Territorial Passerine Research Articles

Conspecific density and habitat quality drive the defence and vocal behaviour of a territorial passerine

Territorial defence depends on highly interrelated factors such as food abundance and conspecific density. We used Dupont's Lark Chersophilus duponti as a model species to evaluate the response of a territorial bird to a foreign male playback, examining how conspecific density, habitat quality and male body condition impact responses. The study was conducted in central Spain with variable male density. Response (yes/no), latency time, distance to the playback speaker, and the number of songs and other vocalizations were monitored for 5 min. Habitat quality was estimated using BlueNDVI vegetation index extracted from high‐resolution drone imagery, which is a proxy for arthropod prey biomass. Conspecific density (Kernel Density Estimator) and male body condition were calculated to assess their effect on response and intensity. We applied generalized linear mixed models (GLMMs) to determine which factors predicted the response and its intensity. There was a greater response probability in areas with a higher density of conspecifics and in areas of poorer habitat quality (i.e. lower BlueNDVI values). In contrast, latency time was longer in areas with lower conspecific density. Intrasexual communication (singing and calling rates) increased with habitat quality. Intraspecific communication (other vocalizations) increased in poorer quality habitats and at a higher density of conspecifics. Body condition was not related to any variables. Our results suggest that male density, sometimes used as an indicator of an area being well conserved for the species, may reflect areas of poorer habitat quality occupied by unpaired floater males, whereas paired territorial males would occupy and defend higher quality areas, leading to lower density.

Distinguishing individual quality from habitat preference and quality in a territorial passerine

Theory predicts that animals breeding in heterogeneous landscapes preferentially occupy habitats likely to maximize individual fitness, but identifying those habitats has proved problematic. Many studies develop metrics of habitat quality linked to site-specific reproductive output measured in successive years, but few separate the independent effects of individual "intrinsic quality" from those due solely to the attributes of the habitats themselves. In many populations, processes such as competitive territory defense, longevity, site-fidelity, and variation in breeding density and territory size over time have the potential to limit the degree to which individual and habitat quality will be positively related in nature. However, the effects of these processes on estimates of habitat or site-specific reproductive output have not been thoroughly investigated. We show that, in an insular population of Song Sparrows (Melospiza melodia), females nested preferentially in breeding sites with high mean reproductive output assessed over 35 years, and that variation in site-specific reproductive output was positively related to female intrinsic quality, measured here as the lifetime reproductive success of individual females relative to others hatched the same year (rLRS). In contrast, vegetation traits (shrub cover, edge, and soil depth) predicted female preference for breeding sites but did not predict site-specific variation in annual reproductive output. Female quality also did not predict which females occupied more- or less-preferred breeding sites over the study period. However, mean annual reproductive output of breeding sites estimated over 35 years was strongly positively related to the quality of the females that nested in them. Overall, these results indicate that site-specific estimates of habitat quality that do not consider the quality of the individuals occupying those sites may include substantial bias due to variation in occupant quality, and thus may not reliably predict the intrinsic effects of habitat quality on individual or population fitness.

Detection heterogeneity and abundance estimation in populations of Golden-cheeked Warblers (Setophaga chrysoparia)

ABSTRACT. Abundance estimators that account for imperfect detection, such as N-mixture models, assume that detection of individuals is independent of abundance. Using spot-mapping and N-mixture models applied to point-count data, we estimated abundance of Goldencheeked Warblers (Setophaga chrysoparia) in two years at six study sites at the Balcones Canyonlands Preserve, Austin, Texas. N-mixture model estimates deviated from spot-mapping estimates at the site level by overestimating at low abundances, and at the survey-station level by underestimating at high abundance, which suggests that model assumptions may have been violated. We tested whether detection of individuals is influenced by abundance by assessing per capita song rate in relation to abundance. Per capita song rate increased with abundance, illustrating how the behavior of a territorial passerine may violate the independent-detectability assumption. We next explored violation of this assumption at the survey-station level by applying N-mixtur...

Adoption by a Territorial Passerine

We observed and video-taped active- ly nesting adult Eastern Bluebirds (Sialia sialis) repeatedly feeding abandoned conspecific juveniles in a nearby nest. We used molecular techniques to confirm these nestlings were unrelated to the adults that fed them. The most likely explanation for the observed behavior is that it was a mistake resulting from parental

Social determinants of songbird vocal activity and implications for the persistence of small populations

AbstractConspecific attraction is an important aspect of animal behaviour and several avian studies have shown that vocalizations may be used as an inadvertent cue to locate areas of suitable habitat. By studying the metapopulation system of a territorial passerine, the Dupont's lark Chersophilus duponti, we analysed the demographic correlates of population vocal activity, and the relationships between the occurrence of immigration and the availability of social information (e.g. vocal activity, population size, density and productivity) in 22 local populations. We found that the proportion of active singing days in spring and territorial call advertisement after breeding were positively related to the number of males within local populations. In turn, the intensity of vocal activity was associated with the likelihood of receiving immigrants, better explaining immigration than other kinds of social or public information. Because of depressed signalling, small local populations could experience reduced rescuing from others, thus compromising population persistence. In such cases, habitat management alone may not be enough to overcome this behavioural constraint. Because we found that the occurrence of inter‐patch movements also depended on the size of nearby local populations, understanding regional processes may be as important as controlling social and environmental factors for the maintenance of small populations.

DHEA levels and social dominance relationships in wintering brent geese (Branta bernicla bernicla)

After testosterone, dehydroepiandrosterone (DHEA) is the main hormone involved in aggressive behaviour in birds. While the role of DHEA has been verified for wintering territorial passerines, it has not been shown for gregarious species. In wintering geese species, both sexes present very low testosterone levels and aggression in a non-sexual context is not testosterone-related. Therefore, testosterone does not seem to be responsible for aggressive behaviour by geese during winter and the role of DHEA must be explored. We used brent geese (Branta bernicla bernicla) to examine the roles of testosterone and DHEA in dominance relationships. For the first time, we highlighted the presence of plasma DHEA in free-living geese. As the level of DHEA was lower than that of testosterone, and there was no obvious impact of DHEA level on dominance status, our results failed to confirm the role of plasma DHEA in the social hierarchies of this species during winter. Nevertheless, because DHEA levels were greater in singletons than in paired birds, we discuss the need to explore hormonal and/or behavioural mechanisms implicated within dominance status acquisition and maintenance within each reproductive status class, to underline the role of the presence of relatives as a signal of dominance abilities. We also acknowledge and discuss the possibility that the long handling time may have affected DHEA levels and masked subtle differences between individuals.

Estimating nests not present at the time of breeding surveys: an important consideration in assessing nesting populations

Counts of nest starts are often used as indicators of the size of avian nesting populations, or of avian productivity. However, the accuracy of single or repeated counts of unmarked nests over time for estimating seasonal numbers of nests may be strongly affected by nest events that fall in between dates, or that occurred prior to or after the period. Accuracy may also be affected by uncertainty in the interpretation of counts due to overlap between starting and ending dates of asynchronous nests during the intervisit interval. To measure the combined magnitude of these effects on accuracy, we overlaid a monthly survey regime on known initiation and ending dates of 2055 nests of ciconiiform birds. Assuming all nests present on the date of simulated were counted, monthly surveys underestimated the true number of nest starts by 24-64%, depending on species and year. Using a simple model, we also demonstrate that accuracy does not increase much as frequency increases, and that significant estimation error can occur over a wide range of nest success values and degrees of asynchrony. We suggest that (1) these biases can be significant for surveys of many kinds of nesting birds including some territorial passerines, (2) this bias cannot be addressed by increasing frequency, and (3) the degree of renesting may be of critical interest for inferring breeding population size from nest count data. We suggest three possible approaches for modeling this error.

Duet song and sex roles during territory defence in a tropical bird, the black-bellied wren, Thryothorus fasciatoventris

A diverse array of bird species show a behaviour known as duet singing, in which mated pairs sing temporally coordinated songs. Several studies have shown that simulating territory intrusion with conspecific song playback evokes duet song from duetting bird species, but the adaptive significance of coordinated song during territorial defence remains unclear. The function of duetting is further obscured by our poor understanding of the roles of the sexes in territory defence among year-round territorial species. We developed a simple optimality model that predicts the conditions under which intersexual territorial defence is most likely to occur. We used male solo song and female solo song playback as well as a novel method called ‘stereo duet playback’ to test the predictions of the model and to explore the function of song initiation and song answering during territorial encounters. We conducted these experiments in the field on the black-bellied wren, a year-round territorial passerine. Birds of both sexes responded to all treatment types. Males initiated more songs during opposite-sex playback than during same-sex playback and both sexes were more likely to answer their mates' songs when mates were physically closer. We argue that the acoustic mate-guarding hypothesis does not adequately account for these results, and suggest that duetting during territorial encounters allows mates to identify one another, thus preventing intrapair aggression. As predicted by our model, females showed a strong same-sex bias in territory defence, whereas males approached playback of both sexes. Also in support of the model, males with dependent juveniles showed stronger intersexual territoriality than males not involved in a breeding effort.

Sex‐specific patterns in body condition and testosterone level changes in a territorial migratory bird: the Bluethroat Luscinia svecica

The Bluethroat Luscinia svecica is a migratory passerine that exhibits a socially monogamous pair bond and a high level of parental care. Males are territorial both when wintering and breeding whereas females are territorial only in winter. We investigated changes in body condition and testosterone levels during successive life‐history stages and determined their relationships. Sex‐specific patterns were observed in the variation in body condition and testosterone level. Male body condition varied mainly during the winter. It peaked at the onset of the prenuptial moult and then decreased, whereas it remained stable throughout breeding. In contrast, female body condition varied mainly during the breeding season. It increased during the prelaying stage and then abruptly decreased until fledgling provisioning. As in other monogamous and territorial passerines, testosterone levels in Bluethroat males were low during winter, increased in late winter, peaked during the prelaying stage and then decreased when provisioning young. In wintering females, territorial competition caused testosterone levels to rise. These females were able to produce territorial vocalizations and exhibit aggressive postures. Females showed higher mean testosterone levels than males when wintering whereas the opposite was observed when breeding. Our data from wintering female Bluethroats support the ‘challenge hypothesis’ under which high testosterone levels are associated with periods of social instability, and testosterone can regulate female territorial behaviour during this period.

Sex-specific dispersal in spatially varying environments leads to habitat-dependent evolutionary stable offspring sex ratios

When the environment varies spatially, so that some habitats are more favorable to reproduction than others, an individual should attempt to increase the number of offspring establishing in high-quality habitats. Hence, if male and female dispersal behavior differ, it may be adaptive to produce more offspring of the more dispersing sex in low-quality habitats, since these offspring are likely to disperse to another patch, and more offspring of the most philopatric sex in high-quality habitats, since these offspring are likely to remain in that patch. Such a strategy is shown to be evolutionarily stable provided that male and female dispersal rates are different and that reproductive success varies between habitats (lack of ideal free distribution). Highly biased sex ratios are predicted (1) in rare habitats, (2) in poor habitats, (3) when difference between habitat quality is large, (4) when at least one sex disperses at a rate close to random with respect to habitat availability, (5) when both sexes disperse at a high rate, (6) when individuals are unable to select their reproducing habitat, and, presumably, (7) with moderate temporal variation of habitat quality. The model appears to be a good candidate to explain the pattern of sex ratio variation in a variety of species: phytophagous arthropods, species with environmental sex determination, and territorial passerines. Key words: environmental sex determination, ideal free distribution model, phytophagous arthropods, reaction norm, territorial passerines. [Behav Ecol 11:421–428 (2000)]

Assessment of auditory distance in a territorial songbird: accurate feat or rule of thumb?

Territorial passerines presumably benefit from their ability to use auditory cues to judge the distance to singing conspecifics, by increasing the efficiency of their territorial defence. Here, we report data on the approach of male territorial chaffinches, Fringilla coelebs, to a loudspeaker broadcasting conspecific song simulating a rival at various distances by different amounts of song degradation. Songs were degraded digitally in a computer-simulated forest emulating distances of 0, 20, 40, 80 and 120m. The approach distance of chaffinches towards the loudspeaker increased with increasing amounts of degradation indicating a perceptual representation of differences in distance of a sound source. We discuss the interindividual variation of male responses with respect to constraints resulting from random variation of ranging cues provided by the environmental song degradation, the perception accuracy and the decision rules.

Lack of Mate‐guarding in a Territorial Passerine Bird with a Low Intensity of Sperm Competition, the Pied Flycatcher; (Ficedula hypoleuca)

AbstractWe examined a Norwegian population of pied flycatchers (Ficedula hypoleuca) for behaviour associated with the paternity‐protection tactic of mate‐guarding and found that this tactic was absent. The absence of mate‐guarding and the fact that pied flycatchers do not have a high rate of copulation make this population unusual to the extent that the current literature implies that, in birds, one or other paternity guard should be present. Comparison of our population with a Swedish population that does exhibit mate‐guarding lends some support to the notion that differences in nesting density may drive the relative risk of extra‐pair copulation, leading to the need for paternity guards such as mate‐guarding. However, we conclude that, whilst the results of both our study and the Swedish study are consistent with the nesting‐density hypothesis, there is at least one other plausible hypothesis that fits the available data. Specifically, we direct attention to the role of the female in determining overall patterns of pair behaviour and, potentially, the rate of extra‐pair paternity, challenging the primacy of male—male competition as the driving force behind the sperm‐competition dynamics of this species.

Juvenile dispersion and use of habitat by the endangered Kakerori Pomarea dimidiata (Monarchinae) on Rarotonga, Cook Islands

The Kakerori (Rarotongan flycatcher) Pomarea dimidiata is a small, territorial passerine, endemic to Rarotonga, Cook Islands. Now listed as endangered, the total known population in August 1991 was 48 individuals. Kakerori are found in the steep, forested inland of southern Rarotonga. Newly fledged young remain high in the canopy near the natal nest. However, 4?5 months later they occupy spurs and small ridges immediately surrounding the parental territory, while 7?8 months after fledging, these young birds are most often found on high, exposed ridges where they form small cohorts. The daily activity budget for Kakerori is dominated by calling and looking, and the most common method of feeding is gleaning. Overall, successful territories (those that produce fledged young) have a lower canopy, fewer ferns, and trees with a larger total basal area, than unsuccessful territories (those that do not produce fledged young). Survival of this species depends on continued management of the population, including further research on habitat requirements and effective predator control.

Effects of neighbours: costs of polyterritoriality in the Fan-tailed WarblerCisticola juncidis

The Fan-tailed Warbler Cisticola juncidis is a polygynous and territorial passerine. Eleven territories were established at an edge habitat over the 3 years of study. All of the owners which were adjacent to only one neighbouring territory attempted to hold secondary territories and four males succeeded in establishing secondary territories. On the contrary, five males adjacent to two neighbours did not attempt to leave their primary territories. The probability of establishing a secondary territory in the Fan-tailed Warbler seems to depend on the risk of intrusion by neighbours.

The use of a bird territory mapping method for detecting mortality following pesticide application

Two experimental and two control sites were selected on sheltered and exposed farmland and a census of territorial passerines was conducted using a territory mapping method. A proportion of the territorial bird population was then removed by mist netting to simulate mortality following a pesticide application. Following the removal, censuses were conducted at frequent intervals over a 60-day period to establish whether the census method would reveal the number of birds removed and to assess the rate of reinvasion from surrounding areas. Satisfactory censuses and removal were achieved for the blackbird Turdus merula, chaffinch Fringilla coelebs, song thrush Turdus philomelos and dunnock Prunella modularis. In each case, the percentage of the population removed was revealed by the 48-h post-removal census. The reinvasion rates for these species varied from rapid but incomplete for the blackbird to no reinvasion after 60 days for the chaffinch. The results of the present study suggest that the territory mapping method can be used in field trials to assess the hazards presented by agricultural chemicals to wild bird populations.

Recognition of Neighbors' Duets By Stripe-Backed Wrens Campylorhynchus Nuchalis

Stable groups of stripe-backed wrens Campylorhynchus nuchalis occupy exclusive territories and perform stereotyped duets and choruses in the same contexts in which many individually territorial passerines sing. When duets are broadcast from a speaker just inside a group's territorial boundary, the principal pair in the subject group reacts much more strongly to playbacks of strangers' duets than neighbors' duets. In addition, they respond slightly more intensely to a neighbor's duet on the wrong side of the subject group's territory than to the same duet on the correct side of the territory. These results indicate that stripe-backed wrens can differentiate neighboring groups' duets and associate each with a customary direction. The duets of this species have nearly the same effects on territorial residents as do the advertising songs of passerines with individual territories.