Fox Sparrow Research Articles

Climate-sensitive forecasts of marked short-term and long-term changes in the distributions or abundances of Northwestern boreal landbirds

Climate change presents a major threat to biodiversity globally. Northern ecosystems, such as Canada's boreal forest, are predicted to experience particularly severe climate-induced changes. These changes may reduce the carrying capacity and habitat suitability of the boreal forest for many wildlife species. Boreal birds are susceptible to both direct and indirect effects of climate change, and several studies have predicted northward shifts in species distributions as temperatures become warmer. We forecasted spatially-explicit changes in the densities of 72 boreal landbird species using integrated climate change projections and a forest dynamics model in the Taiga Plains ecozone of the Northwest Territories (NT), Canada, over the 2011–2091 horizon. We 1) identified "winner," "loser," and "bellringer" species over short (2031) and long-term (2091) forecasts, 2) mapped landbird range and density changes under three contrasting Global Circulation Models (GCMs), and 3) quantify differences in landbird density predictions across a latitudinal gradient. Species that showed a moderate increase or decrease in their predicted abundance were considered "winners" and "losers," respectively. Species that showed a marked increase or decrease – a doubling or halving – of their predicted abundance in all three GCMs, were termed "bellringers". From 2011–2031, only 2/72 (2.8%) were considered winners, and 3/72 (4.2%) were losers. From 2011–2091, the abundance of more species was predicted to change: 26/72 (36.1%) were winners, and 10/72 species (13.9%) were losers. Four species were considered bellringers: Gray-cheeked Thrush, White-crowned Sparrow, Fox Sparrow, and American Tree Sparrow. Overall, projected range shifts were strongly oriented along a southeast-to-northwest axis. Shifts to the north and south were evenly distributed among all three GCMs. Our results suggest that future climate-mitigated distribution shifts and population declines of boreal landbirds will require targeted conservation actions. They also highlight the importance of the NT as a potential refugium for many boreal-breeding landbird species in Canada.

Fox Sparrow Fallout in Southeast Saskatchewan in April 2021

Fox Sparrow Fallout in Southeast Saskatchewan in April 2021

A Contact Zone between Coastal and Interior Fox Sparrows in South-Central Alaska

I report observations, supported by museum voucher specimens, photographs, and audio recordings, from an area of breeding contact between two divergent groups of subspecies of the Fox Sparrow (Passerella iliaca) in upper Cook Inlet, south-central Alaska. In this area, covering ~50 km2, the interior iliaca group comes in contact with the Pacific coast unalaschcensis group in lowland mixed boreal forest. Phenotypically pure P. i. zaboria and P. i. sinuosa (subspecies representing the iliaca and unalaschcensis groups, respectively) occur in approximately equal abundances and outnumber intermediate phenotypes. These subspecies co-occur on a fine scale, males of zaboria and sinuosa often holding adjacent territories. I conclude that some form of pre- or post-zygotic isolating mechanism between these two subspecies-groups is hindering free interbreeding.

Locality and Date of Collection of the Type Specimen of the Slate-colored Fox Sparrow

Locality and Date of Collection of the Type Specimen of the Slate-colored Fox Sparrow

THE IMPORTANCE OF RESTORATION AT AN URBAN RIPARIAN SITE IN SOUTHWESTERN OREGON FOR NON-BREEDING BIRDS

Riparian systems support disproportionately high biodiversity and are critical for avian conservation in the western United States, yet much historic riparian habitat has been lost or degraded. Restoration of riparian vegetation and associated ecological processes is a high priority, and birds can serve as indicators of restoration success. Here, we analyze data from 17 y of constant-effort mist netting at an urban riparian restoration site to examine changes in capture rates from 2001 to 2017. Capture rates of 4 of 17 species commonly using the site during autumn, and 6 of 14 species using the site during winter, increased over time, and more so in the restored area compared to the mature riparian reference area. This suggests that these birds increased use of the restored habitat area as riparian tree and shrub plantings matured. Birds that responded positively to restoration included several species that breed in and are considered indicators of coniferous forest habitats (Purple Finch, Oregon Junco, Hermit Thrush, and Fox Sparrow) that often inhabit riparian areas during the non-breeding season. Species that breed in riparian habitat did not show as consistent or as strong of a non-breeding season response to restoration. Riparian birds associated with open, wet herbaceous vegetation (Common Yellowthroat and Lincoln's Sparrow) declined in the restored area in autumn while maintaining stable capture rates in the reference area. Three species of finch (Spinus spp.) used the restored habitat, and the site as a whole, in far fewer numbers during the autumn dispersal and migration period in the years following restoration, likely owing to invasive teasel removal efforts. Taken together, these results suggest that restoration actions can result in trade-offs that benefit some birds while reducing suitable habitat for others. Birds complete important activities in the autumn and winter, including dispersal, migration stopover, and molt, so restoration work that supports bird populations during the non-breeding season, even at relatively small urban sites, can make important contributions to their conservation. However, managers should consider the goals of restoration actions in the context of species- or guild-specific responses to restoration-associated changes in vegetation structure.

The recent expansion of Fox Sparrow (Passerella iliaca iliaca) breeding range into the northeastern United States.

The breeding range of the Eastern Fox Sparrow (Passerella iliaca iliaca) is generally recognized as comprising the boreal forest of Canada. However, recent observations suggest that the species is present during the summer months throughout much of the northeastern US, unexpected for a species characterized as a passage migrant in the region. To clarify, I conducted a literature review to document the historical status of the species in the northeastern US and then analyzed observations submitted to eBird to describe its recent and current status in the region. Historical accounts consistently identify Fox Sparrow as a passage migrant through the region during early spring and late fall. Beginning in the early 1980s, observers began noting regular extralimital records of Fox Sparrow in northern Maine. A single nest was discovered in the state in 1983, and another in northern New Hampshire in 1997. Despite the paucity of breeding records, observations submitted to eBird suggest that the southern limit of the breeding range of Fox Sparrow has expanded rapidly to the south and west in recent years. The proportion of complete checklists submitted to eBird that contained at least one observation of Fox Sparrow grew at an annual rate of 18% from 2003–2016 and was independent of observer effort. Fox Sparrow now occurs regularly on mountaintops and in young stands of spruce (Picea spp.) and balsam fir (Abies balsamea) during the breeding season throughout northern and western Maine and northern New Hampshire, with occasional records from the Green Mountains of Vermont and the Adirondack Mountains of New York. The cause of this rapid expansion of its breeding range is unknown, but may be related to an increase in the amount of young conifer forest in the northeastern US created by commercial timber harvest.

Extensive Distribution of the Lyme Disease Bacterium, Borrelia burgdorferi Sensu Lato, in Multiple Tick Species Parasitizing Avian and Mammalian Hosts across Canada

Lyme disease, caused by the spirochetal bacterium, Borrelia burgdorferi sensu lato (Bbsl), is typically transmitted by hard-bodied ticks (Acari: Ixodidae). Whenever this tick-borne zoonosis is mentioned in medical clinics and emergency rooms, it sparks a firestorm of controversy. Denial often sets in, and healthcare practitioners dismiss the fact that this pathogenic spirochetosis is present in their area. For distribution of Bbsl across Canada, we conducted a 4-year, tick–host study (2013–2016), and collected ticks from avian and mammalian hosts from Atlantic Canada to the West Coast. Overall, 1265 ticks representing 27 tick species belonging to four genera were collected. Of the 18 tick species tested, 15 species (83%) were positive for Bbsl and, of these infected ticks, 6 species bite humans. Overall, 13 of 18 tick species tested are human-biting ticks. Our data suggest that a 6-tick, enzootic maintenance cycle of Bbsl is present in southwestern B.C., and five of these tick species bite humans. Biogeographically, the groundhog tick, Ixodes cookei, has extended its home range from central and eastern Canada to southwestern British Columbia (B.C.). We posit that the Fox Sparrow, Passerella iliaca, is a reservoir-competent host for Bbsl. The Bay-breasted Warbler, Setophaga castanea, and the Tennessee Warbler, Vermivora peregrina, are new host records for the blacklegged tick, Ixodes scapularis. We provide the first report of a Bbsl-positive Amblyomma longirostre larva parasitizing a bird; this bird parasitism suggests that a Willow Flycatcher is a competent reservoir of Bbsl. Our findings show that Bbsl is present in all provinces, and that multiple tick species are implicated in the enzootic maintenance cycle of this pathogen. Ultimately, Bbsl poses a serious public health contagion Canada-wide.

Classic pattern of leapfrog migration in Sooty Fox Sparrow (Passerella iliaca unalaschcensis) is not supported by direct migration tracking of individual birds

ABSTRACT Leapfrog migration systems, whereby more-northern breeding populations overwinter the farthest south, provide unique opportunities to further our understanding of how environmental variation shapes migratory behavior and the seasonal distributions of birds. Leapfrog migration in a western Fox Sparrow subspecies complex (Passerella iliaca unalaschcensis, Sooty Fox Sparrow) was described as early as 1920, and has served as an exemplar of leapfrog systems in subsequent theoretical work. However, migration behavior within P. i. unalaschcensis has never been studied directly, nor has the proposed leapfrog pattern been confirmed through the tracking of individuals. Using light-level geolocators and GPS tags, we tested the long-standing pattern of leapfrog migration in Sooty Fox Sparrows by determining spatiotemporal movement patterns for individuals originating from a northern (Vancouver Island, British Columbia) and a more southern (Point Reyes, California) overwintering region, where migratory timing...

Demography of Sooty Fox Sparrows (Passerella unalaschcensis) following a shift from a migratory to resident life history

Identifying causes and consequences of variation in species life history has the potential to improve predictions about how climate and land-use change may affect the demography and distribution of species in future. Sooty Fox Sparrows (Passerella unalaschcensis (J.F. Gmelin, 1789); or commonly grouped within the Fox Sparrow, Passerella iliaca (Merrem, 1786)) were migrants that rarely bred in the Georgia Basin of British Columbia prior to ∼1950 but have since established resident populations. Data on 270 color-banded birds and 54 nests on Mandarte Island, British Columbia, allowed us to estimate demographic vital rates and population growth in one recently established population. Annual fecundity (F), estimated as the product of the number of broods initiated (1.5 ± 0.01; mean ± SD), clutch size (2.82 ± 0.44), and probability of survival to fledging (0.68 ± 0.02), exceeded values reported for migrants, supporting the hypothesis that residents invest more in reproduction, on average, than migrants within species. Estimating juvenile and adult overwinter survival (Sj = 0.32 ± 0.06 and Sa = 0.69 ± 0.05) next allowed us to simulate an expected distribution of population growth rates as λexp = Sa + (Sj × F), given parameter error. Our estimate of λexp (1.61 ± 0.57) implies expeditious population growth, consistent with the species’ recent colonization of the region.

Demographic consequences of invasion by a native, controphic competitor to an insular bird population.

Species invasions and range shifts can lead to novel competitive interactions between historically resident and colonizing species, but the demographic consequences of such interactions remain controversial. We present results from field experiments and 45 years of demographic monitoring to test the hypothesis that the colonization of Mandarte Is., BC, Canada, by fox sparrows (Passerella iliaca) caused the long-term decline of the resident population of song sparrows (Melospiza melodia). Several lines of evidence indicate that competition with fox sparrows for winter food reduced over-winter survival in juvenile song sparrows by 48% from 1960 to 2015, enforcing population decline despite an increase in annual reproductive rate in song sparrows over the same period. Preference for locally abundant seeds presented at experimental arenas suggested complete overlap in diet in song and fox sparrows, and observations at arenas baited with commercial seed showed that fox sparrows displaced song sparrows in 91-100% of interactions in two periods during winter. In contrast, we found no evidence of interspecific competition for resources during the breeding season. Our results indicate that in the absence of marked shifts in niche dimension, range expansions by dominant competitors have the potential to cause the extirpation of historically resident species when competitive interactions between them are strong and resources not equitably partitioned.

Subarctic‐breeding passerines exhibit phenological resilience to extreme spring conditions

AbstractThere has been relatively little study of the capacity of subarctic passerines to adjust their phenologies to rapid changes on their breeding grounds. Here, we assess variation in passerine arrival timing in Denali National Park, Alaska, from 1995 to 2015, a period that included both the warmest and coldest recorded mean spring temperatures for the park. Using an open‐population occupancy modeling approach in which arrival events are random variables, we investigated interannual variation in the arrival distribution for 10 Nearctic–Nearctic migrants, three Nearctic–Neotropical migrants, and one Palearctic migrant. Neotropical–Nearctic migrants varied in terms of the flexibility of their arrival timing, but generally showed plastic phenologies, suggesting resilience under extreme spring conditions. In comparison, Nearctic–Nearctic migrants showed similar or greater plasticity in arrival timing. A majority of species showed synchronous–asynchronous fluctuation in arrival (i.e., synchronous arrival in some years, asynchronous in others) in combination with various levels of the mean response (i.e., early, average, and late arrival), suggesting the presence of interactions between environmental conditions at multiple scales and inter‐individual variation. The presence of synchronous–asynchronous fluctuation in arrival suggests that weakening of the north–south temperature gradient under continued Arctic amplification may strongly affect arrival variances. Our results also suggest that complex interactions between distributional and phenological changes may be possible. For example, the arrival distribution of Fox Sparrow (Passerella iliaca) became more synchronized over time, a pattern that coincided with a dramatic increase in occupancy probability through expansion of its elevational distribution. Overall, our findings suggest that monitoring of the mean–variance relationship may lead to a deeper understanding of the factors shaping phenological responses.

Infection Prevalence of <i>Borrelia burgdorferi</i> in Ticks Collected from Songbirds in Far-Western Canada

Worldwide, wild birds play a vital role in the dispersal of ticks that harbour tick-borne pathogens, including Borrelia burgdorferi, the Lyme disease bacterium. Using PCR testing, we found 124 (31%) of 405 ticks (4 species), which were collected from 21 species of birds in far-western Canada, to be infected with B. burgdorferi. Transstadial transmission of B. burgdorferi occurred from larva to nymph, plus nymph to adult, in the avian coastal tick, Ixodes auritulus, collected from songbirds in British Columbia (B.C). Collectively, all 3 motile life stages (larva, nymph, adult) of this tick had an infection prevalence of 31% for B. burgdorferi, which suggests vector competency. A Pacific Wren was highly infested with I. auritulus immatures, and 20 (44%) of 45 ticks (2 nymphs, 43 larvae) were infected with B. burgdorferi. This heavy infestation shows the high potential to initiate a new population of ticks and to disseminate Lyme spirochetes. Epidemiologically, B. burgdorferi-infected I. auritulus larvae collected from the Spotted Towhee, Swainson’s Thrush, Pacific Wren, and Fox Sparrow suggest that these avian hosts act as reservoirs for B. burgdorferi. In this study, the western blacklegged tick, Ixodes pacificus, and Ixodes spinipalpis played a limited role in the enzootic transmission cycle of B. burgdorferi along coastal B.C. We document the first record of I. spinipalpis on a bird in Alberta. Because songbirds widely disperse Lyme disease vector ticks, primary health providers and the general public must be vigilant that outdoors people may be bitten by B. burgdorferi-infected ticks throughout far-western Canada.

Observations of Glaucous-winged Gulls Preying on Passerines at a Pacific Northwest Colony

Breeding within a seabird colony can have positive and negative effects for smaller passerine species. For example, they may profit from the anti-predator behavior of the seabirds as well as from the influx of food and nutrients brought into the community. However, these possible benefits might be offset by seabird predation. For instance, gulls are not only fierce defenders of their colony, but also opportunistic feeders. Here, we report predation of Glaucous-winged Gulls (Larus glaucescens) on Sooty Fox Sparrows (Passerella iliaca fuliginosa), Song Sparrows (Melospiza melodia), and a European Starling (Sturnus vulgaris), observed on Mandarte Island, British Columbia, Canada. These observations provide new evidence for dietary supplements of Glaucous-winged Gulls and for a disadvantage of passerine life in a Glaucous-winged Gull colony.

Accounting for incomplete detection: What are we estimating and how might it affect long-term passerine monitoring programs?

A primary objective of ecological monitoring programs typically includes the efficient detection of population trends. Passerines as a group are important ecological indicators and are often included in such programs to provide information on multiple species with a single survey technique. However, commonly used field and analytical approaches may not provide appropriate inference or sensitivity due to assumption violations and differences in the proportion of the population exposed to sampling. Recent methodological developments utilizing repeated point counts and an N-mixture modeling approach for analysis may produce more consistent and interpretable estimates applicable to the superpopulation of individuals using a site during the breeding season. These estimates should be more useful for monitoring because they are not conditioned on presence or availability as are most single-visit approaches. We used repeated count data collected in Denali National Park and Preserve, Alaska (Denali) between 1995 and 2009 from 12 common passerine species to assess variation in presence and availability throughout the season, estimate trends in superpopulation abundance, and provide recommendations for the design of future monitoring programs. We found that variation in detection due to presence and availability was large and differed among species. After accounting for these sources of variation, we estimated abundance of Wilson’s warblers (Wilsonia pusilla) had declined by approximately 48% and fox sparrow (Passerella iliaca) abundance had increased by approximately 250% over 15years. Combined, our results suggest that if trend estimation is a priority, passerine monitoring programs should formally address all components of the detection process, including the probabilities of presence and availability.

Elevational differences in estimated fattening rates suggest that high-elevation sites are high-quality habitats for fall migrants

ABSTRACT. Many migrant songbird species use high-elevation habitats for stopovers in fall throughout North America, but whether these are good migration habitats as indicated by high fueling rates or other measures has not been previously quantified. At high-quality stopover sites, birds can refuel while maintaining their optimal or preferred migration schedules. We used plasma metabolite analysis to estimate fueling rates of four songbird species during the fall migration period over 3 years at two high-elevation (1,200 m above sea level) and two low-elevation (<25 m above sea level) sites in southwestern British Columbia. For three species with more frugivorous diets during fall—the Fox Sparrow (Passerella iliaca), Golden-crowned Sparrow (Zonotrichia atricapilla), and Hermit Thrush (Catharus guttatus)—estimated fattening rates (defined as residual plasma triglyceride levels) were 37–65% higher at high-elevation sites than at low-elevation sites. By contrast, the largely insectivorous and smaller-bodied ...

Dietary segregation between two cohabiting species of sparrows revealed with stable isotope analysis

Fox Sparrows (Passerella iliaca (Merrem, 1786)) and Song Sparrows (Melospiza melodia (A. Wilson, 1810)) cohabit on many islands along the Pacific coast of North America, and previous studies suggest that they rely on similar prey types. We used δ13C and δ15N stable isotope analysis on blood collected from breeding adults of both species in each of two habitats on Triangle Island, British Columbia, Canada, to test the hypothesis that the two species exhibit a consistent pattern (direction) of dietary segregation in different habitat types. Both δ13C and especially δ15N values differed between habitats, indicating that the two habitats were isotopically distinct. As predicted, δ15N values differed consistently between the two species in the two habitats, averaging ∼1.5‰ higher in the smaller Song Sparrow than in the larger Fox Sparrow in both. We infer that Song Sparrows included more animal matter and less plant matter in their diets than Fox Sparrows, and suggest that fixed traits related to body size might underlie the dietary (trophic) differences. In contrast, δ13C values did not differ between species. We conclude that dietary segregation could help to facilitate the widespread cohabitation of these two species of sparrows.

Use of Data on Avian Demographics and Site Persistence during Overwintering to Assess Quality of Restored Riparian Habitat

Monitoring responses by birds to restoration of riparian vegetation is relatively cost-effective, but in most assessments species-specific abundances, not demography, are monitored. Data on birds collected during the nonbreeding season are particularly lacking. We captured birds in mist nets and resighted banded birds to estimate species richness and diversity, abundance, demographic indexes, and site-level persistence of permanent-resident and overwintering migrants in remnant and restored riparian sites in California. Species richness in riparian remnants was significantly higher than in restored sites because abundances of uncommon permanent residents were greater in remnants. Species richness of overwintering migrants did not differ between remnants and restored sites. Responses among overwintering migrants (but not permanent residents) to remnant and restored riparian sites differed. Capture rates were higher in remnant or restored riparian sites for 7 of 10 overwintering migratory species. For Lincoln's Sparrows (Melospiza lincolnii) and White-crowned Sparrows (Zonotrichia leucophrys) proportions of older birds were significantly higher in remnants, even though capture rates of these species were higher in restored sites. Overwinter persistence of 4 migrant species was significantly higher in remnant than in restored sites. A higher proportion of Hermit Thrushes (Catharus guttatus, 56.3%), older Fox Sparrows (Passerella iliaca, 57.1%), Lincoln's Sparrows (59.7%), and White-crowned Sparrows (67.8%) persisted in remnants than restored sites. Our results suggest restored riparian sites provide habitat for a wide variety of species in comparable abundances and diversity as occurs in remnant riparian sites. Our demographic and persistence data showed that remnants supported some species and age classes to a greater extent than restored sites.

Acceptance of Simulated Cowbird Parasitism in a Northern Population of Red-Winged Blackbirds

Red-winged Blackbirds (Agelaius phoeniceus) may be more likely to desert nests parasitized by Brown-headed Cowbirds (Molothrus ater) when they interact with the parasite, because desertion frequencies are higher at naturally parasitized nests than at artificially parasitized nests in some brood-parasitic hosts. We experimentally parasitized blackbird nests in southern Manitoba with cowbird model eggs and presented taxidermic mounts of adult female cowbirds near their nests. We observed no desertion, which is possibly attributable to constraints of a shorter breeding season at this latitude. We also compared rejection frequencies for blackbirds when a mounted female cowbird was placed near the nest to rejection frequencies recorded when (1) no model egg was added, (2) a model egg was added but no mounted cowbird was presented, and (3) a model egg was added plus presentation of a mount of a non-parasitic and non-threatening species, the Fox Sparrow (Passerella iliaca). Blackbirds struck the cowbird mount more often than the sparrow mount, but responses to the cowbird mount did not influence acceptance or rejection of the model egg. Blackbirds apparently engage in the least costly form of anti-parasite defense (aggression) and accept the costs of acceptance instead of those of renesting during a short breeding season.

Browsing down our natural heritage: Deer impacts on vegetation structure and songbird populations across an island archipelago

Declines in forest and woodland birds have largely been attributed to habitat loss and fragmentation. In the past decade, however, the potential for herbivores to influence bird species abundance and community composition via their direct impact on vegetation structure has also been recognised. We tested the hypothesis that deer influence vegetation structure and bird assemblages in a large island archipelago in western North America using surveys of 18 islands with deer densities ranging from 0 to over 1 deer/ha. Amongst these islands, reduced predation and hunting pressure has allowed deer populations to increase above those likely to have existed in pre-European times. Our results support a growing body of evidence that deer regulate both the cover and architecture of understory vegetation which in turn profoundly affects island bird assemblages. Deer-free islands supported the most abundant and diverse bird fauna. Iconic songbirds such as the rufous hummingbird, song and fox sparrow were abundant on islands with no deer but substantially reduced on islands with high deer densities. Only one bird species, the dark-eyed junco, preferred moderate and high density deer islands. Our observations suggest that current cohorts of palatable shrubs on islands with high deer densities are relatively old and potentially represent an impending extinction debt, where the full effects of high deer density on island biota may take decades to fully unfold. Our results suggest that deer densities below a threshold of 0.1 deer/ha should allow native vegetation to recover and a rich and diverse bird species assemblage to persist. We suggest that adaptive management be used to test the validity of this threshold, and that without active management of deer abundance, local extinctions of native flora and fauna appear likely to accelerate.

Male territorial aggression and androgen modulation in high latitude populations of the Sooty, Passerella iliaca sinuosa, and Red Fox Sparrow, Passerella iliaca zaboria

The Fox Sparrows, Passerella iliaca, include multiple groups and subspecies distributed at several latitudes from the Alaskan arctic to the southwestern United States. As such, this species represents a potential model for investigating latitudinal variation in androgen secretion and aggressive territoriality in male passerines. Breeding male Fox Sparrows from two subspecies within two groups, the Sooty Fox Sparrow, P. i. sinuosa, and the Red Fox Sparrow, P. i. zaboria, were assessed for aggressive territoriality and androgen responsiveness at multiple latitudes in arctic and subarctic Alaska. Subarctic Sooty Fox Sparrows had higher circulating androgen levels in the early (8.54 ng/ml) versus mid–late breeding season (2.44 ng/ml). Males in the mid–late breeding season did not up-regulate androgen secretion in response to social challenge, but were aggressive and spent more time within 5 m of a decoy during a simulated territorial intrusion (STI) than early breeding males. Male subarctic Red Fox Sparrows had slightly higher circulating androgen levels (2.29 ng/ml) than arctic males (1.10 ng/ml) in the mid–late breeding season. However, androgen levels were not correlated with blood collection time after a social challenge in either group, suggesting that neither arctic nor subarctic males up-regulate androgen secretion during the mid–late breeding period. Arctic males spent more time within 5 m of a decoy and sang less than subarctic males during an STI in the mid–late breeding season. These findings demonstrate that the Fox Sparrow is a tractable model for investigating the latitudinal regulation of aggressive territoriality and androgen responsiveness in passerines.