Seasonal differences in the hormonal control of territorial aggression in free-living European stonechats

In birds, territorial aggression during the breeding season is regulated by testosterone (T). However, many bird species also express aggressive behavior during the nonbreeding season, when plasma levels of T are low. It has been suggested that during this period estrogens might play a major role in regulating territorial aggression. In the present study we compared the effects of simultaneous blockage of androgenic and estrogenic actions on territorial aggression during the breeding and nonbreeding seasons in free-living male European stonechats (Saxicola torquata rubicola). European stonechats are of particular interest since they establish territories and form pairs during both the breeding and the nonbreeding seasons. Thus territorial aggression and its endocrine control can be compared between reproductive and non-reproductive contexts. Inhibition of androgenic and estrogenic actions by simultaneous application of Flutamide and ATD reduced territorial aggression during the breeding season, but not during the nonbreeding season. Our results show that androgens and/or estrogens are involved in the endocrine control of territorial aggression in stonechats only in a reproductive context, but not in a non-reproductive one.     

Territorial aggression does not feed back on testosterone in a multiple-brooded songbird species with breeding and non-breeding season territoriality,the European stonechat

Testosterone mediates reproductive behaviours in male vertebrates. For example, breeding season territoriality depends on testosterone in many species of birds and in some, territorial interactions feed back on testosterone concentrations. However, the degree to which territorial behaviour and testosterone are associated differs even between species with seemingly similar life histories, especially between species that also defend territories outside the breeding season. Here, we investigate the link between territorial behaviour and testosterone in European stonechats. Previous studies found that territorial aggression in stonechats depends on testosterone in a breeding, but not in a non-breeding context. We investigated whether stonechats show a rise in testosterone during simulated territorial intrusions (STI) during the breeding season. Post-capture testosterone concentrations of males caught after an STI were not higher than those of males caught in a control situation regardless of breeding stage. However, most of the males would have been able to mount a testosterone response because the same individuals that did not increase testosterone during the STI showed a substantial increase in testosterone after injections of gonadotropin-releasing hormone (GnRH). GnRH-induced and post-capture testosterone concentrations were positively correlated and both decreased with successive breeding stages. Further, territory owners with a short latency to attack the decoy expressed higher post-capture testosterone concentrations than males with a longer latency to attack the decoy. Thus, there is no evidence for behavioural feedback on testosterone concentrations during male-male interactions in stonechats. In combination with previous studies our data suggest that testosterone functions as an on/off switch of high intensity territorial aggression during the breeding season in stonechats. The among-species variation in the androgen control of territorial behaviour may be only partly a result of environmental differences. Instead, potential differences in how territoriality evolved in different species may have influenced whether and how a reproductive hormone such as testosterone was co-opted into the mechanistic control of territorial behaviour.     

Territorial aggression and hormones during the non-breeding season in a tropical bird

The hormonal control of territorial aggression in male and female vertebrates outside the breeding season is still unresolved. Most vertebrates have regressed gonads when not breeding and do not secrete high levels of sex steroids. However, recent studies implicate estrogens in the regulation of non-breeding territoriality in some bird species. One possible source of steroids during the non-breeding season could be the adrenal glands that are known to produce sex steroid precursors such as dehydroepiandrosterone (DHEA). We studied tropical, year-round territorial spotted antbirds (Hylophylax n. naevioides) and asked (1). whether both males and females are aggressive in the non-breeding season and (2). whether DHEA is detectable in the plasma at that time. We conducted simulated territorial intrusions (STIs) with live decoys to male and female free-living spotted antbirds in central Panama. Non-breeding males and females displayed robust aggressive responses to STIs, and responded more intensely to decoys of their own sex. In both sexes, plasma DHEA concentrations were detectable and higher than levels of testosterone (T) and 17beta-estradiol (E(2)). In males, plasma DHEA concentrations were positively correlated with STI duration. Next, we conducted STIs in captive non-breeding birds. Captive males and females displayed robust aggressive behavior. Plasma DHEA concentrations were detectable in both sexes, whereas T was non-detectable (E(2) was not measured). Plasma DHEA concentrations of males were positively correlated with aggressive vocalizations and appeared to increase with longer STI durations. We conclude that male and female spotted antbirds can produce DHEA during the non-breeding season and DHEA may serve as a precursor of sex steroids for the regulation of year-round territorial behavior in both sexes.     

Oestrogen regulates male aggression in the non-breeding season

Extensive research has focused on territorial aggression during the breeding season and the roles of circulating testosterone (T) and its conversion to 17beta-oestradiol (E2) in the brain. However, many species also defend territories in the non-breeding season, when circulating T-levels are low. The endocrine control of non-breeding territoriality is poorly understood. The male song sparrow of Washington State is highly territorial year-round, but plasma T is basal in the non-breeding season (autumn and winter). Castration has no effect on aggression in autumn, suggesting that autumnal territoriality is independent of gonadal hormones. However, non-gonadal sex steroids may regulate winter territoriality (e.g. oestrogen synthesis by brain aromatase). In this field experiment, we treated wild non-breeding male song sparrows with a specific aromatase inhibitor (fadrozole, FAD) using micro-osmotic pumps. FAD greatly reduced several aggressive behaviours. The effects of FAD were reversed by E2 replacement. Treatment did not affect body condition or plasma corticosterone, suggesting that all subjects were healthy These data indicate that E2 regulates male aggression in the non-breeding season and challenge the common belief that aggression in the non-breeding season is independent of sex steroids. More generally, these results raise fundamental questions about how sexual and/or aggressive behaviours are maintained in a variety of model vertebrate species despite low circulating levels of sex steroids or despite castration. Such non-classical endocrine mechanisms may be common among vertebrates and play an important role in the regulation of behaviour.     

Autumnal territorial aggression is independent of plasma testosterone in mockingbirds

Mockingbirds (Mimus polyglottos) show intense territorial activity in the autumn as newcomers attempt to establish space within resident populations. Examination of autumnal territorial behavior showed that unmated males sing more and engage in more territorial fights than mated males. Newcomers that have just acquired space also sing more and show more territorial fights than birds resident to the population for at least one prior season. Among established residents, the average number of territorial fights was greater in birds that shared more territory boundaries with new residents. Radioimmunoassay of plasma samples taken from males during the molt and following the onset of territorial defense showed that during both periods plasma concentrations of testosterone (T), dihydrotestosterone (DHT), and estradiol were basal or below the sensitivity of the assay system. Moreover, groups of males that differed in song and territorial aggression did not differ in plasma concentrations of T, DHT, or luteinizing hormone (LH). Hormone analyses confirm measurements on several other avian species suggesting that sex steroid concentrations are low in the fall and winter and that variations in aggressive behavior at this time of year may be unrelated to LH and androgen levels. Our observations contribute to a growing body of work in temperate passerines indicating that the role of androgens in mediating aggressive challenge may be restricted to the breeding season. The possible hormonal basis (if any) of song and territorial aggression in mockingbirds outside the breeding season remains obscure.     

Testosterone and corticosterone during the breeding cycle of equatorial and European stonechats (Saxicola torquata axillaris and S. t. rubicola)

Northern-temperate male birds show seasonal changes in testosterone concentrations with a peak during the breeding season. Many tropical birds express much lower concentrations of testosterone with slight elevations during breeding. Here we describe testosterone and corticosterone concentrations of male stonechats from equatorial Kenya during different substages of breeding and molt. This tropical species has a short breeding season of approximately 3 months. We compare their hormone concentrations to previously published data of males of a northern-temperate relative, the European stonechat, also a seasonal breeder but with a breeding season of approximately 5 months. Equatorial stonechats show a pronounced peak of testosterone during the nest-building and laying stage. During all other stages, testosterone concentrations are low, similar to other year-round territorial tropical bird species. Corticosterone concentrations peak also during the nest-building and laying stage suggesting that this period of maximum female fecundity is a demanding period for the male. Equatorial stonechats have significantly lower concentrations of testosterone than European stonechats during all stages, except during the nest-building and laying stage. During this stage of maximum female fertility, testosterone levels tend to be higher in equatorial than in European stonechats. Our results suggest that equatorial stonechats belong to a group of tropical bird species that are characterized by a short breeding season and a brief high peak of testosterone during the female's fertile period. Such brief, but substantial peaks of testosterone may be common in tropical birds, but they may easily be missed if the exact breeding stage of individual birds is not known.     

Females competing to reproduce: dominance matters but testosterone may not

The associations among aggression, testosterone (T), and reproductive success have been well studied, particularly in male birds. In many species, males challenged with simulated or real territorial intrusions increase T and levels of aggression, outcomes linked to higher dominance status and greater reproductive success. For females, the patterns are less clear. Females behave aggressively towards one another, and in some species, females respond to a social challenge with increases in T, but in other species they do not. Prior work on female dark-eyed juncos (Junco hyemalis) had shown that experimental elevation of T increases social status and intrasexual aggression. Here, we conducted two experiments designed to answer three questions: Are endogenous concentrations of T associated with dominance status in captive female juncos? Does dominance status influence readiness to breed in female juncos? And do captive females increase T in response to a challenge? In the first experiment, we introduced two females to a breeding aviary, allowed them to form a dominance relationship and then introduced a male. We found that dominant females were more likely to breed than subordinates, but that dominance status was not predicted by circulating T. In the second experiment, we allowed a resident male and female to establish ownership of a breeding aviary (territory) then introduced a second, intruder female. We found that resident females were aggressive towards and dominant over intruders, but T did not increase during aggressive interactions. We suggest that during the breeding season, intrasexual aggression between females may influence reproductive success, but not be dependent upon fluctuations in T. Selection may have favored independence of aggression from T because high concentrations of T could interfere with normal ovulation or produce detrimental maternal effects.     

Spring and autumn territoriality in song sparrows: same behavior, different mechanisms?

Vertebrates show a diverse array of social behaviors associatedwith territoriality. Field and laboratory experiments indicatethat underlying themes—including mechanisms—mayexist. For example in birds, extensive evidence over many decadeshas implicated a role for testosterone in the activation ofterritorial aggression in reproductive contexts. Territorialityat other times of the year appeared to be independent of gonadalhormone control. One obvious question is—why this diversityof control mechanisms for an apparently similar behavior? Controlof testosterone secretion during the breeding season must balancethe need to compete with other males (that tends to increasetestosterone secretion), and the need to provide parental care(that requires lower testosterone concentrations). Regulationof aggressive behaviors by testosterone in the non-breedingseason may incur substantial costs. A series of experimentson the male song sparrow, Melospiza melodia morphna, of westernWashington State have revealed possible mechanisms to avoidthese costs. Song sparrows are sedentary and defend territoriesin both breeding and non-breeding seasons. Dominance interactions,territorial aggression and song during the non-breeding seasonare essentially identical to those during the breeding season.Although in the non-breeding season plasma testosterone andestradiol levels are very low, treatment with an aromatase inhibitordecreases aggression and simultaneous implantation of estradiolrestores territorial behavior. These data suggest that the mechanismby which testosterone regulates territorial behavior at theneural level remains intact throughout the year. How the hormonalmessage to activate such behavior gets to the brain in differentseason does, however, appear to be different.     

The role of androgen receptors in regulating territorial aggression in male song sparrows

This paper examines the role that androgen receptors (ARs) play in modulating aggressive behavior in male song sparrows, Melospiza melodia morphna. Song sparrows are seasonally breeding, territorial birds that maintain year-round territories with male-female pair bonds formed during the spring breeding season. Plasma testosterone levels peak as territories are established and mates acquired. In late summer, testosterone levels fall and remain basal during the non-breeding season. We examined the role of ARs in regulating territorial aggression in captive song sparrows under short- and long-day conditions as well as just prior to, and at the start of the breading season in freely living birds using the nonsteroidal antiandrogen flutamide to block AR function. Birds were implanted with either empty or drug filled silastic implants for 18 to 42 days and then challenged with a novel male decoy to assess the individual birds level of male-male aggression. Freely living birds remained on their home territory and underwent a simulated territorial intrusion, whereas laboratory-held birds were assessed using a laboratory simulated territorial intrusion and remained in their home cage. Experimental treatment of male song sparrows decreased aggressive behavior during the pre-breeding life history substage (March-April) in freely living birds as well as in laboratory-held birds under long-day (16L:8D) conditions. During the early breeding substage (April-May) there was no measurable effect of flutamide treatment on aggressive behavior, nor was there a difference in behavior in the (8L:16D) laboratory birds. This demonstrates that ARs are an important component of the neuroendocrine control of aggressive behavior. Given that flutamide only affected aggression during the pre-breeding substage and in LD birds, the results suggest that AR dependent control of aggressive behavior changes as song sparrow life history states change.     

Pair territoriality in the longnose filefish,oxymonacanthus longirostris

The longnose filefish,Oxymonacanthus longirostris, usually lives in heterosexual pairs, the male and female swimming together and sharing the same territory. Pair territoriality in the species was examined in detail in relation to sexual differences in territorial defense activities. Rigorous pair territoriality was maintained only during the breeding season, although pairs used their home ranges exclusively to a certain extent, during the non-breeding season. The frequency of aggression against other conspecific pairs in the breeding season was higher than in the non-breeding season. Agonistic interactions appear to be over both mates and food resources, the strict pair territoriality in the breeding season possibly being due to mutual mate guarding. In intraspecific aggressive interactions, males usually led their partner females when attacking intruders. The feeding frequency of males was much lower than that of females in the breeding season. Mate removal experiments indicated that females could not defend their original territories solitarily and their feeding frequency decreased. Conversely, males could defend territories solitarily without a decrease in feeding frequency. These results suggest that males contribute most to the defense of the pair territory, with females benefiting from territorial pair-swimming with their partner males.     

Non-invasive administration of 17β-estradiol rapidly increases aggressive behavior in non-breeding,but not breeding,male song sparrows

17β-Estradiol (E₂) acts in the brain via genomic and non-genomic mechanisms to influence physiology and behavior. There is seasonal plasticity in the mechanisms by which E₂ activates aggression, and non-genomic mechanisms appear to predominate during the non-breeding season. Male song sparrows (Melospiza melodia) display E₂-dependent territorial aggression throughout the year. Field studies show that song sparrow aggression during a territorial intrusion is similar in the non-breeding and breeding seasons, but aggression after an intrusion ends differs seasonally. Non-breeding males stop behaving aggressively within minutes whereas breeding males remain aggressive for hours. We hypothesize that this seasonal plasticity in the persistence of aggression relates to seasonal plasticity in E₂ signaling. We used a non-invasive route of E₂ administration to compare the non-genomic (within 20 min) effects of E₂ on aggressive behavior in captive non-breeding and breeding season males. E₂ rapidly increased barrier contacts (attacks) during an intrusion by 173% in non-breeding season males only. Given that these effects were observed within 20 min of E₂ administration, they likely occurred via a non-genomic mechanism of action. The present data, taken together with past work, suggest that environmental cues associated with the non-breeding season influence the molecular mechanisms through which E₂ influences behavior. In song sparrows, transient expression of aggressive behavior during the non-breeding season is highly adaptive: it minimizes energy expenditure and maximizes the amount of time available for foraging. In all, these data suggest the intriguing possibility that aggression in the non-breeding season may be activated by a non-genomic E₂ mechanism due to the fitness benefits associated with rapid and transient expression of aggression.     

Variation in fecal testosterone levels, inter-male aggression, dominance rank and age during mating and post-mating periods in wild adult male ring-tailed lemurs (Lemur catta)

In primate species exhibiting seasonal reproduction, patterns of testosterone excretion in adult males are variable: in some species, peaks correlate with female receptivity periods and heightened male-male aggression over access to estrous females, in others, neither heightened aggression nor marked elevations in testosterone have been noted. In this study, we examined mean fecal testosterone ( f T) levels and intermale aggression in wild adult male ring-tailed lemurs residing in three groups at Beza Mahafaly Reserve, Madagascar. Results obtained from mating and post-mating season 2003 were compared to test Wingfield et al. [1990. Am Nat 136:829-846] "challenge hypothesis", which predicts a strong positive relationship between male testosterone levels and male-male competition for access to receptive females during breeding season. f T levels and rates of intermale aggression were significantly higher during mating season compared to the post-mating period. Mean f T levels and aggression rates were also higher in the first half of the mating season compared with the second half. Number of males in a group affected rates of intermale agonism, but not mean f T levels. The highest-ranking males in two of the groups exhibited higher mean f T levels than did lower-ranking males, and young males exhibited lower f T levels compared to prime-aged and old males. In the post-mating period, mean male f T levels did not differ between groups, nor were there rank or age effects. Thus, although male testosterone levels rose in relation to mating and heightened male-male aggression, f T levels fell to baseline breeding levels shortly after the early mating period, and to baseline non-breeding levels immediately after mating season had ended, offsetting the high cost of maintaining both high testosterone and high levels of male-male aggression in the early breeding period.     

Corticosterone levels as indicators of habitat quality: effects of habitat segregation in a migratory bird during the non-breeding season

During the non-breeding season, many species of territorial migratory birds exhibit a non-random pattern of habitat distribution, with males and females occupying different habitats. In this study, we examined possible physiological consequences arising from such habitat segregation in one migrant passerine species, the American redstart (Setophaga ruticilla), on its non-breeding grounds in Jamaica, West Indies. For 2 years, we measured concentrations of corticosterone, at the time of capture (baseline) and 30 min after capture (profile of acute corticosterone secretion), in redstarts in two distinct habitats, one occupied predominately by males and one mostly by females. All redstarts in both habitat types exhibited similar concentrations of baseline corticosterone levels in fall (October), whereas in spring (March–April), redstarts in female-biased habitat exhibited significantly higher baseline levels regardless of age or sex. In fall, all individuals in both habitats exhibited significant increases in corticosterone concentration with capture and handling, but in spring only redstarts (both sexes) in male-biased habitat continued to exhibit acute corticosterone secretion. Redstarts in female-biased habitat had elevated baseline corticosterone levels and reduced acute corticosterone secretion. In spring, baseline corticosterone concentration was negatively correlated with body mass, suggesting muscle catabolism associated with high corticosterone concentrations or possibly that birds are leaner as a result of increased foraging effort. These results indicate that redstarts (primarily females) in female-biased habitats suffered a decline in physiological condition, which could in turn influence their departure schedules, migration patterns and even their condition and arrival schedules on the breeding grounds. Thus, segregation of populations into habitats of different quality during the non-breeding period may have ramifications throughout the annual cycle of such migratory species. Furthermore, these results show the usefulness of plasma corticosterone levels as indicators of physiological condition and thus habitat quality for birds during the non-breeding period. Received: 14 November 1997 / Accepted: 9 March 1998     

Impact of season and social challenge on testosterone and corticosterone levels in a year-round territorial bird

Plasma testosterone increases during breeding in many male vertebrates and has long been implicated in the promotion of aggressive behaviors relating to territory and mate defense. Males of some species also defend territories outside of the breeding period. For example, the European nuthatch (Sitta europaea) defends an all-purpose territory throughout the year. To contribute to the growing literature regarding the hormonal correlates of non-breeding territoriality, we investigated the seasonal testosterone and corticosterone profile of male (and female) nuthatches and determined how observed hormone patterns relate to expression of territorial aggression. Given that non-breeding territoriality in the nuthatch relates to the reproductive context (i.e., defense of a future breeding site), we predicted that males would exhibit surges in plasma testosterone throughout the year. However, we found that males showed elevated testosterone levels only during breeding. Thus, testosterone of gonadal origin does not appear to be involved in the expression of non-breeding territoriality. Interestingly, territorial behaviors of male nuthatches were stronger in spring than in autumn, suggesting that in year-round territorial species, breeding-related testosterone elevations may upregulate male-male aggression above non-breeding levels. In females, plasma testosterone was largely undetectable. We also examined effects of simulated territorial intrusions (STIs) on testosterone and corticosterone levels of breeding males. We found that STIs did not elicit a testosterone response, but caused a dramatic increase in plasma corticosterone. These data support the hypothesis that corticosterone rather than testosterone may play a role in the support of behavior and/or physiology during acute territorial encounters in single-brooded species.     

Testosterone and its effects on courtship in golden-collared manakins (Manacus vitellinus): seasonal, sex, and age differences

Male golden-collared manakins gather on leks and perform an acrobatic display to attract females. In temperate breeding species, testosterone (T) activation of courtship displays has been well studied. Few studies have examined T activation of displays in tropical species; even fewer have explored the activational role of T in elaborate courtship displays such as in the manakin. In some tropical species, including manakins, territorial aggression or song behavior are uncoupled from T. We have previously shown that T activates display behavior in manakin males when endogenous T levels are low in the non-courtship season. To understand how T functions in breeding birds, we examined T levels in a large group of manakins sampled during the courtship and non-courtship season. In addition, during the courtship season, we gave T implants to adult males, juvenile males, and females. We found that T levels were low during the non-courtship season and comparatively higher on average during the courtship season. However, T levels were low in many adult males during the courtship season, especially when compared to temperate breeding species. Regardless of initial endogenous T levels during the courtship season, T implants did not increase the display frequency of adult males. T-treated females and juvenile males did display under similar conditions. Our data suggest that the effects of T on manakin display vary with season, sex, and age and that high T is not necessary for display.     

Territoriality and testosterone in an equatorial population of rufous-collared sparrows, Zonotrichia capensis

Territorial aggression, displayed by male vertebrates in a reproductive context, is generally thought to be mediated by testosterone. The challenge hypothesis predicts that in socially monogamous species, territorial challenges should induce an increase in plasma testosterone concentrations, which will enhance aggressive behaviour and territory defence. This hypothesis is based on northern latitude birds and needs to be tested in tropical birds before it can be universally accepted. We tested the challenge hypothesis in an equatorial population of rufous-collared sparrows in Papallacta, Ecuador. This population shows an extended breeding period during which males aggressively guard territories. During the early breeding season, males were challenged with conspecific or heterospecific simulated territorial intrusions (STIs) lasting 10 min. Conspecific-challenged males responded more aggressively than heterospecific-challenged males. However, there was no increase in plasma testosterone in response to the conspecific STI. During the breeding season, males were challenged with conspecific STIs lasting 0, 10 or 30 min. Males behaved aggressively regardless of STI duration, and did not differ in plasma testosterone or luteinizing hormone concentrations. During the breeding season, males were implanted with testosterone-filled or empty silastic tubes and subsequently challenged with a conspecific STI. Testosterone implants significantly raised plasma testosterone concentrations, but testosterone-implanted males were not more aggressive than blank-implanted controls. Combined, these findings suggest that testosterone concentrations above breeding baseline are not related to territorial aggression in this population and therefore do not support the challenge hypothesis.     

Territorial aggression, circulating levels of testosterone, and brain aromatase activity in free-living pied flycatchers

Testosterone (T) is a critical endocrine factor for the activation of many aspects of reproductive behavior in vertebrates. Castration completely eliminates the display of aggressive and sexual behaviors that are restored to intact level by a treatment with exogenous T. There is usually a tight correlation between the temporal changes in plasma T and the frequency of reproductive behaviors during the annual cycle. In contrast, individual levels of behavioral activity are often not related to plasma T concentration at the peak of the reproductive season suggesting that T is available in quantities larger than necessary to activate behavior and that other factors limit the expression of behavior. There is some indication from work in rodents that individual levels of brain aromatase activity (AA) may be a key factor that limits the expression of aggressive behavior, and in agreement with this idea, many studies indicate that estrogens produced in the brain by the aromatization of T may contribute to the activation of reproductive behavior, including aggression. We investigated here in pied flycatcher (Ficedula hypoleuca) the relationships among territorial aggression, plasma T, and brain AA at the peak of the reproductive season. In a first experiment, blood samples were collected from unpaired males holding a primary territory and, 1 or 2 days later, their aggressive behavior was quantified during standardized simulated territorial intrusions. No relationship was found between individual differences in aggressive behavior and plasma T or dihydrotestosterone levels but a significant negative correlation was observed between number of attacks and plasma corticosterone. In a second experiment, aggressive behavior was measured during a simulated territorial intrusion in 22 unpaired males holding primary territories. They were then immediately captured and AA was measured in their anterior and posterior diencephalon and in the entire telencephalon. Five males that had attracted a female (who had started egg-laying) were also studied. The paired males were less aggressive and correlatively had a lower AA in the anterior diencephalon but not in the posterior diencephalon and telencephalon than the 22 birds holding a territory before arrival of a female. In these 22 birds, a significant correlation was observed between number of attacks/min displayed during the simulated territorial intrusion and AA in the anterior diencephalon but no correlation was found between these variables in the two other brain areas. Taken together, these data indicate that the level of aggression displayed by males defending their primary territory may be limited by the activity of the preoptic aromatase, but plasma T is not playing an important role in establishing individual differences in aggression. Alternatively, it is also possible that brain AA is rapidly affected by agonistic interactions and additional work should be carried out to determine whether the correlation observed between brain AA and aggressive behavior is the result of an effect of the enzyme on behavior or vice versa. In any case, the present data show that preoptic AA can change quite rapidly during the reproductive cycle (within a few days after arrival of the female) indicating that this enzymatic activity is able to regulate rapid behavioral transitions during the reproductive cycle in this species.     

Weak migratory connectivity,loop migration and multiple non-breeding site use in British breeding Whinchats Saxicola rubetra

Determining the links between breeding populations and the pressures, threats and conditions they experience presents a challenge for the conservation of migratory birds which can use multiple sites separated by hundreds to thousands of kilometres. Furthermore, migratory connectivity – the connections made by migrating individuals between networks of breeding and non-breeding sites – has important implications for population dynamics. The Whinchat Saxicola rubetra is declining across its range, and tracking data from a single African non-breeding site implies high migratory spread. We used geolocators to describe the migration routes and non-breeding areas of 20 Whinchats from three British breeding populations. As expected, migratory spread was high, with birds from the three populations overlapping across a wide area of West Africa. On average, in non-breeding areas, British breeding Whinchats were located 652 km apart from one another, with some likely to share non-breeding areas with individuals from breeding populations as far east as Russia. Four males made a direct non-breeding season movement to a second, more westerly, non-breeding location in January. Autumn migration was through Iberia and around the western edge of the Sahara Desert, whereas spring migration was more direct, indicating an anticlockwise loop migration. Weak migratory connectivity implies that Whinchat populations are somewhat buffered against local changes in non-breeding conditions. If non-breeding season processes have played a role in the species’ decline, then large-scale drivers are likely to be the cause, although processes operating on migration, or interactions between breeding and non-breeding processes, cannot be ruled out.     

Evaluating seasonal patterns of female aggression: Case study in a cavity‐nesting bird with intense female–female competition

Seasonal plasticity in aggression is likely to be shaped by the contexts in which aggression is beneficial, as well as the constraints inherent in its underlying mechanisms. In males, seasonal plasticity in testosterone (T) secretion is thought to underlie seasonal plasticity in conspecific aggression, but it is less clear how and why female aggression may vary across different breeding stages. Here, we integrate functional and mechanistic perspectives to begin to explore seasonal patterns of conspecific aggression in female tree swallows (Tachycineta bicolor), a songbird with intense female–female competition and T‐mediated aggression. Female tree swallows elevate T levels during early breeding stages, coinciding with competition for nest boxes, after which time T levels are roughly halved. However, females need to defend ownership of their nesting territory throughout the breeding season, suggesting it may be adaptive to maintain aggressive capabilities, despite low T levels. We performed simulated territorial intrusions using 3D‐printed decoys of female tree swallows to determine how their aggressive response to a simulated intrusion changes across the breeding season. First, we found that 3D‐printed decoys produce data comparable to stage‐matched studies using live decoys, providing researchers with a new, more economical method of decoy construction. Further, female aggressiveness remained relatively high through incubation, a period of time when T levels are quite low, suggesting that other mechanisms may regulate conspecific female aggression during parental periods. By showing that seasonal patterns of female aggression do not mirror the established patterns of T levels in this highly competitive bird, our findings provide a unique glimpse into how behavioural mechanisms and functions may interact across breeding stages to regulate plasticity.     

Testosterone, paternal behavior, and aggression in the monogamous California mouse (Peromyscus californicus)

Testosterone (T) mediates a trade-off, or negative correlation, between paternal behavior and aggression in several seasonally breeding avian species. However, the presence or absence of a T-mediated trade-off in mammals has received less attention. We examined the relationship between paternal behavior and territorial aggression in the biparental California mouse, Peromyscus californicus. In contrast to seasonally breeding birds, T maintains paternal behavior in this year-round territorial species. Castration reduced paternal behavior, whereas T replacement maintained high levels of paternal behavior. We hypothesize that T is aromatized in the brain to estradiol, which in turn stimulates paternal behavior. In contrast to paternal behavior, aggressive behavior was not reduced by castration. Interestingly, only sham males showed an increase in aggression across three aggression tests, while no change was detected in castrated or T-replacement males. Overall, trade-offs between aggression and paternal behavior do not appear to occur in this species. Measures of paternal behavior and aggression in a correlational experiment were actually positively correlated. Our data suggest that it may be worth reexamining the role that T plays in regulating mammalian paternal behavior.