Loner or socializer? Ravens' adrenocortical response to individual separation depends on social integration

Non-breeding common ravens (Corvus corax) live in complex social groups with a high degree of fission–fusion dynamics. They form valuable relationships and alliances with some conspecifics, while taking coordinated action against others. In ravens, affiliates reconcile their conflicts, console each other after conflicts with a third party, and provide each other with social support — all behaviors that presumably reduce corticosterone levels and alleviate stress. However, how well an individual is socially integrated in a (sub)group might vary substantially. This raises the question whether the social integration of a raven affects its stress responses to fission–fusion dynamics. The present study aims to investigate this effect experimentally by separating single ravens (n = 16) individually from their group for four days and subsequently reintroducing them. To determine stress response patterns in the separated individuals we measured the amounts of immunoreactive corticosterone metabolites (CM) in droppings. We compared two enzyme immunoassays, which we validated by conducting an ACTH challenge, and finally decided to apply an 11-oxoetiocholanolone enzyme immunoassay. Additionally, we determined levels of social integration using focal observations. Our findings suggest that a strong social integration is related to low CM levels when the individuals are within the group and high levels during separations, implying that separation leads to stress in these birds. In contrast, poorly socially integrated ravens seem to exhibit the opposite pattern, indicating that to them group living is more stressful than being temporarily separated. We, therefore, conclude that the birds' adrenocortical activity is modulated by their social integration.     

Fox Odour Increases Vigilance in European Rabbits: A Study under Semi-Natural Conditions

The recognition of predator odours is a well‐known mechanism in many prey species which may lead to various behavioural and physiological responses. This has been shown for many mammal species under laboratory conditions, but efforts to validate the results in the field often have led to inconclusive results. We investigated the behavioural reactions and the physiological stress response of European rabbits (Oryctolagus cuniculus) to the odour of a mammalian predator (red fox, Vulpes vulpes) under semi‐natural conditions. The study was conducted on a rabbit population living in an outdoor enclosure of 2 ha. We compared the rates of vigilance and exploration, the time allocated to self‐directed behaviours, the home range sizes and the physiological responses of an experimental and a control group. Only animals from the experimental group were confronted with fox faeces. These animals increased their vigilance rate whereas the control animals did not respond. The increase did not differ between adult and subadult individuals. Furthermore, the experimental animals frequently approached the odour of the predator which might indicate an increase in investigative behaviour. Home‐range size, feeding and other self‐directed behaviours did not change in response to fox odour. Moreover, the animals of the experimental and the control group did not differ in serum corticosterone concentrations (measured after adrenocorticotrophic hormone challenge) that we determined in the beginning and in the end of the experiment. We suggest that the observed behavioural responses represent a low‐cost strategy for lowering the individual risk of predation.     

Individual Variation in Behavioural Reactions to Unfamiliar Conspecific Vocalisation and Hormonal Underpinnings in Male Chimpanzees

It has been established that various species exhibit personality, defined as intra‐individual consistency and inter‐individual variation in behavioural phenotypes. For example, certain individuals may demonstrate consistently greater behavioural reactions and elevated stress responses. We conducted playback experiments and hormonal analyses on male chimpanzees (Pan troglodytes) in captivity to investigate the patterns and proximate mediators of individual variations in behavioural reactions. We found intra‐individual consistency and inter‐individual variation in behavioural reactions (intensive vigilance towards the direction of speakers) to vocalisations by unfamiliar chimpanzees. This behavioural reaction was positively correlated with changes in salivary cortisol concentration, suggesting that stress is a proximate factor mediating the variation in behavioural reactions. The males who were highly responsive to the conspecific vocalisation also exhibited high behavioural reactions towards the neutral broadcast stimulus (the jungle crow’s Corvus macrorhynchos ‘ka’ vocalisation). This observation is consistent with the notion that male chimpanzees vary in intrinsic behavioural tendency to different stimuli.     

Life‐history evolution under fluctuating density‐dependent selection and the adaptive alignment of pace‐of‐life syndromes

We present a novel perspective on life‐history evolution that combines recent theoretical advances in fluctuating density‐dependent selection with the notion of pace‐of‐life syndromes (POLSs) in behavioural ecology. These ideas posit phenotypic co‐variation in life‐history, physiological, morphological and behavioural traits as a continuum from the highly fecund, short‐lived, bold, aggressive and highly dispersive ‘fast’ types at one end of the POLS to the less fecund, long‐lived, cautious, shy, plastic and socially responsive ‘slow’ types at the other. We propose that such variation in life histories and the associated individual differences in behaviour can be explained through their eco‐evolutionary dynamics with population density – a single and ubiquitous selective factor that is present in all biological systems. Contrasting regimes of environmental stochasticity are expected to affect population density in time and space and create differing patterns of fluctuating density‐dependent selection, which generates variation in fast versus slow life histories within and among populations. We therefore predict that a major axis of phenotypic co‐variation in life‐history, physiological, morphological and behavioural traits (i.e. the POLS) should align with these stochastic fluctuations in the multivariate fitness landscape created by variation in density‐dependent selection. Phenotypic plasticity and/or genetic (co‐)variation oriented along this major POLS axis are thus expected to facilitate rapid and adaptively integrated changes in various aspects of life histories within and among populations and/or species. The fluctuating density‐dependent selection POLS framework presented here therefore provides a series of clear testable predictions, the investigation of which should further our fundamental understanding of life‐history evolution and thus our ability to predict natural population dynamics.     

Stress associated with group living in a long-lived bird

Many long-lived avian species adopt life strategies that involve a gregarious way of life at juvenile and sub-adult stages and territoriality during adulthood. However, the potential associated costs of these life styles, such as stress, are poorly understood. We examined the effects of group living, sex and parasite load on the baseline concentration of faecal stress hormone (corticosterone) metabolites in a wild population of common ravens (Corvus corax). Corticosterone concentrations were significantly higher in non-breeding gregarious ravens than in territorial adults. Among territorial birds, males showed higher stress levels than their mates. Parasite burdens did not affect hormone levels. Our results suggest a key role of the social context in the stress profiles of the two population fractions, and that group living may be more energetically demanding than maintaining a territory. These findings have implications for understanding hormonal mechanisms under different life styles and may inspire further research on the link between hormone levels and selective pressures modulating gregarious and territorial strategies in long-lived birds.     

Poor Nutritional Conditions During the Early Larval Stage Reduce Risk‐Taking Activities of Fire Salamander Larvae (Salamandra salamandra)

Environmental conditions experienced early in life have been shown to significantly affect growth trajectories at later stages in many vertebrate species. Amphibians typically have a biphasic life history, with an aquatic larval phase during early development and a subsequent terrestrial adult phase after completed metamorphosis. Thus, the early conditions have an especially strong impact on the future survival and fitness of amphibians. We studied whether early nutritional conditions affect the behavioural reaction of fire salamander larvae (Salamandra salamandra) before completion of metamorphosis. Fire salamander larvae reared under rich nutritional conditions were heavier and larger, displayed better body condition overall throughout the first three month of life and metamorphosed earlier compared with larvae raised under poor nutritional conditions. Specifically, we tested whether larvae reared under these different conditions differed with respect to their risk‐taking behaviour and activity. We found no differences in the activity of larvae with respect to their experienced early food conditions. However, larvae reared under poor nutritional conditions hid significantly more often in a risk‐taking test than larvae reared under rich food conditions. This increase in shelter‐seeking behaviour might be an adaptation to reduce the risk of larval drift or an adaptation to compensate for physiological deficits in part by appropriate behavioural reactions. Our results indicate that environmental conditions, such as food availability, may lead to different behavioural strategies.     

Effects of larval density and food stress on life‐history traits of Cnaphalocrocis medinalis (Lepidoptera: Pyralidae)

High population density and nutrition restriction can lead to phase variation in morphology and development, and subsequently induce changes in the reaction norms of adult flight in migrant insects. However, response of migratory propensity to such stress in Endopterygote insects, especially in several species of Lepidoptera, remains unclear. In this study, larval and adult developmental responses to crowding and food stress were investigated in the migratory moth, Cnaphalocrocis medinalis (Guenée). A high larval rearing density significantly reduced pupal mass, survival rate and female fecundity. Larvae developed rapidly under crowding conditions, and time to pupation was 2 days earlier than individuals reared alone. By contrast, short‐term starvation and associated compensatory growth prolonged larval duration by 3–4 days and pupal duration by 1–2 days. It also reduced the pupal mass, but showed no detectable effects on female reproductive performance. Both sexes had similar development strategies; however, females seemed to be more sensitive to crowding and food shortage than males. A positive effect was expected if such stress factors acted as cues that triggering a behavioural or physiological shift to a distinct migratory phase. To the contrary, we found no proof that crowding and starvation caused maturation delay in female reproductive development. All treatments did not significantly increase female pre‐oviposition period. Therefore, we concluded that life developmental responses to crowding and food shortage in this species were different. Adult migration propensity was not enhanced under such stress conditions during the larval phase.     

Early Stress Causes Sex-Specific,Life-Long Changes in Behaviour,Levels of Gonadal Hormones,and Gene Expression in Chickens

Early stress can have long-lasting phenotypic effects. Previous research shows that male and female chickens differ in many behavioural aspects, and respond differently to chronic stress. The present experiment aimed to broadly characterize long-term sex differences in responses to brief events of stress experienced during the first weeks of life. Chicks from a commercial egg-laying hybrid were exposed to stress by inducing periods of social isolation during their first three weeks of life, followed by a broad behavioural, physiological and genomic characterization throughout life. Early stressed males, but not females, where more anxious in an open field-test, stayed shorter in tonic immobility and tended to have delayed sexual maturity, as shown by a tendency for lower levels of testosterone compared to controls. While early stressed females did not differ from non-stressed in fear and sexual maturation, they were more socially dominant than controls. The differential gene expression profile in hypothalamus was significantly correlated from 28 to 213 days of age in males, but not in females. In conclusion, early stress had a more pronounced long-term effect on male than on female chickens, as evidenced by behavioral, endocrine and genomic responses. This may either be attributed to inherent sex differences due to evolutionary causes, or possibly to different stress related selection pressures on the two sexes during commercial chicken breeding.     

The role of Euglena gracilis paramylon in modulating xylem hormone levels,photosynthesis and water‐use efficiency in Solanum lycopersicum L

β‐1,3‐glucans such as paramylon act as elicitors in plants, modifying the hormonal levels and the physiological responses. Plant hormones affect all phases of the plant life cycle and their responses to environmental stresses, both biotic and abiotic. The aim of this study was to investigate the effects of a root treatment with Euglena gracilis paramylon on xylem hormonal levels, photosynthetic performance and dehydration stress in tomato (Solanum lycopersicum). Paramylon granules were processed to obtain the linear fibrous structures capable to interact with tomato cell membrane. Modulation of hormone levels (abscisic acid, jasmonic acid and salicylic acid) and related physiological responses such as CO₂ assimilation rate, stomatal and mesophyll conductance, intercellular CO₂ concentration, transpiration rate, water‐use efficiency, quantum yield of photosystem II and leaf water potential were investigated. The results indicate a clear dose‐dependent effect of paramylon on the hormonal content of xylem sap, photosynthetic performance and dehydration tolerance. Paramylon has the capability to enhance plant defense capacity against abiotic stress, such as drought, by modulating the conductance to CO₂ diffusion from air to the carboxylation sites and improving the water‐use efficiency.     

Assortative mating patterns of multiple phenotypic traits in a long‐lived seabird

Choosing the right mate is crucial for successful breeding, particularly in monogamous species with long and extensive bi‐parental care, and when the breeding pair is presumed to last many seasons. We investigated the degree of assortative mating in the Little Auk Alle alle, a long‐lived seabird with long‐term pair bonds and bi‐parental care for fixed (morphological) and labile (physiological, behavioural) traits. Using randomization tests, we suggest assortative mating with respect to wing length, extent of the white area on the upper eyelid and hormonal stress response (the difference between stress‐induced and baseline corticosterone levels). We discuss how the assortative mating patterns that we found in the Little Auk may be adaptive.     

Experimental Manipulation of Food Accessibility Affects Conflict Management Behaviour in Ravens

Conflict management strategies such as reconciliation and bystander affiliation have been described for a variety of species. A common determinant seems to be a ‘complex’ social life, with individuals relying on affiliate relationships or social bonds. Little is known, however, about the strategic and flexible use of conflict management skills in experimental settings in species other than primates. We here investigated conflict and post‐conflict behaviour of ravens by manipulating the accessibility of food and, thus, the likelihood of aggressive interactions while foraging. Specifically, we presented birds with a certain amount of highly preferred food that varied in the number of pieces (one piece, two pieces or, as a control, small pieces matching the number of participating birds) and observed their agonistic behaviour during feeding and their affiliative behaviour afterwards. The results showed that high levels of conflicts during feeding in the 1‐piece and 2‐piece conditions led to high levels of affiliation after feeding. Depending on the experimental condition, this effect is best explained (a) by the affiliative behaviour of former aggressors (1‐piece condition) and (b) by the affiliation directed to the receivers of aggression after feeding (2‐piece condition). Those dyads that engaged in allo‐preening after feeding also engaged in allo‐preening outside the experimental setting, suggesting that socially bonded individuals provided third‐party affiliation to victims of aggression. Moreover, socially bonded ravens fed close to each other in the experiment when food was clumped, indicating that they actively coordinated their behaviour when there was a high conflict potential. Taken together, these findings support the assumption that ravens use their social bonds to avoid conflicts by choosing with whom to feed, and to buffer effects of conflicts by engaging in third‐party affiliation as post‐conflict behaviour.     

Behavioural and hormonal stress responses during chick rearing do not predict brood desertion by female in a small Arctic seabird

We examined behavioural and hormonal stress responses in a small seabird (little auk, Alle alle), which exhibits a transition from biparental to male-only care towards the end of the nesting period, in order to understand the mechanisms underlying this parental strategy. We hypothesized that the male staying with the chick should be less sensitive to stressors. As such the male might offer the offspring more efficient protection during the fledging period than the female. We tested this hypothesis by observing male and female behaviour in a neophobia test. We also measured the birds' baseline and stress-induced levels of corticosterone and prolactin using the standardized capture-and-restraint protocol. Both sexes respond rapidly to foreign objects, delaying the entry time to the nest with food, consuming the food load, and/or temporarily abandoning feeding. However, we did not find any differences between the sexes in the frequency of each behaviour or in the time of the first reaction to the experimental treatment. Level of both corticosterone and prolactin increased after the experimental treatment. However, we did not find sex differences in baseline and stress-induced hormone levels. The results indicate that the males are as much sensitive to the stress situation as the females. Thus, the pattern of male and female behavioural and hormonal responses to stress does not predict their behaviour at the final breeding stage.     

High temperatures change the perspective: Integrating hormonal responses in citrus plants under co‐occurring abiotic stress conditions

Plants growing in the field are subjected to multiple stress factors acting simultaneously. Abnormally high temperatures are expected to affect wild plants and crops in the next years due to global warming. In this work, we have studied physiological, hormonal and molecular responses of the citrus rootstock, Carrizo citrange (Poncirus trifoliata L. Raf. × Citrus sinensis L. Osb.) subjected to wounding or high salinity occurring individually or in combination with heat stress. According to our results, combination of high salinity and heat stress aggravated the negative effects of salt intoxication in Carrizo. The high transpiration rate caused by high temperatures counteracted physiological responses of plants to salt stress and increased Cl^? intake in leaves. In addition, 12‐oxo‐phytodienoic acid accumulated specifically under combination of wounding and heat stress, whereas at low temperatures, wounded plants accumulated jasmonic acid (JA) and JA‐isoleucine (JA‐Ile). Moreover, an antagonism between salicylic acid (SA) and JA was observed, and wounded plants subjected to high temperatures did not accumulate JA nor JA‐Ile whereas SA levels increased (via isochorismate synthase biosynthetic pathway). Wounded plants did not accumulate abscisic acid (ABA) but its catabolite phaseic acid. This could act as a signal for the upregulation of (ABA)‐RESPONSIVE ELEMENT (ABRE)‐BINDING TRANSCRIPTION FACTOR 2 (CsAREB2) and RESPONSIVE TO DISSECATION 22 (CsRD22) in an ABA‐independent way. This work uncovers some mechanisms that explain Carrizo citrange tolerance to high temperatures together with different hormonal signals in response to specific stresses. It is suggested that co‐occurring abiotic stress conditions can modify (either enhance or reduce) the hormonal response to modulate specific responses.     

Food level and sex shape predator-induced physiological stress: immune defence and antioxidant defence

Despite the potential impact on prey fitness and predator–prey interactions, most studies of predation risk ignore physiological responses and their dependence upon food level and sex. Therefore, we reared male and female larvae of the damselfly Lestes viridis under predator stress (dragonfly larvae) at high and low food levels, and subsequently scored for important variables of insect immune defence (i.e. phenoloxidase) and antioxidant defence [i.e. superoxide dismutase, and catalase (CAT)]. Under predation risk, larvae did not decrease growth rate or immune defence, and only slightly reduced food intake in the high food treatment, probably because of time stress, i.e. little time available to complete the larval development. However, larvae facing predator stress did show an upregulation of antioxidant enzymes. This upregulation was dependent upon food level for CAT and both food level and sex for SOD, consistent with energetic constraints and sex differences in the link between longevity and adult fitness. Our results illustrate that predator stress can influence life history, behavioural and physiological responses differentially and in a context-dependent way. This implies that non-consumptive physiological effects of predators on their prey show independent yet similar complexities in behavioural and life history response variables. In general, our results advocate that mechanistic studies on predator–prey interactions may benefit from including physiological variables.     

Repeated maternal separation in the neonatal rat: cellular mechanisms contributing to brain growth sparing.

Separation of neonates from their dam has been shown to evoke acutely a variety of biochemical responses, typified by depression of ornithine decarboxylase (ODC) activity. In the current study where rat pups were removed from their nursing dams for 6 h, ODC activities in the liver, heart, kidney and lung were markedly suppressed, but the enzyme in the brain was not altered during the early postnatal ages. These data suggest that the brain was protected from maternal separation insults, a homeostatic response mediated in part, by an increase of circulating corticosterone and glycogen mobilization from peripheral tissues, particularly the liver. In addition, we examined whether these responses were extended to pups who were subject to repeated episodes of maternal deprivation, and whether this stress paradigm might be associated with corresponding changes of cellular growth and maturation. Pups were removed from their dams for 6 h daily beginning at 4 days of age until weaning at 21 days. Plasma corticosterone levels of the deprived pups were elevated significantly at the end of each stress episode but returned to basal (control) levels subsequently. The repeating stress paradigm did not influence the magnitude of this hormonal response at the ensuing ages. Consistent with findings observed in the single episodes of stress, ODC activities in the peripheral tissues were significantly depressed in pups subject to repeated maternal deprivation, but the enzyme appeared to recover to control levels 18 h after each insult. In contrast, brain ODC activity did not exhibit any change throughout the period examined. Moreover, while ontogenetic gains of DNA and protein in the peripheral tissues of the deprived rats lagged slightly but consistently behind those of controls, these macromolecules in the brain were not affected appreciably. These results thus suggest that brain growth was, by and large, spared from insults associated with repeated maternal separation; but this stressful paradigm did produce marked, though reversible biochemical and physiological responses in the peripheral tissues of neonates, which cumulatively led to a lag of cellular development.     

Socially Driven Consistent Behavioural Differences during Development in Common Ravens and Carrion Crows

Consistent individual differences in behaviour, or ‘personality’, are likely to be influenced by development, social context, and species ecology, though few comparative, longitudinal studies exist. Here, we investigated the role of development and social context on personality variation in two identically reared, social corvids: common ravens and carrion crows. We repeatedly presented subjects with a variety of novel food and objects, while alone and in a primarily sibling subgroup, from fledging to sub-adulthood. We predicted that consistent individual differences would emerge later in development, and that conspecific presence would facilitate behavioural similarities. In contrast to our predictions, we found that individuals of both species were highly inconsistent in their behavioural responses throughout the development period. In line with our predictions, though in the ravens only, conspecific presence promoted behavioural similarities as individuals were strongly shaped by their subgroup, and it is likely that these effects were driven by social context rather than relatedness. We discuss these findings in relation to developmental steps and the role of social relations in these species. Overall, our findings highlight that these two species are highly adaptable in their behaviour, and the ravens in particular are strongly influenced by their social environment, which may facilitate cooperation and social learning.     

Effect of rearing style on the development of social behaviour in young ravens (Corvus corax)

Early social experiences can affect the development and expression of individual social behaviour throughout life. In particular, early-life social deprivations, notably of parental care, can later have deleterious consequences. We can, therefore, expect rearing procedures such as hand-raising—widely used in ethology and socio-cognitive science—to alter the development of individual social behaviour. We investigated how the rearing style later affected (a) variation in relationship strength among peers and (b) individuals’ patterns of social interactions, in three captive groups of juvenile non-breeders consisting of either parent-raised or hand-raised birds, or a mix of both rearing styles. In the three groups, irrespectively of rearing style: strongest relationships (i.e., higher rates of association and affiliations) primarily emerged among siblings and familiar partners (i.e., non-relatives encountered in early life), and mixed-sex and male–male partners established relationships of similar strength, indicating that the rearing style does not severely affect the quality and structure of relationships in young ravens. However, compared to parent-raised ravens, hand-raised ravens showed higher connectedness, i.e., number of partners with whom they mainly associated and affiliated, but formed on average relationships of lower strength, indicating that social experience in early life is not without consequences on the development of ravens’ patterns of social interaction. The deprivation of parental care associated with the presence of same-age peers during hand-raising seemed to maximize ravens’ propensity to interact with others, indicating that besides parents, interactions with same-age peers matter. Opportunities to interact with, and socially learn from peers, might thus be the key to the acquisition of early social competences in ravens.     

To change or not to change experimenters: caveats for repeated behavioural and physiological measures in Montagu's harrier

Animals facing anthropogenic disturbances may exhibit different anti‐predator responses depending on their perception of the risk. Experimental measures of behavioural and physiological traits may be impacted by the disturbance due to experimenter handlings and activities. In this study, we assessed the behavioural and physiological responses of Montagu's harrier chicks Circus pygargus from 14 nests, visited four times from hatching to fledgling by either the same (group I) or different (group NI) experimenters. Escape, aggressiveness and stress‐induced corticosterone concentration increased for both groups over time but chicks from group I showed a higher increase in their behavioural and physiological responses than chicks from group NI. This increase occurred after the second visit when experimenters took the first blood sampling on nestlings. Handling by the same experimenters throughout the study did not result in habituation but in sensitization. Our results suggest that Montagu's harrier nestlings may discriminate between the people who visit their nest and respond to experimenters’ identity. Therefore, we urge researchers working on behaviour and/or the physiology of stress including repeated measurements (such as animal personality studies for instance) to consider whether the experiments should or should not be conducted by the same experimenters (as usually done to minimize bias) as it can influence the outcomes of the experiments.     

Rehabilitation of research chimpanzees: stress and coping after long-term isolation

We report on the permanent retirement of chimpanzees from biomedical research and on resocialization after long-term social isolation. Our aim was to investigate to what extent behavioral and endocrine measures of stress in deprived laboratory chimpanzees can be improved by a more species-typical social life style. Personality in terms of novelty responses, social dominance after resocialization and hormonal stress susceptibility were affected by the onset of maternal separation of infant chimpanzees and duration of deprivation. Chimpanzees, who were separated from their mothers at a younger age and kept in isolation for more years appeared to be more timid personalities, less socially active, less dominant and more susceptible to stress, as compared to chimpanzees with a less severe deprivation history. However, permanent retirement from biomedical research in combination with therapeutic resocialization maximizing chimpanzees' situation control resulted in reduced fecal cortisol metabolite levels. Our results indicate that chimpanzees can recover from severe social deprivation, and may experience resocialization as less stressful than solitary housing.     

Boldness behavior and stress physiology in a novel urban environment suggest rapid correlated evolutionary adaptation

Novel or changing environments expose animals to diverse stressors that likely require coordinated hormonal and behavioral adaptations. Predicted adaptations to urban environments include attenuated physiological responses to stressors and bolder exploratory behaviors, but few studies to date have evaluated the impact of urban life on codivergence of these hormonal and behavioral traits in natural systems. Here, we demonstrate rapid adaptive shifts in both stress physiology and correlated boldness behaviors in a songbird, the dark-eyed junco, following its colonization of a novel urban environment. We compared elevation in corticosterone (CORT) in response to handling and flight initiation distances in birds from a recently established urban population in San Diego, California to birds from a nearby wildland population in the species' ancestral montane breeding range. We also measured CORT and exploratory behavior in birds raised from early life in a captive common garden study. We found persistent population differences for both reduced CORT responses and bolder exploratory behavior in birds from the colonist population, as well as significant negative covariation between maximum CORT and exploratory behavior. Although early developmental effects cannot be ruled out, these results suggest contemporary adaptive evolution of correlated hormonal and behavioral traits associated with colonization of an urban habitat.