Nests in the cities: adaptive and non-adaptive phenotypic plasticity and convergence in an urban bird

Phenotypic plasticity plays a critical role in adaptation to novel environments. Behavioural plasticity enables more rapid responses to unfamiliar conditions than evolution by natural selection. Urban ecosystems are one such novel environment in which behavioural plasticity has been documented. However, whether such plasticity is adaptive, and if plasticity is convergent among urban populations, is poorly understood. We studied the nesting biology of an ‘urban-adapter’ species, the dark-eyed junco (Junco hyemalis), to understand the role of plasticity in adapting to city life. We examined (i) whether novel nesting behaviours are adaptive, (ii) whether pairs modify nest characteristics in response to prior outcomes, and (iii) whether two urban populations exhibit similar nesting behaviour. We monitored 170 junco nests in urban Los Angeles and compared our results with prior research on 579 nests from urban San Diego. We found that nests placed in ecologically novel locations (off-ground and on artificial surfaces) increased fitness, and that pairs practiced informed re-nesting in site selection. The Los Angeles population more frequently nested off-ground than the San Diego population and exhibited a higher success rate. Our findings suggest that plasticity facilitates adaptation to urban environments, and that the drivers behind novel nesting behaviours are complex and multifaceted.     

Acorn mast drives long-term dynamics of rodent and songbird populations

Resource pulses can have cascading effects on the dynamics of multiple trophic levels. Acorn mast is a pulsed resource in oak-dominated forests that has significant direct effects on acorn predators and indirect effects on their predators, prey, and pathogens. We evaluated changes in acorn mast, rodent abundance, raptor abundance, and reproductive success of a ground-nesting songbird over a 24-year period (1980–2004) in the southern Appalachian Mountains in an effort to determine the relationships among the four trophic levels. In particular, we examined the following: acorn mast from red oaks (Quercus rubra) and white oaks (Q. alba), abundance of white-footed mice (Peromyscus leucopus) and deer mice (P. maniculatus), population estimates of seven raptor species from three feeding guilds, and nest failure and number of juveniles of dark-eyed juncos (Junco hyemalis). Finally, we recorded seasonal temperature and precipitation to determine the effects of weather on each trophic level. We found that weather patterns had delayed effects of up to 3 years on these trophic interactions. Variation in acorn mast, the keystone resource in this community, was explained by weather conditions as far back as 2 years before the mast event. Acorn mast, in turn, was a strongly positive predictor of rodent abundance the following year, whereas spring and summer temperature and raptor abundance negatively affected rodent abundance. Dark-eyed junco nests were more likely to fail in years in which there were more rodents and raptors. Nest failure rate was a strong predictor of the number of juvenile juncos caught at the end of the summer. Our results improve our understanding of the complex ecological interactions in oak-dominated forests by illustrating the importance of abiotic and biotic factors at different trophic levels. Ethan D. Clotfelter and Amy B. Pedersen contributed equally to the writing of this paper.     

Nesting Ecology of Boreal Forest Birds Following a Massive Outbreak of Spruce Beetles

Abstract: We studied breeding dark-eyed juncos (Junco hyemalis), yellow-rumped warblers (Dendroica coronata), and spruce-nesting birds from 1997 to 1998 among forests with different levels of spruce (Picea spp.) mortality following an outbreak of spruce beetles (Dendroctonus rufipennis) in Alaska, USA. We identified species using live and beetle-killed spruce for nest sites and monitored nests to determine how the outbreak influenced avian habitat selection and reproduction. We tested predictions that 1) nesting success of ground-nesting juncos would increase with spruce mortality due to proliferation of understory vegetation available to conceal nests from predators, 2) nesting success of canopy-nesting warblers would decrease with spruce mortality due to fewer live spruce in which to conceal nests, and 3) both species would alter nest-site selection in response to disturbance. Juncos did not benefit from changes in understory vegetation; nesting success in highly disturbed stands (46%) was comparable to that in undisturbed habitats throughout their range. In stands with low spruce mortality, nesting success of juncos was low (5%) and corresponded with high densities of red squirrels (Tamiasciurus hudsonicus). Yellow-rumped warblers nested exclusively in spruce, but success did not vary with spruce mortality. As disturbance increased, nesting warblers switched from selecting forest patches with high densities of live white spruce (Picea glauca) to patches with beetle-killed spruce. Warblers also placed nests in large-diameter live or beetle-killed spruce, depending on which was more abundant in the stand, with no differences in nesting success. Five of the 12 other species of spruce-nesting birds also used beetle-killed spruce as nest sites. Because beetle-killed spruce can remain standing for >50 years, even highly disturbed stands provide an important breeding resource for boreal forest birds. We recommend that boreal forest managers preserve uncut blocks of infested forest within managed forest landscapes and practice partial harvest of beetle-killed spruce rather than commercial clear-cutting of infested stands in order to sustain breeding bird populations until natural reforestation occurs. Because breeding densities do not always reflect fitness, assessing impacts of a massive natural disturbance should include measuring impacts of changes in vegetation on both reproductive success and predator-prey dynamics.     

Birds adjust acoustic directionality to beam their antipredator calls to predators and conspecifics

Animals in many vertebrate species vocalize in response to predators, but it is often unclear whether these antipredator calls function to communicate with predators, conspecifics or both. We evaluated the function of antipredator calls in 10 species of passerines by measuring the acoustic directionality of these calls in response to experimental presentations of a model predator. Acoustic directionality quantifies the radiation pattern of vocalizations and may provide evidence about the receiver of these calls. We predicted that antipredator calls would have a lower directionality if they function to communicate with surrounding conspecifics, and a higher directionality and aimed at the receiver if they function to communicate with the predator. Our results support both of these functions. Overall, the birds produce antipredator calls that have a relatively low directionality, suggesting that the calls radiate in many directions to alert conspecifics. However, birds in some species increase the directionality of their calls when facing the predator. They can even direct their calls towards the predator when facing lateral to it—effectively vocalizing sideways towards the predator. These results suggest that antipredator calls in some species are used to communicate both to conspecifics and to predators, and that birds adjust the directionality of their calls with remarkable sophistication according to the context in which they are used.     

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.     

Androgen control of vocal control region volumes in a wild migratory songbird (Junco hyemalis) is region and possibly age dependent

Previous laboratory studies have shown that photoperiodic adult songbirds experience seasonal variations in singing frequency that correlate with plasma androgen levels, as well as changes in the brain regions that control singing (vocal control regions). The present study investigates naturally occurring seasonal changes in the sizes of these regions in a wild migratory species (dark-eyed junco, Junco hyemalis), with samples from adolescence to post-breeding fall migration. In adult males, the volumes of the vocal control regions area X and the higher vocal center (HVC) were large during the breeding season when birds were singing and androgen levels were high, and decreased in size after the breeding season when singing had stopped and androgen levels were low. HVC volume in adolescent males caught in the fall (no singing), when plasma androgen levels were low, was smaller than in breeding adults, thereby following the seasonal pattern of change in plasma androgen levels. In adolescent males, however, area X volume was the same as in breeding adults. Thus, area X size in adolescent male juncos may be testosterone independent. The seasonal pattern of robust nucleus of the archistriatum volume was similar to that of the HVC. The volumes of neither the magnocellular nucleus of the anterior neostriatum nor the nucleus rotundus, a control region, differed seasonally. Castration of breeding adult males caused both area X and HVC volumes to decrease compared to castrated controls with testosterone replacement, indicating that maintenance of these two region volumes is testosterone dependent in adults. © 1997 John Wiley & Sons, Inc. J Neurobiol 32: 391–402, 1997.     

Photoperiod and testosterone independently affect vocal control region volumes in adolescent male songbirds

Previously, we found that, unlike adults, adolescent male dark-eyed juncos (Junco hyemalis) maintained large Area X volumes despite having low plasma testosterone concentrations. Other studies indicate that photoperiod may act independently of testosterone to modulate vocal control region (VCR) volumes in adult songbirds. In the present study, we investigated the effects of testosterone and photoperiod on the volumes of four VCRs in adolescent male juncos. To test the hypothesis that VCR volumes in these males are testosterone independent, we treated birds exposed to short days with testosterone and later compared their VCR volumes with those of birds exposed to short days without testosterone. To examine whether photoperiod alone could affect VCR volumes independent of testosterone, we measured these volumes in photorefractory birds exposed to long photoperiod without testosterone. Administering testosterone induced singing, yet increased the volume of only one VCR, the robust nucleus of the anterior neostriatum (RA). In contrast, long photoperiod increased several VCR volumes (Area X, higher vocal center, and RA) despite low testosterone levels, but did not induce singing. Our results suggest a limited role for testosterone, but an important role for photoperiod, in controlling VCR volumes in adolescent male juncos. In addition, the results demonstrate that singing behavior can be induced in adolescent males without a concomitant increase in most VCR volumes. © 1998 John Wiley & Sons, Inc. J Neurobiol 36: 550–558, 1998     

Individual variation in testosterone and parental care in a female songbird; The dark-eyed junco (Junco hyemalis)

When competition for sex-specific resources overlaps in time with offspring production and care, trade-offs can occur. Steroid hormones, particularly testosterone (T), play a crucial role in mediating such trade-offs in males, often increasing competitive behaviors while decreasing paternal behavior. Recent research has shown that females also face such trade-offs; however, we know little about the role of T in mediating female phenotypes in general, and the role of T in mediating trade-offs in females in particular. Here we examine the relationship between individual variation in maternal effort and endogenous T in the dark-eyed junco, a common songbird. Specifically, we measure circulating T before and after a physiological challenge (injection of gonadotropin releasing hormone, GnRH), and determine whether either measure is related to provisioning, brooding, or the amount of T sequestered in egg yolk. We found that females producing more T in response to a challenge spent less time brooding nestlings, but provisioned nestlings more frequently, and deposited more T in their eggs. These findings suggest that, while T is likely important in mediating maternal phenotypes and female life history tradeoffs, the direction of the relationships between T and phenotype may differ from what is generally observed in males, and that high levels of endogenous T are not necessarily as costly as previous work might suggest.     

Inferring performance in the songs of dark-eyed juncos (Junco hyemalis)

Within bird species, songs differ in their attractiveness tofemales or effectiveness in male–male interactions. Somesongs are more difficult to sing than others, and receiversmay use a singer's performance of difficult songs as a meansfor evaluating the quality of the singer. The concept of songperformance aims at quantifying how physiologically demandingare different songs. Using variation between song types of dark-eyedjuncos, Junco hyemalis, we show that some song traits tradeoff with costly aspects of song output—short intervalsbetween syllables or loud sound amplitude—suggesting thatthose traits are difficult to sing. First, after controllingfor other traits, long syllables require longer intervals forrecovery. This supports the idea that a measure of "respiratoryperformance" could be based on the relative lengths of syllablesand intervals. Second, some syllable traits trade off stronglywith sound amplitude, suggesting that these traits may be difficultto sing at high amplitudes. The ratio of frequency bandwidthand trill rate has been used to infer performance in other birdspecies, but we found no evidence that frequency bandwidth tradesoff with any aspect of song output in the junco. The negativeassociation of bandwidth with trill rate may instead be a passiveconsequence of syllable length, with longer syllables randomlyaccumulating frequency modulation. We conclude that bird receiversmay best evaluate how well a song is performed if they integratemultiple cues and discuss how researchers may similarly devisemeasures of song performance.