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.     

Reedbed structure and habitat preference of reed passerines during the post-breeding period

Post-breeding habitats of nine passerine species were studied in the permanently flooded reed beds of ponds near the village Pusté úlany (SW Slovakia) in 2002. Structural features of reed beds and the abundance of all birds studied were sampled at two sites and eight study plots. The reed bed structure and abundance of four passerines differed considerably between the individual study plots. The variations in vegetation structure were also caused by winter reed burning in the SE part of study area. Stems in burnt reed beds were smaller and thinner than unburnt. Abundance of the reed warbler and to a lesser extent sedge warbler was higher in tall reed beds, while the bearded tit preferred thick reed stems. Position of reed beds along the shore — open water gradient was an important factor in terms of abundance at the study plots in the case of the reed bunting. This species showed a strong preference for the reed bed edge with open water. Horizontal distribution of other species seems to be random, however, the birds with the exception of Savi’s warbler preferred the upper stratum of reed bed in vertical distribution. Our field data corroborate with those found in experimental studies concerning with ecomorphological characteristics of birds. Some inconsistencies, however, could also be caused by other factors (e.g. foraging preferences). Comparison of breeding and post-breeding habitat requirements of the studied bird species confirmed a more or less random distribution of the majority of species in the post-breeding period. Some variation was found in the cases of only the reed warbler and bearded tit.     

Extrapair paternity is influenced by breeding synchrony and density in the common yellowthroat

The effects of breeding synchrony and density on levels of extrapair paternity in birds are controversial. We used multilocus DNAfingerprinting and microsatellite analysis to examine the effectsof breeding synchrony and density on levels of extrapair paternityin the common yellowthroat (Geothlypis trichas). As in manyNeotropical migrants, breeding synchrony was greatest at thebeginning of the breeding season. Levels of extrapair paternitywere higher after the peak in synchrony, leading to an overallnegative relationship between extrapair paternity and breeding synchrony. However, there was a significant interaction betweenbreeding synchrony and density, as levels of extrapair paternitywere higher only for males breeding when both synchrony anddensity were low. We discuss several possible explanationsfor this interaction, including lower quality males or territoriesin low density areas and greater demands on mate guarding among males with larger territories. Most studies have not consideredsimultaneously the effects of breeding synchrony and densityon extrapair paternity. Our results suggest that ecologicalcorrelates of paternity may be revealed only after testingfor interactions in multivariate analyses.     

Polyphyly of the genus Apalis and a new generic name for the species pulchra and ruwenzorii

The genus Apalis is a member of the African forest warblers clade of the Cisticolidae. In view of its morphological diversity, it was suggested that this genus needs a taxonomic revaluation. For this, we sequenced a nuclear intron (myoglobin intron 2) and two mitochondrial protein‐coding genes (ND2 and ND3). The 2016 bp of sequence data obtained were aligned and subjected to parsimony, maximum likelihood and Bayesian inference. All three genes strongly reject the monophyly of Apalis but support the placing of all apalises within a broader clade of forest cisticolids which also includes Urolais. Within this forest clade, a subclade is defined which includes the genera Urolais, Schistolais and a well‐supported clade comprising three afromontane species, the Black‐collared Apalis Apalis pulchra, the Ruwenzori Apalis Apalis ruwenzorii and the African Tailorbird Artisornis. This subclade is sister to other members of Apalis, including the type species of the genus the Bar‐throated Apalis Apalis thoracica. A new generic name, Oreolais, is suggested for the Black‐collared and Ruwenzori Apalises.     

Associations Among Breeding Birds and Gambel Oak in Southwestern Ponderosa Pine Forests

Abstract: Ponderosa pine (Pinus ponderosa) forests with Gambel oak (Quercus gambelii) are associated with higher bird abundance and diversity than are ponderosa pine forests lacking Gambel oak. Little is known, however, about specific structural characteristics of Gambel oak trees, clumps, and stands that may be important to birds in ponderosa pine-Gambel oak (hereafter pine-oak) forests. We examined associations among breeding birds and structural characteristics of Gambel oak at a scale similar in size to individual bird territories in pine-oak forests in northern Arizona and western New Mexico, USA. Avian species richness and occurrence of some bird species were associated with specific growth forms of Gambel oak. Estimated probability of Virginia's warblers (Vermivora virginiae), black-headed grosbeaks (Pheucticus melanocephalus), and red-faced warblers (Cardellina rubrifrons) occurring at points increased with increasing density of pole-sized Gambel oak 7–15 cm in diameter at breast height. We also found evidence that large Gambel oak trees (≥23 cm dbh) were associated with increased occurrence of yellow-rumped warblers (Dendroica coronata) at points. Some avian associations with oak were influenced by characteristics of ponderosa pines. For example, bird species richness was positively associated with the abundance of large Gambel oak when density of large pine trees ≥23 cm in diameter at breast height was low. Because large oak trees are rare and their numbers are thought to be declining, efforts should be made to retain and promote growth of additional oaks in this size class. Forest management practices that maintain forest openings, such as prescribed burning, could promote growth of pole-sized Gambel oak, which appears important to some bird species in pine-oak forests.     

Evolution of a colour pattern: history,development, and selection

Patterns of melanin pigmentation in birds are extremely varied. Nevertheless it is easy to think of many patterns that are never observed, and others that frequently recur in diverse and distantly related species. Using as our model the avian genus Phylloscopus we ask how the restricted range of observed patterns might be attributable to a restricted range of variants produced by developmental perturbations. The patterns we consider consist of unmelanized patches on the wings, crown and rump on otherwise pigmented upperparts. We use reaction-diffusion models to show that gross features of the pattern can be simply predicted from considerations of embryo shape. We suggest that birds are expected to have more patterned heads, because the head region is relatively larger than other regions in the developing embryo. A comparative analysis across many species of birds and a phylogenetic analysis within the genus Phylloscopus show that the component elements of the pattern have repeatedly been lost and gained during evolution. A shift in a threshold reading could explain the appearance and disappearance of the unmelanized patches, perhaps through changes in the sensitivity of melanocytes to epidermal signals. Such threshold shifts would make the transition between patterned and unpatterned forms particularly easy once the patterns have been exposed to selection in some distant ancestor. This partitioning of the roles of selection and development implies that many features of the patterns reflect developmental mechanisms in both immediate and more distant ancestors.     

A rare predator exploits prey escape behavior: the role of tail-fanning and plumage contrast in foraging of the painted redstart (Myioborus pictus)

Escape response, triggered by an approaching predator, is acommon antipredatory adaptation of arthropods against insectivores.The painted redstart, Myioborus pictus, represents insectivorousbirds that exploit such antipredatory behaviors by flushing,chasing, and preying upon flushed arthropods. In field experimentsI showed that redstarts evoke jump and flight in prey by spreadingwings and tail: this display increased frequency of aerial chasesby redstarts. Artificial models with spread tails also elicitedescape responses more often than models with closed tails and wings.The white patches on black wings and tails additionally help:the frequency of chases decreased when the white patches werecovered with black dye. Black models also tended to elicit escaperesponse less often than black-and-white models did, at leastin some situations. Hence, the prey's ability to detect birdsand to flee could cause the evolution of predators specializedat using conspicuous behavior and contrast in body colorationto elicit and exploit such antipredatory responses. Redstartsconstitute only a small proportion of the predatory guild, andtheir adaptations to exploit the prey's behavior illustratethe theoretically modeled "rare enemy" effect present in multispeciespredator-prey systems. This is the first experimental studyof morphological and behavioral adaptations of a rare predatorthat both elicits and exploits antipredator escape behaviorof its prey against more common predators. Hence, the studydocuments a behavior that could be evolutionarily explainedonly if indirect interactions in predator-prey communities aretaken into account.     

Connecting the dots: Stopover strategies of an intercontinental migratory songbird in the context of the annual cycle

The phases of the annual cycle for migratory species are inextricably linked. Yet, less than five percent of ecological studies examine seasonal interactions. In this study, we utilized stable hydrogen isotopes to geographically link individual black‐and‐white warblers (Mniotilta varia) captured during spring migration with breeding destinations to understand a migrant's stopover strategy in the context of other phases of the annual cycle. We found that stopover strategy is not only a function of a bird's current energetic state, but also the distance remaining to breeding destination and a bird's time‐schedule, which has previously been linked to habitat conditions experienced in the preceding phase of the annual cycle. Birds in close proximity to their breeding destination accumulate additional energy reserves prior to arrival on the breeding grounds, as reflected by higher migratory condition upon arrival, higher refueling rates measured via blood plasma metabolites, and longer stopover durations compared to birds migrating to breeding destinations farther from the stopover site. However, late birds near their breeding destination were more likely to depart on the day of arrival (i.e., transients), and among birds that stopped over at the site, the average duration of stopover was almost half the time of early conspecifics, suggesting late birds are trying to catch‐up with the overall time‐schedule of migration for optimal arrival time on the breeding grounds. In contrast, birds with long distances remaining to breeding destinations were more likely to depart on the day of arrival and primarily used stopover to rest before quickly resuming migration, adopting similar strategies regardless of a bird's time‐schedule. Our study demonstrates that migrants adjust their en route strategies in relation to their time‐schedule and distance remaining to their breeding destination, highlighting that strategies of migration should be examined in the context of other phases of the annual cycle.     

Effect of winter cutting on the passerine breeding assemblage in French Mediterranean reedbeds

Common reed is increasingly harvested from the Mediterranean region toprovide thatching material to north European countries. The impact of thesemanagement practices on the fauna is poorly known. The aim of this study was toquantify the effect of reed cutting in the Mediterranean region through acomparative analysis of water regime, vegetation structure, arthropoddistribution and passerine assemblage at cut and uncut reedbeds in southernFrance. Cut reedbeds were characterised by a lower salinity, higher water levelin spring, and higher reed biomass than uncut reedbeds. Arthropod distributiondiffered consistently between cut and uncut sites, leading to a higher index offood available to passerines in cut reedbeds. Cut reedbeds had a similar birdspecies richness but a lower bird abundance, due to the significant decrease inMoustached Warblers and Bearded Tits at cut sites. The mild Mediterranean winterfavoured early growth of reed in spring, making harvested reedbeds suitable forbreeding of long-distance migrants such as the Great Reed Warbler and ReedWarbler. However, for the resident species that breed earlier in the season, cutreedbeds presumably lack sufficient vegetation cover to provide adequate nestingand feeding sites. Although biennial cutting (double wale) is considered as agood compromise between conservation and commercial interests in the UK, thejuxtaposition of annually cut and never cut reed patches appears as the onlysustainable alternative for the Mediterranean region. We further hypothesisethat an optimal mosaic design of cut/uncut reed patches could provide as high aconservation value as unmanaged reedbeds.