Aegolius Acadicus Research Articles

Winter occurrence and roosting behavior of the secretive Northern Saw-whet Owl (Aegolius acadicus) at the southern edge of its range

Winter occurrence and roosting behavior of the secretive Northern Saw-whet Owl (Aegolius acadicus) at the southern edge of its range

Documenting Northern Saw-Whet Owls Breeding in Mixed-Deciduous Forest

Documenting Northern Saw-Whet Owls Breeding in Mixed-Deciduous Forest

Irruptive fall migrations are linked to elevated breeding abundance but are not associated with body condition or stopover duration in Northern Saw-whet Owls

Abstract Bird species that undertake irruptive migrations are good candidates for assessing density-dependent effects on stopover ecology because the number of birds using stopover sites varies considerably from year to year. Using morphometric data from a 9-year banding program and radiotelemetry (n = 25 females), we found that increases in the annual density of Northern Saw-whet Owls (NSWO; Aegolius acadicus) encountered at a fall stopover site in Nova Scotia, Canada, were not linked to female body condition or minimum stopover duration. Rather, most NSWO spent no more than 1 or 2 full days at the stopover site following radio-tagging and during their return visits to the site. Body condition indices were highest for NSWO captured near the end of the migration monitoring season, possibly reflecting birds that had recently established wintering ranges and were elevating energy stores. We used breeding abundance indices derived from Birds Canada’s Atlantic Nocturnal Owl Survey to help test the hypothesis that irruptive fall migrations in NSWO were driven by elevated breeding productivity and dispersal of immature birds (breeding success hypothesis). Indeed, irruptive fall migrations were characterized by elevated densities of immatures, but not adults, and mean breeding abundance indices for the Maritime provinces during years with irruptive migrations were higher than those for non-irruptive years. We hypothesize that prey abundance during years with irruptive migrations was sufficient to enable high breeding densities and minimize effects of elevated fall densities of NSWO on rates of mass gain and stopover duration.

Does migration phenology of Northern Saw-whet Owls (Aegolius acadicus) vary over time?

Does migration phenology of Northern Saw-whet Owls (Aegolius acadicus) vary over time?

Sky Islands in the Distance

Sky Islands in the Distance J. Novalis Wolfe There is clear support for the idea that mental states are subjective if they are ascribed to creatures who can ascribe them to themselves without observation, by other creatures who can ascribe similar states to themselves in the same way. —Thomas Nagel Patterned undulations, he saw them asLike his own skeined skin of folded flesh inscribedAgainst a blue aureole of void, while up Close a saw-whet owl hooting tunes on keenMesquite supposed these back-lit hills to be itsGod, the zenith of its own marrow being. What caused them so, man, mountain and fawning birdMixed in a world of sense, of thoughtful sense,This pure fiction of cosmic seem? Surely They are themselves the rich stuff of Neither-Land(of here nor there); the simple froth of mereThings shared within a space of imagination Of mind so each one’s three-fold part can knowA new whole of harmonium, our worldIn its reasoned red of twilight being there. [End Page 110] J. Novalis Wolfe Hereford, Arizona Copyright © 2023 Johns Hopkins University Press

Sample point variation in gastrointestinal bacteria of migrating Northern Saw-whet Owls (Aegolius acadicus) of eastern North America

We characterize bacteria of the gastrointestinal tract of 28 Northern Saw-whet Owls (Aegolius acadicus) netted on the Mohonk Preserve near New Paltz, New York, during autumn of 2017. We used selective culture media to determine the presence and assess the point infection level of Escherichia coli, Pseudomonas aeruginosa, Salmonella spp., and Staphylococcus aureus in the buccal, cloaca, feces, and pellets of Northern Saw-whet Owls. We tested for associations between bacteria type, sample point location, and infection levels and then assessed the influence of bacterial infection on the health of owls using multivariate analysis of mass/wing chord body condition index estimates and heterophil to lymphocyte ratios. All Northern Saw-whet Owls tested positive for at least one type of bacteria. We found an inverse relationship between the number of bacteria types detected per individual and the percentage of Northern Saw-whet Owls affected, with 46.43%, 32.14%, and 3.57% of individuals showing presence of 2, 3, and 4 types of bacteria, respectively. Multivariate analysis revealed a significantly heightened presence of S. aureus in the buccal cavity, and Spearman's rho analysis revealed positive correlations between site colonization levels and bacterial species levels. We found no significant relationship between bacterial load and mass/wing chord body condition indexes or heterophil to lymphocyte ratios. We did, however, find a positive relationship between heterophil to lymphocyte ratios and Leucocytozoon infections. Providing baseline information of common microflora can help future research recognize potentially harmful gastrointestinal pathogens. Therein, as the first study to characterize bacteria of the Northern Saw-whet Owl, this research adds to the information portfolio of the species and may benefit future research efforts.

Effects of Frequency on the Directional Auditory Sensitivity of Northern Saw-Whet Owls (Aegolius acadicus)

Many animals use sound as a medium for detecting or locating potential prey items or predation threats. Northern saw-whet owls (Aegolius acadicus) are particularly interesting in this regard, as they primarily rely on sound for hunting in darkness, but are also subject to predation pressure from larger raptors. We hypothesized that these opposing tasks should favor sensitivity to low-frequency sounds arriving from many locations (potential predators) and high-frequency sounds below the animal (ground-dwelling prey items). Furthermore, based on the morphology of the saw-whet owl skull and the head-related transfer functions of related species, we expected that the magnitude of changes in sensitivity across spatial locations would be greater for higher frequencies than low frequencies (i.e., more “directional” at high frequencies). We used auditory-evoked potentials to investigate the frequency-specific directional sensitivity of Northern saw-whet owls to acoustic signals. We found some support for our hypothesis, with smaller-magnitude changes in sensitivity across spatial locations at lower frequencies and larger-magnitude changes at higher frequencies. In general, owls were most sensitive to sounds originating in front of and above their heads, but at 8 kHz there was also an area of high sensitivity below the animals. Our results suggest that the directional hearing of saw-whet owls should allow for both predator and prey detection.

Bottom–up trophic effects on fisher Pekania pennanti harvest age structure: associations with mast, voles and owls

Harvest management quotas for fishers Pekania pennanti in some jurisdictions are estimated from the previous year's harvest, and stem from the hypothesis that age ratios in the harvest are largely influenced by ‘top–down’ trapping pressure. The influence of ‘bottom–up’ food supply on fisher harvest age ratios might be underappreciated, which could result in a misallocation of quotas in management planning. We assessed a variety of data sources to test the influence of bottom–up processes on fisher populations in Ontario, Canada. We found evidence that bottom–up trophic effects influence the harvested fisher age structure in some regions of Ontario. Evidence also suggests that harvest pressure had little top–down influence on age ratios over the course of our study, and that basing management strategies on this assumption may lead to unintentional overharvest in years of low productivity. We suggest several trophic linkages with potential to facilitate fisher management, including connections among berry and seed crops, small mammals and northern saw-whet owls Aegolius acadicus.

Differential landscape use by forest owls two years after a mixed‐severity wildfire

AbstractOwls are important avian predators in forested systems, but little is known about landscape use by most forest‐adapted owl species in environments impacted by mixed‐severity wildfire. To better understand species‐specific patterns of post‐wildfire landscape use within an owl guild, we used passive acoustic monitoring using autonomous recording units. The technology is effective for multi‐species surveys, especially if some species are rare, nocturnal, or difficult to detect by traditional means. In 2017, we surveyed the interior and adjacent unburned areas of a 10,700‐ha mixed‐severity wildfire that burned in 2015 in southwest Oregon. We used occupancy modeling to identify patterns of landscape use by five species of forest owls: barred owls (Strix varia), great horned owls (Bubo virginianus), western screech‐owls (Megascops kennicottii), northern pygmy‐owls (Glaucidium gnoma), and northern saw‐whet owls (Aegolius acadicus). Our results showed a positive relationship between increasing fire severity and probability of use by western screech‐owls and a similar but somewhat weaker relationship for northern pygmy‐owls. Barred owls were rarely detected in severely burned areas and their use decreased with increased fire severity. We observed generally low landscape use for great horned owls, which decreased with increased fire severity and at higher elevations. Thus, four out of the five species appeared to use recently burned forests at different levels, with only northern saw‐whet owls showing near‐complete avoidance of the burned area. These findings increase our understanding of the basic ecology of each species and highlight the varied use of burned areas within this community. These previously undocumented patterns of landscape use in burned landscapes should provide insights to managers and policymakers in the Pacific Northwest as climate shifts, and fires may increase in size, frequency, and severity.

Delineating the origin of migratory Northern Saw-whet Owls (Aegolius acadicus) in Indiana using stable isotope analysis

The Northern Saw-whet Owl (Aegolius acadicus) is a highly migratory owl that occurs throughout North America. Banding stations have studied the migratory habits of this species, but these studies rarely collect significant data regarding the origin of migrant birds captured in the midwestern United States. In the last 20 years Northern Saw-whet Owl ecology has been studied broadly; however, little is known how different populations may interact with one another. Many avian species with large geographic ranges often have populations that breed and winter in separate areas. Little has been documented to establish if this pattern exists in Northern Saw-whet Owls, a species where individuals are migratory, sedentary, and nomadic. Stable isotope ratios using deuterium δ2H in feathers have been successfully used to estimate the breeding origin of birds. We used stable isotope analysis of δ2H in conjunction with an isoscape map to determine breeding origins for Indiana Northern Saw-whet Owls. We collected 41 feather samples from migrating owls at 6 different banding sites across Indiana from October to November 2017. Because higher enrichment values in adult owl feathers complicate analysis, we only analyzed samples from young (hatching-year) individuals. After analysis and exclusion of birds identified as outliers, the model suggested that the birds originated along 44°N around the Great Lakes Region and as far east as Nova Scotia, Canada. When including outliers, the model suggested that individuals originated along 40°N between Iowa and New York. The outliers-excluded model remained more consistent with the known breeding range of this species than the outliers-included model. We successfully used this technique and our data suggests that the majority of Northern Saw-whet Owls originated from the Great Lakes Region. We recommend further investigation on the physiology and behavior of owls on the breeding grounds, which could aid in the ability to better understand differences in δ2H enrichment of this species. Better understanding of this would allow these models to be plotted more precisely onto the landscape.

Delineating the origin of migratory Northern Saw-whet Owls (Aegolius acadicus) in Indiana using stable isotope analysis

The Northern Saw-whet Owl (Aegolius acadicus) is a highly migratory owl that occurs throughout North America. Banding stations have studied the migratory habits of this species, but these studies rarely collect significant data regarding the origin of migrant birds captured in the midwestern United States. In the last 20 years Northern Saw-whet Owl ecology has been studied broadly; however, little is known how different populations may interact with one another. Many avian species with large geographic ranges often have populations that breed and winter in separate areas. Little has been documented to establish if this pattern exists in Northern Saw-whet Owls, a species where individuals are migratory, sedentary, and nomadic. Stable isotope ratios using deuterium δ2H in feathers have been successfully used to estimate the breeding origin of birds. We used stable isotope analysis of δ2H in conjunction with an isoscape map to determine breeding origins for Indiana Northern Saw-whet Owls. We collected 41 feather samples from migrating owls at 6 different banding sites across Indiana from October to November 2017. Because higher enrichment values in adult owl feathers complicate analysis, we only analyzed samples from young (hatching-year) individuals. After analysis and exclusion of birds identified as outliers, the model suggested that the birds originated along 44°N around the Great Lakes Region and as far east as Nova Scotia, Canada. When including outliers, the model suggested that individuals originated along 40°N between Iowa and New York. The outliers-excluded model remained more consistent with the known breeding range of this species than the outliers-included model. We successfully used this technique and our data suggests that the majority of Northern Saw-whet Owls originated from the Great Lakes Region. We recommend further investigation on the physiology and behavior of owls on the breeding grounds, which could aid in the ability to better understand differences in δ2H enrichment of this species. Better understanding of this would allow these models to be plotted more precisely onto the landscape.

Abundant Natural Cavities in a Deciduous Forest Leads to a Lack of Nest-Box Occupancy by Northern Saw-Whet Owls

Nest boxes are commonly used for species that utilize tree cavities, including small owls. Boxes were installed in 2011 to facilitate study of Northern Saw-whet Owls in Pennsylvania. We checked the 11 boxes that remained in 2020 after a hiatus of 8 years and found 3 rodent nests, but no evidence of owl usage. Playbacks revealed owls were present at 8 of 11 boxes checked. Cavity surveys yielded an average of 7.7 large cavities within 50 m of each box; extrapolation of cavity densities to a minimal territory size of 150 ha suggested that suitable cavities are an abundant, non-limiting resource here. Cavity abundance was likely a consequence of beech bark disease complex having recently top-killed most large American Beech trees locally, providing abundant resources for excavating woodpeckers. Assessing cavity abundance should be an essential first step for any management project involving cavity-nesting birds, and nest boxes should be used only in situations where cavities appear to be limited.

First Specimen of the Northern Saw-whet Owl from Baja California, Mexico, with Data on Its Abundance in the Sierra San Pedro Mártir

First Specimen of the Northern Saw-whet Owl from Baja California, Mexico, with Data on Its Abundance in the Sierra San Pedro Mártir

Twenty-five year population trends in Northern Saw-whet Owl (Aegolius acadicus) in eastern North America

Due to the low detectability of Northern Saw-whet Owls (Aegolius acadicus; hereafter, NSWO) throughout their annual cycle, standardized monitoring during migration allows for population assessments over time. We assessed age-class population trends in NSWO throughout eastern North America using banding data from 7 sites over a 25 year period. Using a mixed linear model, we did not detect any significant trends over time for the total owl count, adult owl count, and juvenile owl count from 1992 to 2017. During the period when all 7 sites were active from 2001 to 2017, trend estimates remained nonsignificant despite showing negative slopes. We confirmed this nonsignificant, negative trend through a similar mixed linear model of NSWO data from Christmas Bird Counts. Our results suggest that NSWO populations across eastern North America have been relatively stable since 1992 throughout their migration and winter ranges and demonstrate the value of standardized banding data for monitoring the regional population status of NSWO.

Account and Utilization of Blood Lipid Profiles: Lipid Levels Predicted Hemosporidian Infection in Migrating Northern Saw-Whet Owls of Eastern North America.

Noting lipidomic changes following the parasitism of migrating birds, the metabolic needs of which are primarily fueled by lipids, can deepen our understanding of host-parasite interactions. We identified lipids of migrating Northern saw-whet owls (Aegolius acadicus) using collision-induced dissociation mass spectrometry, compared the lipidomic signatures of hemoparasite-infected and noninfected individuals, and performed cross-validation analyses to reveal associations between parasite infection and lipid levels. We found significantly lower levels of lipid classes phosphatidic acid (PA), phosphatidylglycerol (PG), phosphatidylinositol (PI), phosphatidylserine (PS), phosphatidylethanolamine (PE), phosphatidylcholine (PC), and sphingomyelin (SM) in infected Northern saw-whet owls than in the noninfected individuals. Conversely, we found higher levels for certain lysoPS and lysoPE species, and variable lipid level changes for free fatty acid (FFA) species. Reporting lipidomic changes observed between hemosporidian-infected and noninfected Northern saw-whet owls can strengthen our understanding of the mechanisms governing parasite proliferation in this species. Furthermore, our analysis indicated that lipidomic signatures are better predictors of parasite infection than the log-adjusted mass/wing chord body index, a metric commonly used to assess the influence of hemosporidia infection on the health of birds. Establishing a lipidomic profile for Northern saw-whet owls that provides baseline lipid levels during fall migration may assist future studies assessing causes of reductions in breeding brought about from subtle differences in behaviors such as delayed migration.

Can you hear me now? The effect of signal degradation on perceived predator threat in black-capped chickadees (Poecile atricapillus).

Avian predators vary in their degree-of-threat to chickadees; for example, smaller owls and hawks are of higher threat to chickadees as they can easily maneuver through the trees, while larger predators cannot. We conducted an operant go/no-go discrimination task to investigate the effect of signal degradation on perceived threat. Chickadees were trained to respond to high-threat northern saw-whet owl (NSWO) or low-threat great horned owl (GHOW) calls that were recorded at short distances, and then tested with high- and low-threat owl calls that were rebroadcast and re-recorded across six distances (25m, 50m, 75m, 100m, 150m, and 200m). Subjects were further tested with high-threat and low-threat synthetic tones produced to mimic the natural calls across the six distances. We predicted that birds would perceive and respond to: (1) high-threat predator calls at longer distances compared to low-threat predator calls, and (2) synthetic tones similarly compared to the stimuli that they were designed to mimic. We believed chickadees would continue to perceive and respond to predators that pose a high threat at further distances; however, only responding to low-threat stimuli was consistent across distance recordings. Synthetic tones were treated similarly to natural stimuli but at lower response levels. Thus, the results of this study provide insights into how chickadees perceive threat.

Boreal Owl (<i>Aegolius funereus</i>) and Northern Saw-whet Owl (<i>Aegolius acadicus</i>) breeding records in managed boreal forests

During the 2016 breeding season we monitored 169 nest boxes suitable for Boreal Owl (Aegolius funereus) and Northern Saw-whet Owl (Aegolius acadicus) in high-latitude (>55°N) boreal forests of northwestern Alberta affected by partial logging. Despite the large number of boxes deployed, the number of boxes used by Boreal and Northern Saw-whet Owls was small. Boreal Owls used nest boxes (n = 4) in conifer-dominated stands with three being in uncut blocks and the other in a 50% green tree retention cut-block. In contrast, Northern Saw-whet Owls used boxes (n = 4) in a broader range of cover types, breeding in boxes placed in stands with at least 20% post-harvest tree retention. Although both species successfully bred in the same landscape, Boreal Owls produced fewer eggs (mean = 2.5) and raised fewer young (mean = 0.5) than Northern Saw-whet Owls (5 and 2.25, respectively). Furthermore, our observed Boreal Owl egg production was lower than has been found for the same species nesting in nest boxes in different regions or forest types. In contrast, breeding parameters of Northern Saw-whet Owls were similar to that found in nest boxes in the eastern boreal region of Canada and in the southern part of its range.

Hearing in 3D: Directional Auditory Sensitivity of Northern Saw-Whet Owls (Aegolius acadicus).

Northern saw-whet owls (Aegolius acadicus) are nocturnal predators that are able to acoustically localize prey with great accuracy; an ability that is attributed to their unique asymmetrical ear structure. While a great deal of research has focused on open loop sound localization prior to flight in owls (primarily barn owls), directional sensitivity of the ears may also be important in locating moving prey on the wing. Furthermore, directionally sensitive ears may also reduce the effects of masking noise, either from the owls' wings during flight or environmental noise (e.g., wind and leaf rustling), by enhancing spatial segregation of target sounds and noise sources. Here, we investigated auditory processing of Northern saw-whet owls in three-dimensional space using auditory evoked potentials (AEPs). We simultaneously evoked auditory responses in two channels (right and left ear) with broadband clicks from a sound source that could be manipulated in space. Responses were evoked from 66 spatial locations, separated by 30° increments in both azimuth and elevation. We found that Northern saw-whet owls had increased sensitivity to sound sources directly in front of and above their beaks and decreased sensitivity to sound sources below and behind their heads. The spatial region of highest sensitivity extends from the lower beak to the crown of the head and 30° left or right of the median plane, dropping off beyond those margins. Directional sensitivity is undoubtedly useful during foraging and predator evasion, and may also reduce the effect of masking noise from the wings during flight due to the spatial segregation of the noise and targets of interest.

Environmental conditions and call-broadcast influence detection of eastern forest owls during standardized surveys

AbstractOwls play crucial roles in the environment and provide ecosystem services making them important to monitor and study. However, standardized methods for most species do not exist, and we lack understanding of the effects of many environmental variables and call-broadcast on detection of owls during surveys. We performed a multispecies occupancy analysis of owl monitoring data collected from 2004 to 2013 across the state of Maine to examine the effects of environmental variables, conspecific and heterospecific call-broadcast, and general survey protocols on detection of 3 forest owls: Northern Saw-whet Owl (Aegolius acadicus), Barred Owl (Strix varia), and Great Horned Owl (Bubo virginianus). We found that environmental variables such as cloud cover, precipitation, temperature, time of night, and wind had species-specific effects on detection probability, and ambient noise decreased detection probability for all species. Snow cover did not affect detection of any species. We also found that conspecific call-broadcast increased detection of each species, while heterospecific call-broadcast had variable effects. Specifically, we found that Long-eared and Barred owl broadcast increased the detection of Northern Saw-whet Owl, and our results suggest additional heterospecific effects may exist. Our study showed that, compared to the protocol of the Maine Owl Monitoring Program, surveys simultaneously examining all 3 of our focal species can increase efficiency and lower disturbance by only broadcasting Long-eared and Barred owl calls during a 10-min survey. We recommend that future owl surveys take into account species-specific effects of conspecific and heterospecific call-broadcast, and use our results when designing survey protocols that include one or more of our focal species.

Call production induces motor-driven ZENK response in the song control system of black-capped chickadees

Imitative vocal learning has now been described in a variety of taxa, but is most extensively studied for birdsong learning. However, songbirds produce a variety of songs and calls that also are learned. For example, black-capped chickadees, Poecile atricapillus, produce a variety of highly complex learned vocalizations that they use in different contexts. The role of the neural song control system, including the HVC and the robust nucleus of the arcopallium (RA), in the production of chickadee calls is unclear. In this experiment, birds were housed in different conditions to elicit fee-bee songs and gargle, chick-a-dee and tseet calls. The birds were exposed to different stimuli (i.e. a novel live chickadee, a stuffed saw-whet owl, Aegolius acadicus, a mirror, or long-day photoperiods) and their behavioural and vocal responses were recorded. Brains were then processed to label immediate early gene (ZENK) immunoreactivity. In both HVC and RA, the gargle call elicited the most ZENK immunoreactivity. Although the fee-bee song is well established to depend on imitative vocal learning, its production did not induce the most motor-driven ZENK. These findings suggest that activity in the song control system may be more closely correlated with the acoustic complexity of learned vocalizations than with the extent of imitative learning required during development. These findings, and those of earlier studies, indicate that the neural circuit commonly referred to as the ‘song control system’ is better described as a vocal control system for learned vocalizations.