Artificial rain and cold wind act as stressors to captive molting and non-molting European starlings (Sturnus vulgaris)

Free-roaming animals continually cope with changes in their environment. One of the most unpredictable environmental phenomena is weather. Being able to respond to weather appropriately is crucial as it can be a threat to survival. The stress response, consisting of increases in heart rate and release of glucocorticoids, is an important mechanism by which animals cope with stressors. This study examined behavioral, heart rate, and corticosterone responses of captive European starlings (Sturnus vulgaris) to two aspects of weather mimicked under controlled conditions, a subtle (3 °C) decrease in temperature and a short, mild bout of rain. Both decreased temperature and exposure to rain elicited increases in heart rate and corticosterone in non-molting starlings. Molt is an important life history stage in birds that affects feather cover and may require a different response to weather-related stressors. We repeated the experiment in molting starlings and found increases in heart rate in response to rain and cold wind. However, the hypothalamic–pituitary–adrenal (HPA)-axis was suppressed during molt, as molting starlings did not increase corticosterone release in response to either stimulus. These data suggest these stimuli induce increased allostatic load in starlings, and that animals may adjust their response depending on the life-history stage.     

Photoperiod-induced testicular apoptosis in European starlings (Sturnus vulgaris)

To determine the extent to which testicular regression involves apoptotic cell death, photosensitive adult starlings were photostimulated for up to 9 wk by exposure to long-day (18 h of light) photoperiods. Apoptotic activity in recrudescing and regressing testes was assessed by in situ TUNEL labeling. Absolute testis mass in male starlings increased after 2 wk of photostimulation and subsequently decreased with continued long-day exposure. Seminiferous tubule diameter also increased after 1-3 wk of photostimulation, then decreased as photorefractoriness developed. Testosterone concentrations increased significantly by Week 2 of photostimulation and declined with further light exposure. TUNEL labeling was significantly elevated in germ cells with 4 wk of photostimulation. An approximate 7-fold increase in the degree of apoptotic cell death was observed over the course of gonadal regression. Incidences of TUNEL labeling in somatic Sertoli cells also increased. Light and electron microscopy examination confirmed that these somatic cells displayed morphological characteristics of apoptotic death. In rodents, Sertoli cells have not been previously reported to die during gonadal regression. These results suggest that seasonal testicular regression in European starlings is mediated by apoptosis.     

Captive European starlings (Sturnus vulgaris) in breeding condition show an increased cardiovascular stress response to intruders

European starlings (Sturnus vulgaris) alter their physiology and behavior between seasons, becoming territorial during the spring/summer and flocking during the fall/winter. We used captive male starlings in breeding (photostimulated to 18L : 6D) and nonbreeding (11L : 13D) conditions to determine whether changing physiology and behavior alters their reaction to crowding. One or five intruders entered a resident's cage without human disturbance. A subcutaneous heart rate transmitter recorded cardiovascular output in residents. Corticosterone and testosterone were measured in plasma samples taken before and after the intrusion. While corticosterone concentrations did not change, heart rate changed significantly, indicating that these responses can be regulated independently. Long-day birds showed a significantly elevated heart rate response to the single-bird intrusion compared to short-day birds. Whereas five intruders elicited an identical peak response in both groups, long-day birds also demonstrated an equivalent response to one intruder. In addition, one intruder induced longer elevation in heart rate for long-day birds. Male starlings in breeding condition, therefore, demonstrate an increased sensitivity to additional conspecifics. This seasonal shift in response suggests that a higher tolerance for intrusion (i.e., considering a nearby starling as less stressful) may facilitate flocking behavior, while a lower tolerance may aid in territoriality.     

Song predicts immunocompetence in male European starlings (Sturnus vulgaris)

According to the immunocompetence handicap hypothesis, sexually selected characteristics predict immune function and this relationship is mediated by testosterone. In the present study, we investigated whether bird song could predict immunity in European starlings (Sturnus vulgaris). We recorded the singing and reproductive behaviours of 16 adult male starlings in an outdoor aviary and then assessed their cell-mediated and humoral immunity in vivo. The males were observed in groups of four for 2 h each day over a 4-day period. For each male, the number of songs produced was recorded and the average song-bout length was computed. Next, cell-mediated and humoral immunity were assessed via cutaneous swelling responses to the T-cell mitogen phytohaemagglutinin and antibody responses to a novel antigen, keyhole limpet haemocyanin. Song rate and song-bout length were positively correlated with cell-mediated and humoral immunity, respectively. Additionally, a negative relationship between plasma testosterone concentration and antibody response was observed. These data demonstrate that male starling song can be used as a predictor of immunocompetence, with more robust singers exhibiting enhanced immunity. Whether this relationship is mediated directly by testosterone requires further investigation.     

Embryonic modulation of maternal steroids in European starlings (Sturnus vulgaris)

In birds, maternally derived yolk steroids are a proposed mechanism by which females can adjust individual offspring phenotype to prevailing conditions. However, when interests of mother and offspring differ, parent–offspring conflict will arise and embryonic interests, not those of the mother, should drive offspring response to maternal steroids in eggs. Because of this potential conflict, we investigated the ability of developing bird embryos to process maternally derived yolk steroids. We examined how progesterone, testosterone and oestradiol levels changed in both the yolk/albumen (YA) and the embryo of European starling eggs during the first 10 days of development. Next, we injected tritiated testosterone into eggs at oviposition to characterize potential metabolic pathways during development. Ether extractions separated organic and aqueous metabolites in both the embryo and YA homogenate, after which major steroid metabolites were identified. Results indicate that the concentrations of all three steroids declined during development in the YA homogenate. Exogenous testosterone was primarily metabolized to an aqueous form of etiocholanolone that remained in the YA. These results clearly demonstrate that embryos can modulate their local steroid environment, setting up the potential for parent–offspring conflict. Embryonic regulation must be considered when addressing the evolutionary consequences of maternal steroids in eggs.     

The effects of chronic psychological and physical stress on feather replacement in European starlings (Sturnus vulgaris)

Corticosterone (CORT) is seasonally modulated in many passerines, with plasma CORT concentrations lowest during the prebasic molt, when all feathers are replaced. Recent evidence indicating that CORT implants slow the rate of feather regrowth in molting birds suggests that plasma CORT concentrations are downregulated during molt in order to avoid the inhibition of feather growth caused by the protein catabolic activity of CORT. To further test this hypothesis, we examined whether endogenous CORT release, stimulated by exposure to either psychological stress or physical stress (food restriction), could inhibit feather regrowth rates or decrease feather quality in birds undergoing an induced molt (feather replacement after plucking). European starlings (Sturnus vulgaris) were exposed to chronic psychological stress or food restriction for three weeks of the feather regrowth period. Throughout this time, the length of growing primary, secondary, and tail feathers was measured and blood samples were collected to measure baseline and stress-induced CORT concentrations. Upon completion of growth, feather quality was analyzed via measurements of mass, rachis length, feather area, and presence of fault bars. Both psychological and physical stress protocols elevated circulating plasma CORT but significantly less than implants from an earlier study did. Psychological stress had no effect on feather regrowth rates or feather quality. Food restriction had no effect on feather growth rate but caused asynchronous feather replacement. When combined with psychological stress, physical stress also resulted in smaller feather area. Results indicate that CORT implants may not accurately represent chronic stress physiology. Additionally, the purpose for downregulating CORT concentrations during molt appears to be more complicated than simply protecting feather production from CORT's catabolic effects.     

Post-hatching dynamics of plasma biochemistry in free-living European starlings (Sturnus vulgaris)

This is the first study of plasma biochemical parameters in free-living altricial birds during an entire developmental period in a nest, represented by European starling (Sturnus vulgaris). Dynamics of postnatal changes from hatching until close to fledging (days 1 to 15) were registered. Parameters of protein metabolism represented by total proteins, albumin and globulin concentrations increased continuously during the observed developmental period. There were two peaks in uric acid concentration on days 5 and 11. To the contrary, the creatinine content did not change throughout the observed period and increased only on day 15. Creatine kinase activity gradually increased until day 11 and then fell before fledging. Parameters of lipid metabolism (concentration of total lipids, triacylglycerols and nonesterified fatty acids) in plasma increased gradually reaching a plateau between days 8 and 11 and then declined on day 15. The cholesterol concentration pattern was similar to maximum value on day 11, then consecutively decreased. Concentration of glucose increased until day 8 and remained unchanged until fledging. Whereas calcium reached the highest concentration during days 8 and 11, phosphorus peaked earlier on day 5. The activity of alkaline phosphatase was similar to the pattern found in calcium concentration. Presented data showed an increase in both protein and lipid metabolism during the phase of rapid growth. A remarkable decrease in parameters of lipid metabolism before fledging may reflect increased physical activity and changes in nutrition.     

Occurrence and attempted transmission of Toxoplasma gondii in European starlings (Sturnus vulgaris)

Serums of 563 fledgling starlings (Sturnus vulgaris) trapped during a 14 month period near Lodi, California were examined for antibodies to Toxoplasma gondii using the microtiter indirect hemagglutination method. Titers ranging from 1:64 to 1:512 were found in 4.8% of the birds. Starlings collected during May through October had a higher prevalence of antibody than those collected during November through April. Rats inoculated with individual heart and brain suspensions from 10 seropositive starlings remained seronegative for T. gondii antibodies when tested at 22 and 82 days post-inoculation. Peritoneal smears made from these rats at post-inoculation day 82 were negative for T. gondii.     

The effects of chronic psychological and physical stress on feather replacement in European starlings (Sturnus vulgaris).

Corticosterone (CORT) is seasonally modulated in many passerines, with plasma CORT concentrations lowest during the prebasic molt, when all feathers are replaced. Recent evidence indicating that CORT implants slow the rate of feather regrowth in molting birds suggests that plasma CORT concentrations are downregulated during molt in order to avoid the inhibition of feather growth caused by the protein catabolic activity of CORT. To further test this hypothesis, we examined whether endogenous CORT release, stimulated by exposure to either psychological stress or physical stress (food restriction), could inhibit feather regrowth rates or decrease feather quality in birds undergoing an induced molt (feather replacement after plucking). European starlings (Sturnus vulgaris) were exposed to chronic psychological stress or food restriction for three weeks of the feather regrowth period. Throughout this time, the length of growing primary, secondary, and tail feathers was measured and blood samples were collected to measure baseline and stress-induced CORT concentrations. Upon completion of growth, feather quality was analyzed via measurements of mass, rachis length, feather area, and presence of fault bars. Both psychological and physical stress protocols elevated circulating plasma CORT but significantly less than implants from an earlier study did. Psychological stress had no effect on feather regrowth rates or feather quality. Food restriction had no effect on feather growth rate but caused asynchronous feather replacement. When combined with psychological stress, physical stress also resulted in smaller feather area. Results indicate that CORT implants may not accurately represent chronic stress physiology. Additionally, the purpose for downregulating CORT concentrations during molt appears to be more complicated than simply protecting feather production from CORT's catabolic effects.     

Effects of dietary quality on basal metabolic rate and internal morphology of European starlings (Sturnus vulgaris)

European starlings (Sturnus vulgaris) were fed either a low- or high-quality diet to test the effects of dietary quality on basal metabolic rate (BMR) and internal morphology. Basal metabolic rate did not differ significantly between the two dietary groups, but internal morphology differed greatly. Starlings fed the low-quality diet had heavier gastrointestinal tracts, gizzards, and livers. Starlings fed the high-quality diet had heavier breast muscles. Starlings on the low-quality diet maintained mass, while starlings on the high-quality diet gained mass. Dry matter digestibility and energy digestibility were lower for starlings fed the low-quality diet, and their food and water intake were greater than starlings on the high-quality diet. The lack of dietary effect on BMR may be the result of increased energy expenditure of digestive organs paralleling a reduction of energy expenditure of organs and tissues not related to digestion (i.e., skeletal muscle). This trade-off in energy allocation among organs suggests a mechanism by which organisms may alter BMR in response to a change in seasonal variation in food availability.     

Castration does not decrease nonreproductive aggression in yearling male European starlings (Sturnus vulgaris)

In the nonbreeding season, some bird species express high levels of aggression despite basal plasma testosterone (T) concentrations. Consequently, nonreproductive aggression is believed to be independent of plasma T. In the present study, we investigated the effect of castration on nonreproductive aggressive behavior in yearling male European starlings (Sturnus vulgaris). We paired castrated (Cx) with control (C) males in dyadic trials during three test periods (December, January-February, and March-April), and by using an infrared camera, we defined which male was the most aggressive one when both males were competing over access to a nest box to roost in. During each of the three periods, Cx males behaved more aggressively than C males but differences between groups did not reach significance. When analyzing the results of the three periods together, Cx males were found to be significantly more aggressive than C males. Likewise, when considering only the second and third period (when plasma T levels of C males were significantly higher than those of Cx males) the same result was obtained. Furthermore, in the third period, aggression tended to be lower than in the first and second period, although T concentrations of C males were highest in this period. Our data thus clearly show that nonreproductive aggression in yearling male starlings is independent of gonadal sex steroids and suggest it even increases by castration.     

Circannual variations in plasma luteinizing hormone levels in castrated male European starlings (Sturnus vulgaris)

Intact and castrated male European starlings were held for about 2 years in a constant 12-hr photoperiod and constant temperature conditions. At 1- to 2-month intervals, testicular width was measured by laparotomy, and blood samples were taken for analysis of plasma luteinizing hormone (LH). Most of the control birds went through at least one circannual cycle of testicular width and plasma LH concentration. In the castrates, a similar proportion of birds went through circannual LH cycles with periods indistinguishable from those of the controls. It is concluded that the testes and their hormones are not essential components of the mechanism that generates circannual gonadal cycles in male European starlings.     

Hand-rearing reduces fear of humans in European starlings, Sturnus vulgaris

Pending changes in European legislation ban the use of wild-caught animals in research. This change is partly justified on the assumption that captive-breeding (or hand-rearing) increases welfare of captive animals because these practices result in animals with reduced fear of humans. However, there are few actual data on the long-term behavioural effects of captive-breeding in non-domestic species, and these are urgently needed in order to understand the welfare and scientific consequences of adopting this practice. We compared the response of hand-reared and wild-caught starlings to the presence of a human in the laboratory. During human presence, all birds increased their general locomotor activity but the wild-caught birds moved away from the human and were less active than the hand-reared birds. After the human departed, the wild-caught birds were slower to decrease their activity back towards baseline levels, and showed a dramatic increase in time at the periphery of the cage compared with the hand-reared birds. We interpret these data as showing evidence of a greater fear response in wild-caught birds with initial withdrawal followed by a subsequent rebound of prolonged attempts to escape the cage. We found no effects of environmental enrichment. However, birds in cages on low shelves were less active than birds on upper shelves, and showed a greater increase in the time spent at the periphery of their cages after the human departed, perhaps indicating that the lower cages were more stressful. In demonstrating reduced fear of humans in hand-reared birds, our results support one of the proposed welfare benefits of this practice, but without further data on the possible welfare costs of hand-rearing, it is not yet possible to reach a general conclusion about its net welfare impact. However, our results confirm a clear scientific impact of both hand-rearing and cage position at the behavioural level.     

Contrasting mitochondrial diversity of European starlings (Sturnus vulgaris) across three invasive continental distributions

European starlings (Sturnus vulgaris) represent one of the most widespread and problematic avian invasive species in the world. Understanding their unique population history and current population dynamics can contribute to conservation efforts and clarify evolutionary processes over short timescales. European starlings were introduced to Central Park, New York in 1890, and from a founding group of about 100 birds, they have expanded across North America with a current population of approximately 200 million. There were also multiple introductions in Australia in the mid‐19th century and at least one introduction in South Africa in the late 19th century. Independent introductions on these three continents provide a robust system to investigate invasion genetics. In this study, we compare mitochondrial diversity in European starlings from North America, Australia, and South Africa, and a portion of the native range in the United Kingdom. Of the three invasive ranges, the North American population shows the highest haplotype diversity and evidence of both sudden demographic and spatial expansion. Comparatively, the Australian population shows the lowest haplotype diversity, but also shows evidence for sudden demographic and spatial expansion. South Africa is intermediate to the other invasive populations in genetic diversity but does not show evidence of demographic expansion. In previous studies, population genetic structure was found in Australia, but not in South Africa. Here we find no evidence of population structure in North America. Although all invasive populations share haplotypes with the native range, only one haplotype is shared between invasive populations. This suggests these three invasive populations represent independent subsamples of the native range. The structure of the haplotype network implies that the native‐range sampling does not comprehensively characterize the genetic diversity there. This study represents the most geographically widespread analysis of European starling population genetics to date.     

Seasonal changes in courtship song and the medial preoptic area in male European starlings (Sturnus vulgaris)

In male starlings (Sturnus vulgaris) courtship song plays a critical role in mate attraction. During the breeding season courtship song occurs prior to copulation and appears to reflect male sexual arousal. Outside the breeding season starlings sing, but song appears unrelated to reproduction. The aromatization of testosterone (T), likely within the medial preoptic nucleus (POM), is critical for the expression of male sexual arousal. The present study was performed to determine whether seasonal changes in the POM might relate to seasonal changes in courtship singing behavior in male starlings. T concentrations, the volume of the POM, and aromatase within the POM were examined both during and outside of the breeding season in male starlings. Song was also recorded at these times both with and without a female present. The POM was largest and contained dense aromatase immunostaining only during the spring breeding season, when T concentrations were highest and males responded to a female with an increase in courtship song. Outside the breeding season the volume of the POM was small, T concentrations were low, and males displayed no changes in song expression in response to female conspecifics. Song bout length was positively related to POM volume, and males sang longer songs in spring. Only males with nestboxes in spring responded to a female, and the POM tended to be larger in these males, suggesting that nestbox possession might influence neuroplasticity within the POM. Overall, the findings suggest that T-dependent plasticity and aromatase activity within the POM might regulate courtship singing in a wild songbird.     

A behavioral analysis of prey detection lateralization and unilateral transfer in European starlings (Sturnus vulgaris)

It has been suggested that birds prefer to use a particular eye while learning to detect cryptic prey and that this eye preference enhances foraging performance. European starlings (Sturnus vulgaris) with the left, right, or both eyes available learned to detect inconspicuous cues associated with the presence of hidden prey. Acquisition scores were not significantly different between left and right-eyed birds; however, performance in the binocular condition was significantly higher than in the two monocular conditions. When binocular birds were tested with familiar and unfamiliar cues present simultaneously, the familiar cue was selected significantly more often than the unfamiliar cue, suggesting that the birds were searching for specific cue features. When monocular birds were tested using only the naïve eye, performance dropped significantly. In right-eyed birds using the naïve left eye, performance remained at chance levels over transfer trials. However, left-eyed birds using the naïve right eye had a superior performance compared to the initial acquisition scores of right-eyed birds and also showed a significant improvement in performance over transfer trials. Thus, although there was no direct evidence of lateralization during acquisition, there was unilateral transfer of the prey detection skill from the right to the left hemisphere.     

Attempted experimental transmission of pseudorabies virus to European starlings (Sturnus vulgaris).

Forty European starlings (Sturnus vulgaris) were inoculated orally with pseudorabies virus. Subsequent attempts to isolate the virus from feces, liver, lung and kidney of the birds were negative; antibody against the virus could not be detected in their serums.     

Influence of testosterone metabolites on song-control system neuroplasticity during photostimulation in adult European starlings (Sturnus vulgaris)

The song-control system is a network of discrete nuclei in the songbird brain that controls the production and learning of birdsong and exhibits some of the best-studied neuroplasticity found in the adult brain. Photoperiodic growth of the song-control system during the breeding season is driven, at least in part, by the gonadal steroid testosterone. When acting on neural tissue, however, testosterone can be metabolized into 5α-dihydrotestosterone (DHT) or 17β-estradiol (E2), which activate different hormonal signaling pathways. By treating adult starlings with both testosterone metabolites and metabolite antagonists, we attempted to isolate the effects of androgen and estrogen treatment on neuroplasticity during photostimulation in male and female European starlings (Sturnus vulgaris). Photostimulation resulted in a large HVC volume typical of the breeding season in all treatments independent of hormone treatment. E2 had additional effects on HVC growth by reducing neuron density and enhancing early survival of new neurons recruited to HVC in females but did not significantly affect HVC volume. Conversely, DHT reduced the migration of new neurons, assessed by the expression of doublecortin, to HVC. DHT also increased syrinx mass and maintained RA (robust nucleus of the arcopallium) cytoarchitecture in the presence of aromatase inhibitors. In addition, we document the first evidence of sex-specific neuroplastic responses of the song-control system to androgens and estrogens. These findings suggest that the contributions of DHT and E2 signaling in songbird neuroplasticity may be regulated by photoperiod and that future studies should account for species and sex differences in the brain.     

Fear and exploration in European starlings (Sturnus vulgaris): a comparison of hand-reared and wild-caught birds

The revision of EU legislation will ban the use of wild-caught animals in scientific procedures. This change is partially predicated on the assumption that captive-rearing produces animals with reduced fearfulness. Previously, we have shown that hand-reared starlings (Sturnus vulgaris) indeed exhibit reduced fear of humans compared to wild-caught conspecifics. Here, we asked whether this reduction in fear in hand-reared birds is limited to fear of humans or extends more generally to fear of novel environments and novel objects. Comparing 6-8 month old birds hand-reared in the lab with age-matched birds caught from the wild as fledged juveniles a minimum of 1 month previously, we examined the birds' initial reactions in a novel environment (a small cage) and found that wild-caught starlings were faster to initiate movement compared to the hand-reared birds. We interpret this difference as evidence for greater escape motivation in the wild-caught birds. In contrast, we found no differences between hand-reared and wild-caught birds when tested in novel object tests assumed to measure neophobia and exploratory behaviour. Moreover, we found no correlations between individual bird's responses in the different tests, supporting the idea that these measure different traits (e.g. fear and exploration). In summary, our data show that developmental origin affects one measure of response to novelty in young starlings, indicative of a difference in either fear or coping style in a stressful situation. Our data contribute to a growing literature demonstrating effects of early-life experience on later behaviour in a range of species. However, since we did not find consistent evidence for reduced fearfulness in hand-reared birds, we remain agnostic about the welfare benefits of hand-rearing as a method for sourcing wild birds for behavioural and physiological research.     

Ratite-like neoteny induced by neonatal thyroidectomy of European starlings, Sturnus vulgaris

The Ratites (ostriches, emus, etc.) are thought to be neotenous descendants of flying birds, rather than primitive birds, even though they became a separate group early in the evolution of birds. This is because of the juvenile, rather than primitive, condition of the palate, skull-sutures and feathers. We report here that European starlings (Sturnus vulgaris) thyroidectomized soon after hatching also show neoteny, retaining many juvenile features (general morphology, palate, skull-sutures, feathers, behaviour), while at the same time becoming sexually mature. The juvenile characters were similar to those found in Ratites, suggesting that hypothyroidism may have been a factor in the evolution of the Ratites.