Song dialects,mate selection,and breeding success in white-crowned sparrows

The songs of 15 colour-marked, mated pairs of white-crowned sparrows (Zonotrichia leucophrys nuttalli) were recorded in a contact zone between the Presidio and San Francisco, California dialect regions. Males were recorded in the field and females were trapped after the breeding season and injected with testosterone to induce singing. The song of each member of a mated pair was classified using two different markers. It was found that most pairs did not match song types, and that the reproductive success of those that matched did not differ from those that did not match. Most of the territorial males sang the Presidio dialect, while most injected females sang the San Francisco dialect. The results fail to support a positive assortative mating hypothesis regarding the functional significance of learning dialects. It is suggested that the mismatching of songs could be due to sexual differences in the disposition to learn songs of neighbours at the breeding site.     

Song learning by prairie warblers: When,where, and from whom

Songbirds of many species acquire their songs by imitating the songs of conspecific singers. Conclusive evidence of such imitation comes from controlled laboratory studies, but such studies do not reveal when and where songbirds learn their songs under natural conditions. To determine the timing and location of song learning in a population of prairie warblers, we compared the songs of yearling prairie warblers of known hatching location to the songs of other birds in the yearlings' natal and first breeding areas. The comparisons yielded a likely model song (and model singer) for each of the song types used by the focal yearlings. We supplemented our findings from the song comparisons with inferences drawn from an analysis of local geographic variation in songs. This analysis revealed that shared song types showed no tendency to be geographically clustered within the study area. Taken together, our data suggest that prairie warblers learn their songs during the hatch year, at locations somewhat distant (mean distance 1,437 m) from their natal site, most likely as birds wander about during the post-fledging period.     

Aspects of risk-aversion in foraging white-crowned sparrows

Two series of aviary experiments with white-crowned sparrows (Zonotrichia leucophrys) are reported. The first series attempts to mimic foraging choices between microhabitats. The second series examines foraging choices made within a single artificial patch, a design intended to resemble dietary selection. In all experiments subjects chose between a constant food reward and a variable reward, the mean of which equalled the constant reward. Each experiment allowed the bird to feed at an overall rate greater than that minimally required for daily energy balance. In both series of experiments, the birds usually preferred the constant reward. Results of the first series do not differ significantly from previously reported preferences of dark-eyed juncos (Junco hyemalis), birds weighing 10 g less than the white-crowned sparrows.     

Energy metabolism, testosterone and corticosterone in white-crowned sparrows

The influence of the steroid hormones testosterone and corticosterone on energy metabolism and activity of birds is largely enigmatic. We measured resting metabolic rate during night and day in 12 long-term castrated and 12 intact male white-crowned sparrows (Zonotrichia leucophrys gambelii) under short-day (8:16 SD), long-day (20:4 LD), LD+testosterone implant and LD−testosterone implant conditions. Each male was sequentially measured under all four conditions. Photostimulation increased testosterone, resting metabolic rate, food intake, hopping activity and body mass in castrates and intact males. Surprisingly, testosterone levels and metabolic rates did not differ between intact and castrated males. Testosterone implantation increased activity and food intake, but decreased body mass and resting metabolic rate in both groups. Removing testosterone implants reversed the effects on resting metabolic rate, activity and food intake. Corticosterone levels, measured immediately at the end of metabolism measurements, showed birds were not stressed. Corticosterone had no apparent relationship with resting metabolic rate and there was no interaction between corticosterone and testosterone. Overall, positive changes in testosterone levels resulted in a decrease of resting metabolic rate. We speculate that testosterone increases activity, and birds compensate for increased activity metabolism by reducing resting metabolic rate. Accepted: 18 July 1999     

Seasonal changes in song nuclei and song behavior in Gambel's white-crowned sparrows

In males of several songbird species, the morphology of forebrain nuclei that control song changes seasonally. The only seasonally breeding songbird in which seasonal changes in the structure of song control nuclei have been reported not to occur is the nonmigratory Nuttall's subspecies of white-crowned sparrow. In the present study, we manipulated photoperiod and plasma testosterone concentrations in captive male white-crowned sparrows of the migratory Gambel's subspeices. Males exposed to photoperiods and plasma testosterone concentrations typical of those experienced by wild breeding males had larger song control nuclei than males held on a winter photoperiod. We also found seasonal change in stereotypy of spectral and temporal parameters of song in wild Gambel's white-crowned sparrows. We hypothesize that seasonal changes in song control nuclei may correlate with seasonal changes in song stereotypy. © 1995 John Wiley & Sons, Inc.     

Regulation of vernal migration in Gambel's white-crowned sparrows: Role of thyroxine and triiodothyronine

Appropriate timing of migratory behavior is critical for migrant species. For many temperate zone birds in the spring, lengthening photoperiod is the initial cue leading to morphological, physiological and behavior changes that are necessary for vernal migration and breeding. Strong evidence has emerged in recent years linking thyroid hormone signaling to the photoinduction of breeding in birds while more limited information suggest a potential role in the regulation of vernal migration in photoperiodic songbirds. Here we investigate the development and expression of the vernal migratory life history stage in captive Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii) in a hypothyroidic state, induced by chemical inhibition of thyroid hormone production. To explore possible variations in the effects of the two thyroid hormones, triiodothyronine and thyroxine, we subsequently performed a thyroid inhibition coupled with replacement therapy. We found that chemical inhibition of thyroid hormones resulted in complete abolishment of mass gain, fattening, and muscle hypertrophy associated with migratory preparation as well as resulting in failure to display nocturnal restlessness behavior. Replacement of thyroxine rescued all of these elements to near control levels while triiodothyronine replacement displayed partial or delayed rescue. Our findings support thyroid hormones as being necessary for the expression of changes in morphology and physiology associated with migration as well as migratory behavior itself.     

Genetic analysis of song dialect populations in Puget Sound white-crowned sparrows

The relationship between cultural variation and biological variationamong natural populations has been the subject of both theoreticaland empirical study. Zonotrichia leucophrys pugetensis is oneof three subspecies of white-crowned sparrow known to form geographicalsong dialects. We investigated whether these dialects correspondto genetic differences among Z. l. pugetensis populations. Wecompared allele frequencies at four microsatellite loci in malesfrom 11 sites spanning six dialects over the subspecies' rangein Oregon and Washington. Cluster analysis and genotype assignmenttests indicated no tendency for sample sites within dialectareas to be genetically more similar than are sites from differentdialect areas. AMOVA tests revealed high within-site variationand low but significant cross-site and cross-dialect-area variation.Finally, genetic distance between sites was not correlated withdialect differences when the effect of geographic distance wascontrolled statistically. We compare our finding of low geneticdifferentiation among Z. l. pugetensis dialect populations toresults of previous studies on Z. l. nuttalli and Z. l. oriantha.Because genetic structuring appears weaker than cultural (songdialect) structure in this species, we discuss the behavioralmechanisms underlying dialect maintenance in the presence ofapparent gene flow.     

Cultural and genetic evolution in mountain white-crowned sparrows: song dialects are associated with population structure

Bird song often varies geographically within a species; when this geographic variation has distinct boundaries, the shared song types are referred to as song dialects. How dialects are produced and their adaptive significance are longstanding problems in biology, with implications for the role of culture in the evolution and ecology of diverse organisms, including humans. Here we test the hypothesis that song dialect, a culturally transmitted trait, is related to the population genetic structure of mountain white-crowned sparrows (Zonotrichia leucophrys oriantha). To address this, we compared microsatellite allele frequencies from 18 sample sites representing eight dialect regions in the Sierra Nevada. Pairwise genetic distances were not significantly correlated with geographic distances either within or between dialects, nor did dialect groups form distinct genetic groups according to neighbor-joining or UPGMA analysis, and most variation in allele frequencies occurred among individuals rather than at higher levels. However, most of the remaining variation was attributable to differences among, rather than within, dialect regions, and this among-dialect component of variance was statistically significant. Moreover, when controlling for the effect of geographic distance, song dissimilarity and genetic distance between site pairs were significantly correlated. Thus, song dialects appear to be associated with reductions in, but not strict barriers to, gene flow among dialect regions.     

Song development in the white-crowned sparrow: social factors and sex differences

Results of earlier studies indicated that hand-raised white-crowned sparrows exposed to taperecorded songs learned conspecific song between ages 10–50 days, but not before or after that age. These studies also indicated that allospecific songs were not learned. We describe song development in 41 male and 22 female hand-raised white-crowned sparrows. Thirty males and 15 females were exposed to a live adult singing male. It was found that most male students learned the song of their live tutor even though tutoring was begun at 50 days of age, an age by which young would have dispersed from the natal to the breeding area. Male students learned allospecific song as easily as they did conspecific song, even though conspecific song was present in the laboratory. Only three females copied any part of the song of either conspecific or allospecific live tutors. Six 50-day-old males and seven females were exposed to taperecorded song and none learned the tutor song. These results indicate that there are sex differences in song learning, and that, if live tutors are used, the sensitive phase for male song learning extends beyond 50 days of age. We conclude that social interaction can override any auditory gating mechanism that prevents inappropriate stimuli from influencing song learning centres.     

Song development in the white-crowned sparrow: modification of learned song

Previous research has shown that laboratory-tutored white-crowned sparrows, Zonotrichia leucophrys nuttalli, can learn tape-recorded song only until they are about 50 days of age. However, they can learn either conspecific or allospecific song after they are 50 days of age when a live tutor is used. Because the live-tutored birds had not been exposed to song during the first 50 days of life, a question regarding the reasonableness of applying these results to the natural, field situation can be raised. In the experiments reported here, hand-raised nestling white-crowned sparrows between 10 and 50 days of age were tutored in the laboratory with one song, and were then placed with a live tutor singing a different dialect when they were over 50 days of age. Eight of 13 males, and none of 10 females, adopted the song of the second tutor. Birds were also captured in the field as fledglings, and were placed with a live tutor singing a song different from that of the natal area: three of seven males, and one of five females, adopted the song of the live tutor. Four birds that had been group-isolated (two males and two females) were exposed to a live tutor when they were 100 days old, and none acquired normal song; all sang isolate song. The results of these studies indicate that there is considerable plasticity in the song learning of whitecrowned sparrows, and that, for many individuals, song can be modified quite readily. The implications of these findings for the nature of the sensitive phase and for mechanisms of song learning are discussed.     

Re-expression of songs deleted during vocal development in white-crowned sparrows, Zonotrichia leucophrys

White-crowned sparrows learn and produce multiple song types as juveniles, but most individuals stop singing all except one by the end of the first singing season. This single song type is generally maintained throughout adulthood. We demonstrate that, at the start of the second and subsequent singing seasons, this species can recall songs that had been deleted during the first singing season. The re-expression of song occurred in both the oriantha and the gambelii subspecies. Although all our males recrystallized the original song in the second year, our results indicate a mechanism for seasonal song change without new song memorization. The traditional dichotomy of closed-ended versus open-ended learning is inadequate for birds that learn early in life but can change their song output seasonally. We suggest that species can exhibit a closed sensitive period for song memorization and first production, with the ability to recall deleted songs later in life. This type of learning, selective attrition followed by subsequent re-expression, may be used by some species currently considered open-ended learners. Copyright 2000 The Association for the Study of Animal Behaviour.     

Dramatic orientation shift of white-crowned sparrows displaced across longitudes in the high Arctic

Advanced spatial-learning adaptations have been shown for migratory songbirds, but it is not well known how the simple genetic program encoding migratory distance and direction in young birds translates to a navigation mechanism used by adults. A number of convenient cues are available to define latitude on the basis of geomagnetic and celestial information, but very few are useful to defining longitude. To investigate the effects of displacements across longitudes on orientation, we recorded orientation of adult and juvenile migratory white-crowned sparrows, Zonotrichia leucophrys gambelii, after passive longitudinal displacements, by ship, of 266-2862 km across high-arctic North America. After eastward displacement to the magnetic North Pole and then across the 0 degrees declination line, adults and juveniles abruptly shifted their orientation from the migratory direction to a direction that would lead back to the breeding area or to the normal migratory route, suggesting that the birds began compensating for the displacement by using geomagnetic cues alone or together with solar cues. In contrast to predictions by a simple genetic migration program, our experiments suggest that both adults and juveniles possess a navigation system based on a combination of celestial and geomagnetic information, possibly declination, to correct for eastward longitudinal displacements.     

Effects of aggressive encounters on plasma corticosteroid-binding globulin and its ligands in white-crowned sparrows

In birds, corticosteroid-binding globulin (CBG) binds corticosterone, progesterone and testosterone. The concentration of each ligand can alter the binding of the other ligands through competitive interactions. Thus, an increase in corticosterone or progesterone may displace testosterone bound to CBG, leading to an increase in bioactive free testosterone levels without affecting total testosterone levels in the circulation. Aggressive interactions increase plasma total testosterone levels in some birds but not in others. Here, we tested the hypothesis that aggressive encounters in the late breeding season would not increase total testosterone levels in plasma, but would alter CBG, total corticosterone or total progesterone levels in such a way as to modify the number of available binding sites and therefore occupancy by testosterone. A marked decrease in CBG occupancy by testosterone would indirectly suggest an increase in free testosterone levels in plasma. Wild male white-crowned sparrows were exposed to a simulated territorial intrusion (STI) or control for 30 min. Subjects were then caught and bled. We measured CBG using a ligand-binding assay and corticosterone, progesterone and testosterone using highly sensitive radioimmunoassays. STI significantly increased aggressive behaviors but did not affect plasma total testosterone levels. STI significantly increased plasma CBG and total corticosterone levels and decreased plasma total progesterone levels. We predict that CBG occupancy by corticosterone will increase slightly following an aggressive encounter. However, this small change is unlikely to increase free testosterone levels, because of the large number of seemingly unoccupied CBG binding sites in these subjects.     

Signals of status in wintering white-crowned sparrows, Zonotrichia leucophrys gambelii

The possibility of plumage status signalling within the social systems of wintering birds has been a controversial issue. Our results are the first to demonstrate conclusively the reality of such signalling. Data from eight groups of captive white-crowned sparrows (Zonotrichia leucophrys gambelii), each with 8 to 11 different individuals, show that immature and adult females with crowns painted to resemble more brightly coloured, dominant adult males consistently win encounters with control birds of their own age and sex. These experiments demonstrate that signals that correlate with age (adult versus immature) and sex (adult male versus adult female) are used by the birds as reliable indicators of relative dominance position. Our demonstration of status signalling draws attention to the need to explain how such a system can be evolutionarily stable and we discuss some suitable models.