Changes in singing behavior of male black-capped chickadees (Parus atricapillus) following mate removal

We removed the mates of ten male black-capped chickadees (Pares atricapillus) during the nest-building period to determine the effect of female presence on dawn singing. During the first dawn chorus following mate removal, males sang significantly longer, increased movement within their territory, and increased the percentage of their territory covered while singing. After the female was returned, these parameters returned to the pre-removal values. Males did not alter the frequency range or modal frequency of their songs when the mate was removed, nor did they change the degree of frequency shifting in the fee-bee song. We conclude that dawn singing in the black-capped chickadee acts, in part, as an intersexual signal, and that the behavior of frequency shifting in the song may be directed more toward rival males than females. Correspondence to: K. Otter     

Northern harriers on feeding territories respond more aggressively to neighbors than to floaters

Summary Territory owners often respond less aggressively towards intruding neighbors than towards intruding floaters, an observation termed the dear enemy phenomenon. Comparisons of territory owners' responses to intruding neighbors versus their responses to intruding floaters usually have been made for owners of multi-purpose and/or breeding territories. Here, I describe responses of female northern harriers Circus cyaneus (owners) on winter feeding territories towards three types of intruders (female neighbors, female floaters, and male floaters) and show that the dear enemy phenomenon does not occur. Owners' responses towards neighbors were more intense (mostly flights rather than calls) than responses towards female floaters, which in turn were more intense than responses towards male floaters. The greater intensity of owners' responses towards neighbors compared to owners' responses towards male and female floaters may be related to differences in the threat posed by each of the three intruder types in terms of fighting ability (RHP) and potential losses from intrusion. Hence, whether owners respond more aggressively towards neighbors or floaters, and whether the dear enemy phenomenon is observed, may depend upon the relative magnitude of threat presented by neighbors and floaters to owners in terms of fighting ability and potential losses from intrusion.     

Responses of wintering humpback whales (Megaptera novaeangliae) to playback of recordings of winter and summer vocalizations and of synthetic sound

Summary Three natural sounds and one synthetic sound were played back to humpback whales during their 1985 and 1986 winter residency in Hawaiian waters. Playback was conducted from a vessel positioned within visual range of an elevated shorestation equipped with a high-precision surveyor's theodolite, used to determine the positions and movements of observed whale and of the playback vessel. A playback session consisted of 20 min of pretest observation with the vessel in place and underwater speaker deployed, followed by a 20-min test phase during which sound, or a blank tape control, was introduced. A total of 143 playback sessions, involving a total of 338 pods (a single whale or a group of whales), were completed over the two winter seasons. The major response observed during playback was a rapid approach to the playback vessel, characterized in some cases by velocities up to 9 km/h and approaches to within 50 m or less. Whales approaching were mainly singletons and, secondly, apparent adult pairs. No cow-calf pair ever approached. The approach was selective: 21.6% of targeted pods approached in response to a feeding sound recorded in summer feeding grounds in Alaska; 8.3% approached in response to social sounds recorded in the Hawaiian winter grounds in the presence of large surface-active pods; 3.4% responded to playback of winter song; and 4.1% responded to playback of synthetic sound. There were no approach responses to the blank tape control. Singing whales have been identified as males by many researchers. Data from Alaska suggested that the feeding sound was produced by a female; data from Hawaii suggests that the social sounds were produced by males. The different rates of response were attributed to the behavior of sexually active males seeking to affiliate with sexually mature females. Although a female may be present in pods producing social sounds, the presence of multiple males exhibiting aggression may inhibit the approach of other males. Song did not serve as an attractant for females, as measured by direct approach, but may still serve as a basis for female choice.     

Effects of floaters on the free surface profiles of river flows

Environmental Fluid Mechanics - The presence of floaters should be taken into account when dealing with hydraulic analyses, e.g. when boats are moored along the river banks or large wood is...     

Interspecific attraction to the mobbing calls of black-capped chickadees (Parus atricapillus)

The species and number of birds attracted to playbacks of mobbing calls and song of the black-capped chickadee, Parus atricapillus, were compared. The chickadee vocalizations were played at abandoned chickadee nests, and the numbers and species of other birds that approached the speaker were tallied. Few non-chickadees were observed in the area when the song was played. That significantly more birds approached the speaker and displayed mobbing behavior during the mobbing call playback suggests that black-capped chickadee mobbing vocalizations carry meaning for at least ten other avian species. This finding supports the idea that species subjected to the same predators in an area may benefit by recognizing each others’ predator calls. This recognition may be facilitated by the similar structure of different species’ mobbing calls and also a learned association of another species’ call with the presence of a predator. Received: 23 March 1995 /Accepted after revision: 28 October 1995     

The effect of prey consumption on territorial defense by harriers: differential responses to neighbors versus floaters

Summary Food consumption may reduce fighting intensity of territory owners by decreasing resource value (additional food cannot be consumed) and/or increasing fighting costs (food in the digestive tract may raise injury risks). A territorial harrier's (Circus cyaneus, adult females) decision to reduce its level of aggression should depend upon whether or not the intruder was a competitor for individual prey items (as are smaller male floaters) or for the territory proper (as are female floaters and especially female neighbors). Accordingly, following meals, aggressive intensity of owners was strongly reduced towards male floaters (more were ignored), slightly reduced towards female floaters (more were called at than chased), and remained unchanged towards neighbors (virtually all were chased). Hence, alterations in aggressive behavior of territory owners following food consumption may depend upon the type of intruder and the resource under contest (a food item or a territory).     

Individual variation in winter foraging of black-capped chickadees

Summary Wintering black-capped chickadees (Paridae: Parus atricapillus) in northwestern Massachusetts showed a high degree of individual variation in foraging behavior. After accounting for the effects of different habitats and weather conditions, individual differences comprised 6–17% of the total observed variation in four measures of foraging location and rate of feeding. Differences between age and sex groups were not significant and explained comparatively little variation (0.0–1.4%). The chickadees did not fall into a few distinct behavioral categories but instead showed continuous variation on all measures of foraging behavior. It appeared that some variation among individuals was a consequence of behavioral convergence within social groups, since birds that were observed together were more similar in their foraging than expected by chance, after taking habitat differences into account. Our results therefore do not support the interpretation that individual variation in feeding behavior serves to reduce exploitation competition within social groups.     

Ranking individual habitat patches as connectivity providers: Integrating network analysis and patch removal experiments

Here we propose an integrated framework for modeling connectivity that can help ecologists, conservation planners and managers to identify patches that, more than others, contribute to uphold species dispersal and other ecological flows in a landscape context. We elaborate, extend and partly integrate recent network-based approaches for modeling and supporting the management of fragmented landscapes. In doing so, experimental patch removal techniques and network analytical approaches are merged into one integrated modeling framework for assessing the role of individual patches as connectivity providers. In particular, we focus the analyses on the habitat availability metrics PC and IIC and on the network metric Betweenness Centrality. The combination and extension of these metrics jointly assess both the immediate connectivity impacts of the loss of a particular patch and the resulting increased vulnerability of the network to subsequent disruptions. In using the framework to analyze the connectivity of two real landscapes in Madagascar and Catalonia (NE Spain), we suggest a procedure that can be used to rank individual habitat patches and show that the combined metrics reveal relevant and non-redundant information valuable to assert and quantify distinctive connectivity aspects of any given patch in the landscape. Hence, we argue that the proposed framework could facilitate more ecologically informed decision-making in managing fragmented landscapes. Finally, we discuss and highlight some of the advantages, limitations and key differences between the considered metrics.     

The structural significance of a vocalization that stimulates gaping in black-capped chickadees

In a variety of avian species, vocalizations used to stimulate gaping in nestlings during feeding share similar features such as broad frequency range, multiple spectral components, and sudden onset. It is currently not understood what functional significance, if any, these acoustic features have for close-range communication. One example of such a vocalization is the “squawk” of the black-capped chickadees (Parus atricapillus). The present study analyzes the significant features of the squawk by observing the response of chickadee nestlings to field playbacks of natural and computer-modified squawks. Of the features tested, frequency range appears most critical, but responses to frequency ranges change with age, such that low frequencies are most stimulating around the time of hatching but cease to elicit gaping after 5 days posthatch. Aperiodic stimuli and computer-modified squawks with gradual onsets tend to be less stimulating than natural squawks at some ages, but these differences are not significant at any age. The functional significance of the broad frequency range of the squawk is discussed, including the possible role of accommodating shifting frequency preferences related to ontogenetic changes in auditory sensitivity and elaboration of the vocal repertoire. Received: 16 January 1996 / Accepted after revision: 28 December 1996     

Efficacy of stable isotope ratios in assigning endangered migrants to breeding and wintering sites.

A primary constraint on effective conservation of migratory animals is our inability to track individuals through their annual cycle. One such animal is the endangered southwestern subspecies of the Willow Flycatcher, which is difficult to distinguish from conspecifics. Identifying wintering regions used by the endangered subspecies would be an important step in formulating an effective conservation strategy. Our objective was to use stable isotope ratios as a means of identifying wintering sites of Southwestern Willow Flycatchers. We analyzed stable isotope ratios of carbon, nitrogen, and hydrogen from feathers of breeding and wintering Willow Flycatchers. Based on winter samples, we document a positive trend in hydrogen isotope ratios across latitude. We also found that Willow Flycatchers use C4 food webs south of 8 degrees N latitude, but we found no evidence of use of C4 food webs farther north. Nitrogen stable isotope ratios of feathers showed no discernable geographic variation. Discriminant function analyses, based on stable isotope ratios of wintering Willow Flycatchers, were only useful (>50% accurate) for assigning individuals to winter regions if the regions were large and the threshold probability for assignment was relatively high. When using these discriminant functions, most breeding samples of Southwestern Willow Flycatchers were assigned to two wintering regions: central Mexico and Ecuador. We think that assignment of Southwestern Willow Flycatchers to Ecuador is unrealistic. Given the large percentages of samples that could not be classified with certainty, we are not confident that these two regions are truly more likely to harbor wintering Southwestern Willow Flycatchers than other winter regions. We think our inconclusive results are due primarily to weak and nonlinear gradients in isotope ratios across the winter range of Willow Flycatchers.     

Habitat-induced changes in song consistency affect perception of social status in male chickadees

The ability of male black-capped chickadees to maintain consistent internal structure between successive iterations of their songs is affected by both their social rank and the quality of their habitat. Lab studies reveal that female chickadees discriminate between songs of dominant and subordinate males, which vary in acoustic structure. We investigate whether males also rely on acoustic structure to assess rival quality during agonistic interactions, and whether habitat-induced differences in song consistency affect the perception of male rank. We conducted a playback experiment to simulate territorial intrusions by dominant, stimuli males into the territories of dominant, subject males; stimuli males were recorded either in low-quality (young forest) or high-quality (mature forest) habitats. Stimuli from low-quality habitat had lower song consistency than those from high-quality habitats, despite being recorded from males of equivalent social rank. Subject males for playbacks (also socially dominant males) were chosen from either habitat type. Subject males in mature forest responded less to young-forest stimuli compared to mature-forest stimuli, despite the stimuli in both cases being recorded from dominant males. Conversely, male subjects in young forest did not differentiate between stimuli, but their response to both stimuli was lower than that of mature-forest subject males to mature-forest stimuli. We demonstrate that the ability to maintain internal song structure in the black-capped chickadee constitutes a signal that appears to be used by males to assess the level of threat of intruders, and that this perception is affected by habitat from which the stimulus males were recorded.     

Experimental test of the effects of a non-native invasive species on a wintering shorebird

The abundance of nearly one-quarter of the world's shorebird species is declining. At the same time, the number of non-native species in coastal ecosystems is increasing rapidly. In some cases, non-native species may affect negatively the abundance and diversity of shorebird prey species. We conducted an experimental study of the effects of the introduced European green crab (Carcinus maenas) on prey consumption by wintering Dunlin (Calidris alpina) in a central California estuary. We placed green crabs and Dunlin sequentially in field enclosures and measured changes in density of benthic invertebrate prey (e.g. polychaetes and small clams), Dunlin biomass, and gut contents of both Dunlin and crabs and observed foraging behavior of Dunlin. Green crabs significantly affected Dunlin foraging success through both direct and indirect multitrophic linkages. In enclosures with high densities of green crabs, crab foraging reduced the availability of polychaetes, and Dunlin consumed significantly fewer polychaetes compared with Dunlin in enclosures without crabs. High densities of green crabs were also associated with increased availability of small clams. Dunlin consumed significantly more small clams compared with Dunlin in enclosures without crabs. In our literature survey of studies of effects of non-native invasive species on shorebirds, we found three prior experiments that addressed the effect of non-native invasive species on shorebirds. Results of two of these studies showed positive direct effects of non-native invertebrates on shorebirds, 1 showed negative direct effects of a non-native plant on shorebirds through habitat conversion, and none showed indirect effects of non-native invertebrates. We suggest future management of shorebirds explicitly examine how non-native marine species, particularly invertebrates, directly and indirectly affect shorebirds.     

Fish Responses to Experimental Fragmentation of Seagrass Habitat

Abstract: Understanding the consequences of habitat fragmentation has come mostly from comparisons of patchy and continuous habitats. Because fragmentation is a process, it is most accurately studied by actively fragmenting large patches into multiple smaller patches. We fragmented artificial seagrass habitats and evaluated the impacts of fragmentation on fish abundance and species richness over time (1 day, 1 week, 1 month). Fish assemblages were compared among 4 treatments: control (single, continuous 9‐m² patches); fragmented (single, continuous 9‐m² patches fragmented to 4 discrete 1‐m² patches); prefragmented/patchy (4 discrete 1‐m² patches with the same arrangement as fragmented); and disturbance control (fragmented then immediately restored to continuous 9‐m² patches). Patchy seagrass had lower species richness than actively fragmented seagrass (up to 39% fewer species after 1 week), but species richness in fragmented treatments was similar to controls. Total fish abundance did not vary among treatments and therefore was unaffected by fragmentation, patchiness, or disturbance caused during fragmentation. Patterns in species richness and abundance were consistent 1 day, 1 week, and 1 month after fragmentation. The expected decrease in fish abundance from reduced total seagrass area in fragmented and patchy seagrass appeared to be offset by greater fish density per unit area of seagrass. If fish prefer to live at edges, then the effects of seagrass habitat loss on fish abundance may have been offset by the increase (25%) in seagrass perimeter in fragmented and patchy treatments. Possibly there is some threshold of seagrass patch connectivity below which fish abundances cannot be maintained. The immediate responses of fish to experimental habitat fragmentation provided insights beyond those possible from comparisons of continuous and historically patchy habitat.     

Demographic Responses by Birds to Forest Fragmentation

Abstract:  Despite intensive recent research on the effects of habitat loss and fragmentation on bird populations, our understanding of underlying demographic causes of population declines is limited. We reviewed avian demography in relation to habitat fragmentation. Then, through a meta-analysis, we compared specific demographic responses by forest birds to habitat fragmentation, providing a general perspective of factors that make some species and populations more vulnerable to fragmentation than others. We obtained data from the scientific literature on dispersal, survival, fecundity, and nesting success of birds. Birds were divided into subgroups on the basis of region, nest site, biogeographical history, and migration strategy. Species most sensitive to fragmentation were ground- or open-nesters nesting in shrubs or trees. Residents were equally sensitive to fragmentation in the Nearctic and Palearctic regions, but Nearctic migrants were more sensitive than Palearctic migrants. Old World species were less sensitive than New World species, which was predicted based on the history of forest fragmentation on these two continents. Pairing success was the variable most associated with fragmentation, suggesting an important role of dispersal. Fledgling number or condition, timing of nesting, and clutch size were not associated with sensitivity to fragmentation, suggesting that negative fragmentation effects on birds do not generally result from diminished food resources with increasing level of fragmentation. Future studies on demographic responses of birds to habitat fragmentation would be more effective if based on a combination of measures that can distinguish among the demographic mechanisms underlying population changes related to habitat fragmentation.     

Pair territoriality of wintering stonechats : behaviour,function and hormones

Migratory stonechats (Saxicola torquata) spend the winter in the northern Negev desert in Israel. After arrival in October most birds defend territories throughout the winter either as heterosexual pairs or as single birds. In this population we examined: (1) aggressive behaviour of pairs; (2) the functional significance of pair territoriality; and (3) hormonal correlates of territorial behaviour. Pairs were aggressive towards both conspecific intruders and stonechat dummies, particularly in autumn when territories were established. Males were more aggressive than females and aggression was primarily intrasexual. More frequent than intraspecific interactions were interspecific encounters, with 94% involving the local mourning wheatear (Oenanthe lugens). With one exception these encounters consisted of attacks of mourning wheatears against stonechats, mainly males. Two functions of winter territoriality of migrant pairs are proposed: (a) a reduction of individual defense costs associated with the maintenance of a territory; (b) an increase of the net vigilance to reduce the risk of attacks by competitors or predators. The levels of testosterone (males and females) and estradiol (females only) were close to or below the lower detection limit of the assay throughout the winter. Therefore, high levels of gonadal steroids are not required for pair formation and aggression in the non-reproductive context. The results indicate that both the function and the proximate control mechanisms of territoriality during the non-breeding season in winter are different from those during the breeding season in spring.     

Pitch shifts and song structure indicate male quality in the dawn chorus of black-capped chickadees

The fee-bee song of male black-capped chickadees (Poecile atricapillus) is considered a single-type song that singers transpose up and down a continuous frequency range. While the ability to shift song pitch in this species provides a mechanism for song matching as an aversive signal in male-male territorial song contests, the functional significance of this behaviour during the solo performances of males during the dawn chorus is unclear. We analysed the dawn chorus songs and singing behaviour of males whose winter-flock dominance status we determined. We used correlation analysis to show that pitch shifts were accompanied by changes to other fine structural characteristics in song, including temporal and relative amplitude parameters. We also found that songs of socially dominant males and songs of their most subordinate flockmates could be distinguished using these methods by the way they performed a between-note frequency measure accompanying pitch shifts. That is, a ratio measure of the internote frequency interval remained constant for songs of high-ranking birds despite changes in absolute pitch, while low-ranking males sang a smaller ratio as they shifted to higher absolute pitches. These findings identify previously unrecognised variation in the songs of black-capped chickadees. More importantly, they indicate a mechanism by which pitch shifting during the dawn chorus of black-capped chickadees could provide a reliable indicator of relative male quality.Communicated by I. Hartley     

海拔梯度上的植物生长与生理生态特性

环境是植物生存和发展的条件,在各种不同的生境中聚生着特定的植物种类.全球气候变化迅速而显著地改变着高海拔地区的生态环境,这必将导致山地植物生理生态适应性、分布界限等发生变化,这些变化反过来又会对气候变化造成深远影响.因此,研究山地植物的生理生态特性,揭示其对全球气候变化的响应和适应性,具有重要的现实与科学意义.海拔梯度由于包含了温度、湿度、光照等诸多环境因子的剧烈变化而成为了研究植物对全球气候变化响应的理想区域.随着海拔升高,气温下降、大气压及CO2分压降低、光强增加等,植物的生态和生理特征将产生巨大的变化,可能影响到植物种类在海拔梯度上的分布、植物群落的结构组成.一些亟待回答的问题包括:全球气候变化将如何影响高山植物的生长环境?环境变化如何影响植物生理生态适应性?植物群落的分布、结构和组成将发生哪些变化?这些变化又将怎么样影响气候进一步变化?众多的研究报导了树木的生理生态特征、生长速率和生产力等对海拔的响应.本文在总结前人研究的基础上,系统地综述了海拔梯度上温度、水分、光照、土壤等环境因子的差异对高山植物的生长及生理生态特性的影响,以及植被分布的变化,旨在阐明植物对全球气候变化的行为与生理生态响应,以期为该领域的深入研究提供参考.