tailed hawks (Buteo jamaicensis) cannot be determined by in-hand methods. Males and females do not differ in plumage and overlap in size. During migration, we collected feather samples and morphological measurements from birds at four sites in the Western United States. Sex was determined for individual birds using sex-specific DNA markers and Polymerase Chain Reaction was used to identify these DNA markers. Through Discriminant Function Analysis, we created equations for determining the sex of Red-tailed hawks using in-hand measurements based on the DNA-determined sexes. We formed two equations, one for adults, which was 98% accurate, and one for hatch-year birds, which was 97% accurate. Our results will aid future studies looking at intra- and intersexual differences in the Western Red-tailed hawk

Plumage Variation, Plasma Steroids and Social Dominance in Male House Finches

In male House Finches (Carpodacusm exicanus), the extent and color of plumage varies depending on access to carotenoid pigments. Colorful males exhibit extensive red pigmentation, while less colorful (i.e., drab ) males exhibit carotenoid pigmentation over a smaller percentage of their plumage, pigmentation of a color besides red (e.g., yellow, gold, orange, or pink), or both. One explanation for maintenance of plumage variation is that it reliably reflects social status, allowing males to correctly assess their status in relation to others and avoid or minimize costly fights. Social relationships may also be related to endogenous factors, such as circulating levels of the hormones testosterone and corticosterone. High levels of testosterone may promote or facilitate increased aggression, and stress associated with receiving aggression from individuals of higher status may increase adrenal activity and secretion of corticosterone. We examined the relationship between plumage variability, steroid hormones, and social status in captive male House Finches during the non-breeding period in: (1) groups of males in which individuals varied by age, size, and plumage, and (2) pairs (dyads) of males matched for several measurable parameters except plumage. Testosterone and social status were not related in males competing in either groups or dyads, and levels of testosterone were routinely low. Corticosterone and status were not related in groups but, in dyads where subordinate individuals had little chance of escaping aggression from more dominant birds, subordinates exhibited significantly greater levels of corticosterone. Although drab males tended to achieve higher status than colorful males in both experiments, which is consistent with previous studies on free-living individuals, we could not reject the null hypothesis that plumage and status were unrelated. We conclude that dominance relationships among male House Finches during the non-breeding season may not be related to testosterone, but they are reflected by levels of corticosterone in some circumstances. Additionally, colorful plumage appears to be a poor predictor of high social status among male House Finches during the non-breeding season

Intrafeather and Intraindividual Variation in the Stable-Hydrogen Isotope (δD) Content of Raptor Feathers

Stable-hydrogen isotope ratios (deuterium:protium; δD) in feathers enable researchers to evaluate patterns of avian movement and to estimate the source areas of migratory birds. However, variation in feather δD remains inadequately described, thus confounding inferences of avian movement and origin. We assessed variation within a feather and among feathers within and between tracts in three species of immature raptors. Within contour feathers, measurements of δD increased from a distal section to an adjacent, proximal section; the magnitude of δD increase varied with raptor species. Furthermore, contour and flight feathers differed systematically in their δD content. Two explanations for intrafeather and intraindividual variation warrant further investigation: (1) hydrogen isotope fractionation associated with feather growth rate, and (2) the incorporation of temporal variation in environmental δD into growing feathers. We consider these explanations for raptors and passerines, which seemingly differ in the incorporation of deuterium into feathers. Additionally, corresponding sections of multiple contour feathers exhibited better measurement repeatability than multiple sections within a contour feather; the variability of multiple δD measurements within a feather tract (geometric SD: ±3.5%) suggests that biological effects on the repeatability of δD measurements from concurrently grown feather material are difficult to distinguish from analytical effects. In most cases, intrafeather and intraindividual variation can be minimized by informed sample selection decisions, but both sources of variation must be considered when stable-hydrogen isotopes are used to infer the geographic origins of migrants, ascertain migratory connectivity, and facilitate avian conservation decisions

Effects of Small Increases in Corticosterone Levels on Morphology, Immune Function, and Feather Development

Stressors encountered during avian development may affect an individual’s phenotype, including immunocompetence, growth, and feather quality. We examined effects of simulated chronic low-level stress on American kestrel (Falco sparverius) nestlings. Continuous release of corticosterone, a hormone involved in the stress response, can model chronic stress in birds. We implanted 13-d-old males with either corticosterone-filled implants or shams and measured their growth, immune function, and feather coloration.We found no significant differences between groups at the end of the weeklong exposure period in morphometrics (mass, tarsus, wing length, and asymmetry), immunocompetence (cutaneous immunity, heterophil/lymphocyte ratio, and humoral immunity), or feather coloration. One week subsequent to implant removal, however, differences were detected. Sham-implanted birds had significantly longer wings and a reduced level of cutaneous immune function compared with those of birds given corticosterone-filled implants. Therefore, increases of only 2 ng/mL in basal corticosterone titer can have small but measurable effects on subsequent avian development

Rejection of Foreign Eggs by Yellow-Headed Blackbirds

Egg rejection behavior is more likely to develop in avian populations that experience high levels of hetero-specific brood parasitism than in populations where parasitism exerts little selection pressure to recognize and respond to foreign eggs (Davies and Brooke 1989, Brown et al. 1990, Soler and Moller 1990 [but see Zufiiga and Redondo 1992], Briskie et al. 1992). Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) are not preferred hosts of the brood-parasitic Brown-headed Cowbird (Molothrus ater; hereafter cowbird) and rarely are parasitized (Willson 1966, Ortega and Cruz 1988, Neudorf and Sealy 1994 [Table 1], L. Beletsky and G. Orians, pers. comm.; but see Lincoln 1920). Consequently, low rates of heterospecific brood parasitism exert little selection pressure on Yellow-headed Blackbirds to recognize parasitic eggs, and individuals from populations with low parasitism rates do not reject nonmimetic eggs placed in their nests (Ortega and Cruz 1988). However, if a population of Yellow-head- ed Blackbirds were to experience significant amounts of brood parasitism, then one might expect to see evidence of egg rejection. I investigated this hypothesis in a population of this species that is heavily parasitized (1991: 7/33 nests [21.2%]; 1992: 5/29 nests [17.2%]) by cowbirds (Dufty, in prep.). I noted responses to the presence of foreign eggs by replacing single eggs in Yellow-headed Blackbird nests with an egg of the Red-winged Blackbird (Agelaius phoeniceus)

Vocalizations and Copulatory Attempts in Free-Living Brown-Headed Cowbirds

Audiospectrograms of vocalizations recorded during 12 copulatory attempts by male Brown-headed Cowbirds (Molothrus ater artemesiae) are presented. All vocal sequences during copulatory attempts included songs, and all but one included flight whistles. In addition, a vocalization not previously described in cowbird copulatory sequences, the Kek Note, was recorded in four instances. Analysis of flight whistles indicated that they did not differ from flight whistles given in other contexts. The functional aspects of producing multiple types of vocalizations during the copulatory attempts are discussed. For example, incorporation of flight whistles during copulatory attempts may reflect conflicting tendencies to approach and to fly off during these times of heightened sexual arousal. Alternatively, flight whistles may have a direct stimulatory effect on the female\u27s willingness to mate

Vocal and Behavioral Responses of Brown-Headed Cowbirds to Flight Whistles from Different Dialects

We examined the vocal and behavioral responses of free-living male Brownheaded Cowbirds (Molothrus ater) to playbacks of flight whistles (FWs) from local, nearforeign, and distant-foreign dialects. Full, partial, and reverse FWs were broadcast to solitary males. Test males responded with their own FWs and approached playbacks of FWs significantly more than playbacks of control heterospecific vocalizations. This suggests that all three dialects were recognized as conspecific. The strongest responses were elicited by playbacks of local FWs, and there was little behavioral evidence that males distinguished between the near-foreign and distant-foreign dialects. Males responded to playbacks of partial or complete local FWs primarily with the next or missing part of the FW. That is, they avoided matching the playback. The FW responses to playbacks of near-foreign and distantforeign FWs were not consistent. Males presumably use matching-avoidance within a FW dialect to initiate social interactions with particular conspecific males