Ian G. Warkentin

Territorial responses by North American red squirrels (Tamiasciurus hudsonicus) to conspecifics vary seasonally with peaks during mating and dispersal periods. Broadcast of squirrel vocalizations during surveys may elicit territorial defense behaviors such as calling and movement that make individuals more available for detection, with implications for subsequent occupancy and abundance analyses. We examined the effect of vocalization broadcasts on detection probability during point counts throughout a 14-month period at two locations (year-round study) and during two summers at a third location (summer-only study) on Newfoundland, Canada. Overall detection probability based on sight and sound varied seasonally but the use of vocalization broadcast consistently enhanced detection probability. Squirrels were also more likely to be seen during vocalization broadcast survey periods than during silent point counts. Response to vocalization broadcast was highest when local population density was lowest. Higher detection probability during the initial silent periods of our surveys, when population density was high, likely reflects the greater chance of spontaneous vocalizations in response to the behavior of neighbors.

Elevational gradients in boreal forest songbird assemblages have received limited attention but may have important conservation implications in the face of climate change–induced habitat alteration and shifts in predator communities. Red squirrels (Tamiasciurus hudsonicus), introduced to Newfoundland in the 1960s and limited to lower elevations of the island, are important nest predators in boreal forests and influence bird assemblages where they are present. Our objectives were (1) to examine the role that elevation plays in the distribution of individual species, (2) to identify bird species for which occupancy rate was correlated with squirrel probability of occurrence, and (3) to investigate the influence of elevation and red squirrel occurrence on the species richness of different nesting guilds. We completed 1960 point count surveys for birds and red squirrels over two years across a 257 km² study area in western Newfoundland, Canada, which spanned southern boreal to northern boreal bioclimatic zones (75–610 m elevation). Among 28 species, eight had greater occupancy at higher elevations, 11 had decreasing occupancy with elevation, and nine showed no relationship. The relationship between red squirrel predicted occurrence and elevation was strongly negative. However, relationships between squirrel predicted occurrence and songbird occupancy varied by species, with occupancy for ten species being negatively related to squirrel probability of occurrence, and nine having squirrel occurrence as a better predictor of distribution than elevation. Richness of ground nesting species was best predicted by squirrel occupancy, but richness of above-ground and cavity-nesting species was best predicted by elevation. Populations of boreal birds in mountainous areas such as western Newfoundland are likely to show significant change and re-distribution in response to climate change as a function of altered forest composition and upslope expansion of red squirrels. Targeted monitoring of montane portions of the boreal forest is needed to assess responses to climate change by boreal songbirds.

Preferential use of one limb through handedness or footedness to carry out particular tasks, or asymmetry in cognitive function through specialization of brain hemispheres, are forms of laterality ...

Breeding bird survey (BBS) data are typically analyzed at the route level in which the average or combined total number of individuals across the 50 stops is treated as a single observation. However, analysis at the stop level may provide information on finer-scale habitat affiliations of species within the broader landscape, as well as changes in species distribution over time. Though once abundant, the Newfoundland Gray-cheeked Thrush (Catharus minimus minimus) underwent a dramatic population collapse across most of the island during the 1980s and 1990s and today is most common in montane forests. The reason for this collapse is not well understood but introduced North American red squirrels (Tamiasciurus hudsonicus), which colonized most forested areas on the island below ~400 m from the late 1970s through the mid-1990s, have been identified as a possible cause. To improve our understanding of changes in the elevational distribution of Gray-cheeked Thrushes, we compared stop-level BBS data from two periods. Our historic period from 1974-1984 corresponds with the initial expansion phase of introduced red squirrels, whereas that from 2000-2015 reflects a period during which the colonization of suitable habitat by squirrels was complete. We used 17 routes (850 stops) where the thrushes had been observed and there were at least 3 surveys completed during each period. Surveys during the 1970s and 1980s indicated that Gray-cheeked Thrushes were abundant at all elevations sampled by the BBS (0-450 m), with higher incidence below 100 m and above 300 m; in contrast, recent surveys indicate that the species is now essentially absent below 350 m. This finding highlights the value of historical stop-level BBS data for reconstructing changes in species distributions and is consistent with the hypothesis that red squirrels have adversely affected Newfoundland Gray-cheeked Thrushes. RESUMEN. Los datos de los muestreos de aves reproductoras (BBS, por sus siglas en inglés) suelen analizarse a nivel de ruta, en el que el número total medio o combinado de individuos a lo largo de las 50 paradas se trata como una única observación. Sin embargo, el análisis a nivel de parada puede proporcionar información sobre asociaciones de hábitat de las especies, a escala más fina dentro de un paisaje más amplio, así como cambios en la distribución de las especies a lo largo del tiempo. Aunque abundante en el pasado, el Zorzal de Cara Gris de Terranova (Catharus minimus minimus) sufrió un dramático colapso poblacional en la mayor parte de la isla durante las décadas de 1980 y 1990 y hoy es más común en los bosques montanos. La razón de este colapso no se conoce bien, pero la introducción de las ardillas rojas norteamericanas (Tamiasciurus hudsonicus), que colonizaron la mayoría de las zonas boscosas de la isla por debajo de ~400 m desde finales de los 70 hasta mediados de los 90, ha sido identificada como una posible causa. Para mejorar nuestra comprensión de los cambios en la distribución altitudinal de los Zorzales de Cara Gris, comparamos los datos del BBS a nivel de parada de dos periodos. Nuestro periodo histórico de 1974-1984 se corresponde con la fase inicial de expansión de las ardillas rojas introducidas, mientras que el de 2000-2015 refleja un periodo durante el cual la colonización del hábitat adecuado por las ardillas fue completa. Se utilizaron 17 rutas (850 paradas) en las que se habían observado los zorzales y se realizaron al menos 3 muestreos durante cada periodo. Las encuestas realizadas durante los años 70 y 80 indicaron que los Zorzales de Cara Gris eran abundantes en todas las elevaciones muestreadas por el BBS (0-450 m), con una mayor incidencia por debajo de 100 m y por encima de 300 m; por el contrario, las encuestas recientes indican que la especie está ahora esencialmente ausente por debajo de 350 m. Este hallazgo pone de relieve el valor de los datos históricos del BBS a nivel de parada para reconstruir los cambios en la distribución de las especies y es consistente con la hipótesis de que las ardillas rojas han afectado negativamente a los Zorzales de Cara Gris de Terranova.

Rusty blackbirds (Euphagus carolinus), once common across their boreal breeding distribution, have undergone steep, range-wide population declines. Newfoundland is home to what has been described as one of just two known subspecies (E. c. nigrans) and hosts some of the highest known densities of the species across its extensive breeding range. To contribute to a growing body of literature examining rusty blackbird breeding ecology, we studied habitat occupancy in Western Newfoundland. We conducted 1960 point counts across a systematic survey grid during the 2016 and 2017 breeding seasons, and modeled blackbird occupancy using forest resource inventory data. We also assessed the relationship between the presence of introduced red squirrels (Tamiasciurus hudsonicus), an avian nest predator, and blackbird occupancy. We evaluated 31 a priori models of blackbird probability of occurrence. Consistent with existing literature, the best predictors of blackbird occupancy were lakes and ponds, streams, rivers, and bogs. Red squirrels did not appear to have a strong influence on blackbird habitat occupancy. We are among the first to model rusty blackbird habitat occupancy using remotely-sensed landcover data; given the widespread availability of forest resource inventory data, this approach may be useful in conservation efforts for this and other rare but widespread boreal species. Given that Newfoundland may be a geographic stronghold for rusty blackbirds, future research should focus on this distinct population.

Bird-window collisions are the second leading cause of human-related avian mortality for songbirds in Canada. Our ability to accurately estimate the number of fatalities caused by window collisions is affected by several biases, including the removal of carcasses by scavengers prior to those carcasses being detected during surveys. We investigated the role of scavenger behavior in modifying perceived carcass removal rate while describing habitat-specific differences for the scavengers present in a relatively scavenger-depauperate island ecosystem. We used motion activated cameras to monitor the fate of hatchling chicken carcasses placed at building (under both windows and windowless walls) and forest (open and closed canopy) sites in western Newfoundland, Canada. We recorded the identity of scavengers, timing of events, and frequency of repeat scavenging at sites. Using 2 treatments, we also assessed how scavenging varied with 2 levels of carcass availability (daily versus every third day). Scavenger activities differed substantially between forest and building sites. Only common ravens (Corvus corax) removed carcasses at building sites, with 25 of 26 removals occurring under windows. Burying beetles (Nicrophorus spp.) dominated scavenging at forest sites (14 of 18 removals), completely removing carcasses from sight in under 24 hours. Availability had no effect on removal rate. These findings suggest that ravens look for carcasses near building windows, where bird-window collision fatalities create predictable food sources, but that this learning preceded the study. Such behavior resulted in highly heterogeneous scavenging rates at fine spatial scales indicating the need for careful consideration of carcass and camera placement when monitoring scavenger activity. Our observations of burying beetle activity indicate that future studies investigating bird collision mortality near forested habitats and with infrequent surveys, should consider local invertebrate community composition during survey design. The high incidence of invertebrate scavenging may compensate for the reduced vertebrate scavenger community of insular Newfoundland.

We examined species limits, admixture, and genetic structure among populations in the Bicknell’s Thrush (Catharus bicknelli)–Gray-cheeked Thrush (C. minimus) species complex to establish the geographic and temporal context of speciation in this group, which is a model system in ecology and a high conservation priority. We obtained mitochondrial ND2 sequences from 186 Bicknell’s Thrushes, 77 Gray-cheeked Thrushes, and 55 individuals of their closest relative, the Veery (C. fuscescens), and genotyped a subset of individuals (n = 72) at 5,633 anonymous single nucleotide polymorphic (SNP) loci. Between-species sequence divergence was an order of magnitude greater than divergence within each species, divergence was dated to the late Pleistocene (420 kbp) based on Bayesian coalescence estimation, and a coalescent model (IMa) revealed almost no gene flow between species based on ND2. SNP data were consistent with mitochondrial results and revealed low levels of admixture among species (3 of 37 Bicknell’s Thrushes, no Gray-cheeked Thrushes, and no Veeries were >2% admixed). Species distribution models projected to the Last Glacial Maximum suggest that Bicknell’s Thrush and Gray-cheeked Thrush resided in primarily allopatric refugia in the late Pleistocene, consistent with the genetic data that support reproductive isolation over an extended period of time. Our genetic data suggest that both species underwent demographic expansions, possibly as they expanded out of Pleistocene refugia into their current ranges. We conclude that Bicknell’s Thrush and Gray-cheeked Thrush are 2 distinct species-level lineages despite low levels of Gray-cheeked Thrush introgression in Bicknell’s Thrushes, and divergence has been maintained by a long history of allopatry in subtly different habitats. Their unique phylogeography among boreal forest birds indicates that either cryptic species breaks in eastern North America are still undiscovered, or another factor, such as divergent natural selection, high migratory connectivity, or interspecific competition, played a role in their divergence.

The Newfoundland subspecies of Gray-cheeked Thrush (Catharus minimus minimus) has declined since the 1980s and degradation of winter habitat has been suggested as a contributing stressor. However, the winter range of this subspecies is not well understood, so we fitted 29 males with archival GPS tags during summer 2016. Four tagged thrushes were recaptured in summer 2017 and, though all tags had missing locations and broken antennae, the data retrieved showed that one thrush wintered in the Sierra Nevada de Santa Marta (SNSM) in northern Colombia, one in the nearby Sierra de Perija in Venezuela, and a third may have settled in the same region. One tag provided locations until April 21 and that thrush was consistently detected within a ~1 ha area through the winter. Locations obtained during fall migration indicated that thrushes travelled to South America via Central America and possibly by directly crossing the Caribbean. Contemporary research indicates that the SNSM is an important migratory stopover for Northern Gray-cheeked Thrushes (C. m. aliciae) but a historical report coupled with our observations suggest winter use of the SNSM and adjacent areas in northern South America by C. m. minimus, though numbers may be lower than during the 1900s.

Insular Newfoundland has no native amphibians. While global amphibian populations are declining at alarming rates, populations of introduced anurans (frogs and toads) continue to expand in western Newfoundland, Canada. We expected the establishment and dispersal of the most recently introduced species, Mink Frog (Lithobates septentrionalis), to be influenced by the presence in the region of the previously introduced and ecologically similar Green Frog (L. clamitans). We used a combination of anuran calling surveys and pond-edge visual surveys to assess the ongoing dispersal, relative regional distribution, and local habitat use for these two species in western Newfoundland. The recently established Mink Frog has dispersed ∼3.8 km/year northeast from the original (2001) discovery location and ∼2.6 km/year southwest; co-occurrence analyses revealed that this population displayed habitat and spatial separation from long-established Green Frog populations, at both landscape and local scales. Although cause and effect is not established, this response appears to be defined, at least in part, by the influence of pH on species occurrence; Mink Frogs may be avoiding acidic environments which are tolerated by Green Frogs.