Vanya G. Rohwer

Migratory double breeding in Neotropical migrant birds

Neotropical migratory songbirds typically breed in temperate regions and then travel long distances to spend the majority of the annual cycle in tropical wintering areas. Using stable-isotope methodology, we provide quantitative evidence of dual breeding ranges for 5 species of Neotropical migrants. Each is well known to have a Neotropical winter range and a breeding range in the United States and Canada. However, after their first bout of breeding in the north, many individuals migrate hundreds to thousands of kilometers south in midsummer to breed a second time during the same summer in coastal west Mexico or Baja California Sur. They then migrate further south to their final wintering areas in the Neotropics. Our discovery of dual breeding ranges in Neotropical migrants reveals a hitherto unrealized flexibility in life-history strategies for these species and underscores that demographic models and conservation plans must consider dual breeding for these migrants.

Variation in the Structure of Bird Nests between Northern Manitoba and Southeastern Ontario

Traits that converge in appearance under similar environmental conditions among phylogenetically independent lineages are thought to represent adaptations to local environments. We tested for convergence in nest morphology and composition of birds breeding in two ecologically different locations in Canada: Churchill in northern Manitoba and Elgin in southeastern Ontario. We examined nests from four families of passerine birds (Turdidae: Turdus, Parulidae: Dendroica, Emberizidae: Passerculus and Fringillidae: Carduelis) where closely related populations or species breed in both locations. Nests of American Robins, Yellow Warblers, and Carduelis finches had heavier nest masses, and tended to have thicker nest-walls, in northern Manitoba compared with conspecifics or congenerics breeding in southeastern Ontario. Together, all species showed evidence for wider internal and external nest-cup diameters in northern Manitoba, while individual species showed varying patterns for internal nest-cup and external nest depths. American Robins, Yellow Warblers, and Carduelis finches in northern Manitoba achieved heavier nest masses in different ways. American Robins increased all materials in similar proportions, and Yellow Warblers and Common Redpolls used greater amounts of select materials. While changes in nest composition vary uniquely for each species, the pattern of larger nests in northern Manitoba compared to southeastern Ontario in three of our four phylogenetically-independent comparisons suggests that birds are adapting to similar selective pressures between locations.

Geographic variation in nests of Yellow Warblers breeding in Churchill, Manitoba, and Elgin, Ontario.

Abstract. Nesting structures are important for successful reproduction in most birds, and, because of this, geographic variation in nest morphology and composition are usually interpreted as adaptations to breeding in different environments. We compared the structure of nests of Yellow Warblers (Dendroica petechia) breeding in Churchill, Manitoba, and Elgin, Ontario, Canada. Churchill is subarctic in habitat and typically much colder during the breeding season than Elgin. We compared temperature, rainfall, and wind speed at these two sites and then tested whether differences in nest structure corresponded to different environments. Yellow Warblers breeding in Churchill built larger, less porous nests that retained heat better but also absorbed more water and took longer to dry than Yellow Warbler nests from Elgin. We suggest that differences in the structure of Yellow Warbler nests represent adaptations to breeding in different environments because the differences in nest morphology and properties of heat retention and water loss correspond to differences between the sites in environmental challenges.

Whistling in caterpillars (Amorpha juglandis, Bombycoidea): sound-producing mechanism and function

SUMMARY Caterpillar defenses have been researched extensively, and, although most studies focus on visually communicated signals, little is known about the role that sounds play in defense. We report on whistling, a novel form of sound production for caterpillars and rare for insects in general. The North American walnut sphinx (Amorpha juglandis) produces whistle ‘trains’ ranging from 44 to 2060 ms in duration and comprising one to eight whistles. Sounds were categorized into three types: broadband, pure whistles and multi-harmonic plus broadband, with mean dominant frequencies at 15 kHz, 9 kHz and 22 kHz, respectively. The mechanism of sound production was determined by selectively obstructing abdominal spiracles, monitoring air flow at different spiracles using a laser vibrometer and recording body movements associated with sound production using high-speed video. Contractions of the anterior body segments always accompanied sound production, forcing air through a pair of enlarged spiracles on the eighth abdominal segment. We tested the hypothesis that sounds function in defense using simulated attacks with blunt forceps and natural attacks with an avian predator – the yellow warbler (Dendroica petechia). In simulated attacks, 94% of caterpillars responded with whistle trains that were frequently accompanied by directed thrashing but no obvious chemical defense. In predator trials, all birds readily attacked the caterpillar, eliciting whistle trains each time. Birds responded to whistling by hesitating, jumping back or diving away from the sound source. We conclude that caterpillar whistles are defensive and propose that they function specifically as acoustic ‘eye spots’ to startle predators.

MOLT SCHEDULING OF WESTERN NEOTROPICAL MIGRANTS AND UP-SLOPE MOVEMENT OF CASSIN'S VIREO

Abstract We investigate the effects of body mass and breeding habitat use on the timing and location of the fall molt of western Neotropical migrant passerines. Most western migrants that molt within their breeding ranges breed in coniferous forests, while most that move south before molting breed in low elevation broadleaf or open habitats. We show that larger passerines take longer to molt than smaller passerines and that larger species are more likely to migrate south before molting, whereas smaller species are more likely to molt in their breeding ranges, presumably because their molts take less time. To test our habitat results, we surveyed Cassins Vireos (Vireo cassinii) during their breeding and molting season in Washington to assess up-slope movements. Vireos that bred in low elevation coniferous forest (usually ponderosa pine [Pinus ponderosa] or Douglas-fir [Pseudotsuga menziesii]) moved up-slope at least 300 m to molt in wetter, high-elevation Douglas-fir forests.

How do Birds Adjust the Time Required to Replace their Flight Feathers

ABSTRACT. Regular replacement of flight feathers is one of the most time-consuming activities in the annual cycle of flying birds. In principle, birds can decrease the time required to renew feathers by increasing the rate at which individual feathers grow and by growing more feathers simultaneously. Using 52 comparisons of 43 species from 26 families of birds that maintain the ability to fly while molting, we show that the average number of primary feathers growing simultaneously explains 60.0% of the variation in mass-corrected molt duration, whereas feather growth rate explains only 4.4% of the variation in mass-corrected molt duration. The trend is even stronger when species that grow all their primaries simultaneously are included in the analysis. We suggest that the rate at which feather tissue is produced cannot increase without compromising feather quality because the size of the feather follicle constrains the speed at which feathers can be generated. Thus, birds that must molt rapidly to meet time constraints in their life cycle may be forced to reduce the time spent molting mostly by growing more feathers simultaneously.

Documenting Molt-Migration in Western Kingbird (Tyrannus verticalis) Using Two Measures of Collecting Effort

ABSTRACT. We used museum specimens to describe the timing and location of the postbreeding molt in Western Kingbird (Tyrannus verticalis), an insectivore that breeds in arid lowlands of western North America, where late summer conditions are exceedingly dry. Like many other western migrants, adult Western Kingbirds depart their breeding grounds and move to molt in the Mexican monsoon region. By contrast, juveniles stay on the breeding grounds in late summer, delaying their eccentric primary molt and body molt until after undertaking part of their fall migration. The high number of juvenile specimens collected on the breeding grounds in late summer confirms that the decrease in adults, measured as the percentage of all Western Kingbirds that are adults, is not an artifact of inactive collectors. We also demonstrate adult departure using museum databases to calculate the percentage of all passerines collected in breeding areas outside the southwestern molting grounds that were adult Western Kingbirds. The close correlation between these indices validates the use of total number of passerines as an index of the collecting effort targeting a specific passerine. Our results provide another example of the importance of the Mexican monsoon region for molting passerines, highlighting the need to preserve habitat in this region.

The functions of adult female begging during incubation in sub-Arctic breeding yellow warblers

Begging behaviour in birds is frequently studied in nestlings, but the function of adult female begging behaviour remains unknown. We examined begging behaviours of adult female yellow warblers, Setophaga petechia, during incubation at the northern limit of their breeding range. Female yellow warblers beg only when their mate arrives at the nest, and vocalization duration, intensity of wing fluttering and position of the females body on the nest varies between begging events. We measured changes in these characteristics for 696 begging events from nine incubating females and examined how begging intensity changed with temperature and time since a females last feeding. Begging intensity increased as the duration between feedings increased especially during colder temperatures. Next, we investigated how male feeding rates changed in response to variation in female begging intensity. Males fed females more quickly in response to intense begging during warm temperatures but not during cold temperatures. Finally, we investigated whether male feeding rates influenced the frequency or duration of the females bouts away from the nest (i.e. ‘off-bouts’) during which the female had the opportunity to forage. Females that were fed more frequently reduced the average duration, but not the frequency, of their off-bouts. These results suggest that variation in the intensity of female begging behaviour signals energetic need, and that begging during incubation may be an important adaptation allowing female yellow warblers to maximize incubation time when breeding in sub-Arctic environments.

Assessing Migratory Double Breeding Through Complementary Specimen Densities and Breeding Records

Abstract. We re-evaluate the plausibility that five species of birds that breed in late summer in northwestern Mexico are migratory double breeders that first bred earlier in the same season to the north. We use data aggregated from scientific collections to generate abundance indices that adjust counts of specimens in collections by collecting effort, which we measure as the number of passerines collected in the same region and time period as the species of interest. Our abundance indices generally show displaced phenologies, such that presumed double breeders arrive and breed early in the north, then later in northwestern Mexico. We also compare breeding records for these regions, but these records could not be corrected for effort. Our phenologies suggest that the breeding populations of the Yellow-billed Cuckoo (Coccyzus americanus) from the western U.S. and northwestern Mexico may be derived from birds that bred earlier in eastern North America. Similarly, Orchard Orioles (Icterus spurius) breeding in late summer in northwestern Mexico and on the Mexican plateau may be derived from birds that attempted to breed earlier in North America. Our abundance indices and other new data suggest migratory double breeding is less likely in the Hooded Oriole (I. cucullatus) and probably not occurring in the Yellow-breasted Chat (Icteria virens) and Cassins Vireo (Vireo cassinii).

Conflict between biotic and climatic selective pressures acting on an extended phenotype in a subarctic, but not temperate, environment.

Climatic selective pressures are thought to dominate biotic selective pressures at higher latitudes. However, few studies have experimentally tested how these selective pressures differentially act on traits across latitudes because traits can rarely be manipulated independently of the organism in nature. We overcame this challenge by using an extended phenotype—active bird nests—and conducted reciprocal transplant experiments between a subarctic and temperate site, separated by 14° of latitude. At the subarctic site, biotic selective pressures (nest predation) favoured smaller, non-local temperate nests, whereas climatic selective pressures (temperature) favoured larger local nests, particularly at colder temperatures. By contrast, at the temperate site, climatic and biotic selective pressures acted similarly on temperate and subarctic nests. Our results illustrate a functional trade-off in the subarctic between nest morphologies favoured by biotic versus climatic selective pressures, with climate favouring local nest morphologies. At our temperate site, however, allocative trade-offs in the time and effort devoted to nest construction favour smaller, local nests. Our findings illustrate a conflict between biotic and climatic selective pressures at the northern extremes of a species geographical range, and suggest that trade-offs between trait function and trait elaboration act differentially across latitude to create broad geographic variation in traits.