Modeling a cross-ecosystem subsidy: forest songbird response to emergent aquatic insects

Context: Resource movements across ecosystem
boundaries are important determinants of the diversity
and abundance of organisms in the donor and recipient
ecosystem. However the effects of cross-ecosystem
movements of materials at broader spatial extents than
a typical field study are not well understood.
Objectives: We tested the hypotheses that (1) variation
in abundance of 57 forest songbird species within
four foraging guilds is explained by modeled emergent
aquatic insect biomass inputs from adjacent lakes and
streams and (2) the degree of association varies across
foraging guilds and species within guilds. We also
sought to determine the importance of emergent
aquatic insects while accounting for variation in local
forest cover and edge.
Methods: We spatially modeled the degree to which
distribution and abundance of songbirds in different
foraging guilds was explained by modeled emergent
aquatic insect biomass. We used multilevel models to
simultaneously estimate the responses of species in
four different insectivorous guilds. Bird abundance
was summarized from point counts conducted over
24 years at 317 points.
Results: Aerial insectivores were more abundant in
areas with high estimated emergent insect biomass
inputs to land (regression coefficient 0.30, P\0.05)
but the overall abundance of gleaners, bark-probers,
and ground-foragers was not explained by estimated
emergent insect abundance. The coursing aerial
insectivores had the strongest association with emergent
insects followed by willow flycatcher, olive-sided
flycatcher, and alder flycatcher.
Conclusions: Modeling cross-ecosystem movements
of materials at broad spatial extents can effectively
characterize the importance of this ecological process
for aerial insectivorous songbirds.

File: Schilke-et-al_2020_Landscape-Ecology_Modeling-a-cross-system-subsidy.pdf

Habitat resilience for songbirds: The role of topographic position in a mixed deciduous forest

Climate change is altering patterns of resource availability and this may have negative effects on insectivorous forest birds in the US upper Midwest. As invertebrate life cycle phenology shifts due to earlier spring leaf-out, nesting birds are vulnerable to phenological mismatches between food supply and demand. Areas with complex topography, and thus a variety of thermal and humidity conditions, may support a greater variety of plant and invertebrate phenological rates and stages within close proximity than are found in areas with simple topography. However, the extent and magnitude of this phenomenon is unclear, as is the degree to which topographic position may influence the ability of species to persist during extreme conditions. We examined the effects of topographic position on the
phenology of a tri-trophic forest system over two years from spring through mid-summer. We hypothesized that in cool microsites the likelihood of trophic mismatches and late season food shortages is lower than in warm microsites. At 70 sites in the Baraboo Hills, part of the Driftless Area of the US Midwest, we recorded leaf-out timing of over 700 deciduous trees, measured weekly changes in invertebrate biomass on understory foliage, and conducted bird point counts to assess avian species richness and density. In stream gorges, cooler temperatures were associated with slight but significant delays in leaf-out timing of canopy and understory deciduous trees relative to upland sites. At all sites, invertebrate biomass was distributed relatively evenly across the study period, in contrast to other temperate zone sites where phenological mismatches have been reported between birds and their invertebrate prey. Invertebrate
biomass was similar in stream gorges and uplands in both study years. Insectivorous bird species richness was greater in stream gorges than in the surrounding upland forest during both seasons and was positively related to Lepidoptera larvae biomass in the understory. Among eight abundant insectivorous bird species, density was similar in uplands and stream gorges, among four species density was higher in uplands, and density of two species was higher in stream gorges. These results suggest that insectivorous birds within this study area are unlikely to experience trophic mismatches, and that despite having cooler microclimates and higher avian species richness, stream gorges did not provide more invertebrate food resources than uplands under the climate conditions of the years in which we
sampled this tri-trophic system.

File: Persche-and-Pidgeon-2020.pdf

The richness–heterogeneity relationship differs between heterogeneity measures within and among habitats

The positive monotonic relationship between habitat heterogeneity and species richness is a cornerstone of ecology. Recently, it was suggested that this relationship should be unimodal rather than monotonic due to a tradeoff between environmental heterogeneity and population sizes, which increases local species extinctions at high heterogeneity levels. Here, we studied the richness–heterogeneity relationship for an avian community using two different environmental variables, foliage-height diversity and cover type diversity. We analyzed the richness–heterogeneity within different habitat types (grasslands, savannas, or woodlands) and at the landscape scale. We found strong evidence that both positive and unimodal relationships exist at the landscape scale. Within habitats we found positive relationships between richness and heterogeneity in grasslands and woodlands, and unimodal relationships in savannas. We suggest that the length of the environmental heterogeneity gradient (which is affected by both spatial scale and the environmental variable being analyzed) affects the type of the richness–heterogeneity relationship. We conclude that the type of the relationship between species richness and environmental heterogeneity is non-ubiquitous, and varies both within and among habitats and environmental variables.

File: Bar-Massada-Wood-2014.pdf

Tropical bird species richness is strongly associated with patterns of primary productivity captured by the Dynamic Habitat Indices

Biodiversity science and conservation alike require environmental indicators to understand species richness and predict species distribution patterns. The Dynamic Habitat Indices (DHIs) are a set of three indices that summarize annual productivity measures from satellite data for biodiversity applications, and include: a) cumulative annual productivity; b) minimum annual productivity; and c) variation in annual productivity. At global scales and in temperate regions the DHIs predict species diversity patterns well, but the DHIs have not been tested in the tropics, where higher levels of productivity lead to the saturation of many remotely sensed vegetation indices. Our goal was to explain bird species richness patterns based on the DHIs in tropical areas. We related the DHIs to species richness of resident landbirds for five guilds (forest, scrub, grassland, generalist, and all resident birds) based on a) species distribution model (SDM) maps for 217 species, and b) range map for 564 species across Thailand. We also quantified the relative importance of the DHIs in multiple regression models that included two measures of topography, and two climate metrics using multiple regression, best-subsets, and hierarchical partitioning analyses. We found that the three DHIs alone explained forest bird richness best (R2adj 0.61 for both SDM- and rangemap based richness; 0.15–0.54 for the other guilds). When combining the DHIs with topography and climate, the richness of both forest birds and all resident bird species was equally well explained (R2adj 0.85 and 0.67 versus 0.81 and 0.68). Among the three DHIs, cumulative annual productivity had the greatest explanatory power for all guilds based on SDM richness maps (R2adj 0.54–0.61). The strong relationship between the DHIs and bird species richness in Thailand suggests that the DHIs capture energy availability well and are useful in biodiversity assessments and potentially bird conservation in tropical areas.

File: Suttidate_etal_RSE_TropicalBirds_DHI_2019.pdf

Biodiversity science and conservation alike require environmental indicators to understand species richness and predict species distribution patterns. The Dynamic Habitat Indices (DHIs) are a set of three indices that summarize annual productivity measures from satellite data for biodiversity applications, and include: a) cumulative annual productivity; b) minimum annual productivity; and c) variation in annual productivity. At global scales and in temperate regions the DHIs predict species diversity patterns well, but the DHIs have not been tested in the tropics, where higher levels of productivity lead to the saturation of many remotely sensed vegetation indices. Our goal was to explain bird species richness patterns based on the DHIs in tropical areas. We related the DHIs to species richness of resident landbirds for five guilds (forest, scrub, grassland, generalist, and all resident birds) based on a) species distribution model (SDM) maps for 217 species, and b) range map for 564 species across Thailand. We also quantified the relative importance of the DHIs in multiple regression models that included two measures of topography, and two climate metrics using multiple regression, best-subsets, and hierarchical partitioning analyses. We found that the three DHIs alone explained forest bird richness best (R2adj 0.61 for both SDM- and rangemap based richness; 0.15–0.54 for the other guilds). When combining the DHIs with topography and climate, the richness of both forest birds and all resident bird species was equally well explained (R2adj 0.85 and 0.67 versus 0.81 and 0.68). Among the three DHIs, cumulative annual productivity had the greatest explanatory power for all guilds based on SDM richness maps (R2adj 0.54–0.61). The strong relationship between the DHIs and bird species richness in Thailand suggests that the DHIs capture energy availability well and are useful in biodiversity assessments and potentially bird conservation in tropical areas.

Conservation of the threatened and endemic Rufous-throated dipper, Cinclus schultzi, in Argentina

The Rufous-throated Dipper Cinclus schulzi is endemic to the Southern Yungas of north-western Argentina and southern Bolivia. The species is categorised as ‘Vulnerable’ on the IUCN Red List on
the basis of small population size and restricted range. The purpose of our study was to determine the distribution of potentially suitable habitat for the Rufous-throated Dipper, estimate its pop-ulation size, and assess potential distribution within strict protected areas, in north-western
Argentina. We surveyed 44 rivers in the Southern Yungas of Argentina from 2010 to 2013 to determine dipper density (i.e. the number of individuals detected per km surveyed). The dipper’s potential distribution was assessed using a maximum entropy modeling approach based on
31 occurrence points and eight bioclimatic and two topographic variables as predictors. The species is dependent on mountain forest rivers, so the potential distribution was restricted to rivers. We estimated dipper population size by multiplying density by the potential distribution along rivers.
Finally, we calculated the extent of suitable habitat contained within the boundaries of Argentina´s National Parks. Dipper density was 0.94  1.55 individuals/km. We estimate that within north-west Argentina there are ~2,815 km of river that are potential habitat, with an area of occupancy of
141 km2 and a population size of 2,657  4,355 dippers. However, of this river extent, less than 5%
is within National Parks. Our results highlight the need to create new and to enlarge existing National Parks that protect the potentially suitable habitat of the species. Although more infor-mation is needed for Bolivia, the country-level area of occupancy and population size of the dipper
found in Argentina provides strong evidence that the IUCN Red List classification of this species as ‘Vulnerable’ is warranted.

File: Politi_et_al_2019_conservation_status_rufousthroated_dipper_cinclus_schulzi_argentina.pdf

On leaves, bugs and birds – how topography influences spring phenology, and habitat quality for birds in the Baraboo Hills

Pine Hollow, one of 4 stream gorge study sites

While walking through a forest in spring we often marvel at the vivid greenness, listen to birdsong, and mind our steps in order not to get into a spider’s web. Enjoying the moment, we usually do not think about the complexity of this environment, nor the intertwined relations among all of its elements. However, what slips our attention is not going unrecognized by Maia Persche – a Master’s candidate in the SILVIS lab. In her research, Maia seeks to discern the role of topography in the timing of vegetation growth onset within forest, and to understand how topographic position potentially shapes songbird habitat.

Spring leaf emergence on a hillside in Baxter’s Hollow

To gain insight into these questions, Maia undertook the challenging tasks of measuring tree phenology, and conducting invertebrate and bird surveys in her study area in the Baraboo Hills of Southern Wisconsin. In order to relate these data to each other, each type of survey was carried out at the same 70 locations during narrow time windows throughout the season. Tree phenology surveys occurred in April and May, and invertebrate and bird surveys were repeated throughout the bird breeding season, or until the end of July. At each location, additional data was collected on temperature, tree species composition, and site characteristics. Over the course of two field seasons, she detected 53 insectivorous bird species, and tracked the seasonal abundance of common invertebrate orders (Lepidoptera, Araneae, Hemiptera, Hymenoptera, Diptera and Coleoptera).

Early May at an upland study site in Devil’s Lake State Park

Based on only a portion of measurements collected, Maia has already drawn some interesting preliminary conclusions. Trees leafed out slightly later in stream gorges than in uplands, and although invertebrate biomass was related to tree phenology, it did not appear to follow a predictable yearly pattern. However, sheltered stream gorges supporteded high invertebrate biomass during the mid- and late summer. This could be important for double-brooded bird species that still have active nests in July and can be limited by food availability in some habitats. Overall, stream gorges supported the highest bird species richness, perhaps due to stable food resources or habitat complexity. Also, a strong association has become evident between particular tree species and invertebrate orders, suggesting that tree composition may be more important than topographic context for some folivorous invertebrates.

Louisiana Waterthrush banded at Baxter’s Hollow

Under shifting climate conditions in deciduous forests, it is important to identify areas where habitat quality for species is likely to remain high. To assess bird territory quality in different topographic situations, Maia used feather growth bar analysis for a few widely distributed forest species (Wood Thrush, Red-eyed Vireo, and Ovenbird). She captured birds throughout her study area using mist nets, playback calls, and bird models. She then banded the birds, took structural measurements, and pulled one tail feather. Growth bars, or horizontal bands along the feather, correspond to diet richness of the bird while the feather was growing, and will be used to assess social dominance and habitat quality. Although this approach provides a detailed look at habitat quality, it is also the most difficult to carry out in the field.

Maia has collected a large amount of data, and analyzing the relationships among different factors and trophic levels is somewhat daunting, but she approaches it with great enthusiasm. Maia is currently working to determine how bird territory density varies according to topographic context. It is definitively worth staying tuned to see what new results Maia uncovers!

A Tale of Two Birds: Determining the success of Steller’s Jay management and its effect on the conservation of the endangered Marbled Murrelet in coastal old-growth redwood forests

Steller’s Jay that has been fitted with a radio-transmitter to track its movements and has been banded with a unique color combination, so it can be identified in the future without having to be re-captured.
Steller’s Jay that has been fitted with a radio-transmitter to track its movements and has been banded with a unique color combination, so it can be identified in the future without having to be re-captured.
Steller’s Jay that has been fitted with a radio-transmitter to track its movements and has been banded with a unique color combination, so it can be identified in the future without having to be re-captured.

Ph.D. student Kristin Brunk works in the old-growth redwoods of central California to understand the efficacy of current management for Steller’s Jays (Cyanocitta stelleri) and the implications of this management for the federally threatened Marbled Murrelet (Brachyramphus marmoratus). Both are native bird species in the redwoods, but Steller’s Jays are a synanthropic species, meaning they benefit from associating with humans, while Marbled Murrelet populations have severely declined. As populations of Steller’s Jays have increased, they have threatened the viability of Marbled Murrelet populations mainly through nest predation. Nowhere has this threat been more dire than in protected campground areas in remnant patches of old-growth forest. These areas have Steller’s Jay populations that are twice as high as in non-human dominated forests, and these campground areas also represent 60% of all remaining Marbled Murrelet nesting habitat in central California.

PhD student Kristin Brunk holds a banded Steller’s Jay before releasing.

Murrelet reproduction is naturally slow, as adults only produce one chick per year, but in central California where Steller’s Jay populations are subsidized by human foods, about 80% of Marbled Murrelet nests fail, due mostly to predation. Reducing corvid predation would help boost murrelet reproduction, which is believed to offer the highest probability of Marbled Murrelet population recovery in central California. In 2013, in an attempt to combat corvid predation, California State Parks implemented multiple non-lethal strategies to manage Steller’s Jays. These included improving trash management, deploying noxious murrelet mimic eggs to create taste aversion to murrelet eggs in corvids, and the “Crumb Clean” campaign, an effort to educate campers about the harm of feeding corvids and eliminate corvid access to camper food. The crumb clean campaign requires campers to properly store or dispose of all foodstuffs in their camp. Through the elimination of this food source in the campgrounds, the hope is that jay populations will decrease, allowing more murrelet nests to succeed. Brunk’s research focuses on comparing jay density, home range size, body condition, and diet between pre- and post-management jay populations to determine if these management strategies have been successful.

The aftermath of Steller’s Jays and other wildlife accessing food that has not been properly stowed by campers at a campsite in Big Basin State Park, where Kristin conducts her field work.
The aftermath of Steller’s Jays and other wildlife accessing food that has not been properly stowed by campers at a campsite in Big Basin State Park, where Kristin conducts her field work.

Brunk focuses her work within the campgrounds of Big Basin Redwoods State Park by capturing Steller’s Jays in mist nets. Once she has captured a jay, Brunk takes a tail feather sample to determine the bird’s body condition and a flight feather sample to determine what the bird has been eating. Body condition is determined by measuring feather growth bars, and Brunk deciphers how much human food the jays eat by performing stable isotope analyses on flight feather samples. Each jay is also banded with a unique color combination, so individuals can be identified without re-capturing them, and males are fitted with a backpack-mounted radio-transmitter. By tracking birds with radio-transmitters, Brunk is able to understand how jay home range sizes have changed since management started. Despite a perpetual battle of wits with the extremely intelligent Steller’s Jays, Brunk has successfully banded about 85% of the jays in her study area.

In addition to her research, Brunk is also incredibly active in education and outreach throughout her study area. She gives talks at Big Basin Campfire Programs to educate the public about Marbled Murrelets and Steller’s Jays. Brunk conducts banding demonstrations so campers can see exactly how she captures the jays, and they can often participate in the release of banded jays. And as she walks through the campgrounds tracking birds with radio-transmitters, she frequently answers campers’ questions about the giant metal antenna she is carrying. Brunk works hard to foster good relationships with the other users of her study area and is passionate about sharing her work to promote better public understanding of management initiatives such as the crumb clean campaign.

The Crumb Clean Campaign is an effort in Big Basin and all the other California State Parks to remind campers to pick up all their trash and every crumb of food, so jays and other potentially problematic critters won’t have access to human foods.
The Crumb Clean Campaign is an effort in Big Basin and all the other California State Parks to remind campers to pick up all their trash and every crumb of food, so jays and other potentially problematic critters won’t have access to human foods.

Brunk plans to complete one more field season in 2019, but her research is far from over. Brunk is also working to evaluate a Habitat Conservation Plan (HCP) and its effectiveness at conserving Marbled Murrelet habitat on private land. Habitat Conservation Plans are a commonly used management strategy, with over 1000 HCPs currently active, but the efficacy of these plans has not been well-tested. Brunk hopes to determine if HCPs are an effective management technique, using the Marbled Murrelet as a case study. Overall, Brunk’s research aims to understand the effectiveness of management strategies and conservation of federally threatened species. Ultimately, what she discovers will be used directly in adaptive management strategies that will be paramount in preventing the extinction of the Marbled Murrelet. Along the way, Brunk hopes to uncover strategies and techniques that will apply to the conservation of other species in the future.

Urbanization’s Effects on Avian Predator Occupancy and Citizen Science’s Contribution

A Cooper’s hawk (Accipiter cooperi) in an urban nature conservancy. Prey abundance seems to be the driving factor in their colonization and persistence in urban areas. Photo: Ashley Olah 2014.
A Cooper’s hawk (Accipiter cooperi) in an urban nature conservancy. Prey abundance seems to be the driving factor in their colonization and persistence in urban areas. Photo: Ashley Olah 2014.
A Cooper’s hawk (Accipiter cooperi) in an urban nature conservancy. Prey abundance seems to be the driving factor in their colonization and persistence in urban areas. Photo: Ashley Olah 2014.

The effect of urbanization on wildlife is varied; some species adapt to urbanization, and others do not. For accipiter hawks, urbanization might not be all that bad. By combining remote sensing with citizen science, UW-Madison researchers recently found that in Chicago, increases in imperviousness and tree cover reduced the probability of colonization of urban areas by accipiter hawks, while increases in prey abundance (i.e. songbirds) increased the probability of colonization. In addition, the stabilization of urbanization coincided with a leveling off of hawk occupancy. These recent findings can help scientists understand how wildlife may respond and adapt to urbanization in other areas. To see if these trends reoccur in urban environments in very different biomes, Sofia Kozidis will conduct research that expands upon the Chicago findings to improve understanding of how avian predators respond to urbanization over a larger range of conditions. She will expand the area of study from one urban area, Chicago, to other major urban areas that have high accipiter hawk occupancy and a wealth of Project FeederWatch data.

A pine warbler (Setophaga pinus) visits a suet feeder in a residential backyard. Abundance of songbird species visiting bird feeders affects where accipiter hawks colonize and persist in urban areas. Photo: Ashley Olah 2014.
A pine warbler (Setophaga pinus) visits a suet feeder in a residential backyard. Abundance of songbird species visiting bird feeders affects where accipiter hawks colonize and persist in urban areas. Photo: Ashley Olah 2014.

Project FeederWatch is a citizen science project in which participants count the birds visiting their backyard bird feeders periodically between November and April. This data is used by scientists to track long-term trends in winter bird distribution and abundance. Using FeederWatch data from 1996 to 2018, Sofia will identify occupancy patterns in urban areas that have a high abundance of accipiter hawks while also seeking to assess impacts of hawk’s presence on songbird populations. The expectation is that, as was found in Chicago, accipiter hawk occupancy will increase over time. However, it is possible that colonization and occupancy patterns could differ between cities depending on factors such as climate, city layout, and the surrounding environment. If patterns in certain urban areas differ from the expectation, Sofia will analyze those areas using remote sensing tools to see if she can determine factors associated with unexpected patterns, ultimately helping to elucidate the range of responses of accipiter hawks to urbanization. Sofia’s research highlights the usefulness of citizen science projects, which generates larger amounts of data than could be collected by one researcher alone and connects citizens to scientific research.

Using texture analysis of Landsat satellite imagery to map habitat heterogeneity and avian biodiversity across the conterminous U.S.

Map of habitat heterogeneity across the conterminous U.S., based on 30-m resolution standard deviation texture (21x21 moving window) of NDVI (index of vegetation greenness) from Landsat 8 imagery. Darker green areas indicate regions with higher habitat heterogeneity.

Humans are rapidly transforming the Earth’s ecosystems, with profound consequences for biodiversity. To predict how species will respond to rapidly changing environments, biodiversity science needs better datasets of biodiversity patterns and species distribution. Dr. Laura Farwell is part of a team on a mission to advance and broaden the use of Landsat satellite data for biodiversity science by characterizing habitat heterogeneity at a medium resolution (30 m), across the conterminous U.S.

Map of habitat heterogeneity across the conterminous U.S., based on 30-m resolution standard deviation texture (21x21 moving window) of NDVI (index of vegetation greenness) from Landsat 8 imagery. Darker green areas indicate regions with higher habitat heterogeneity.
Map of habitat heterogeneity across the conterminous U.S., based on 30-m resolution standard deviation texture (21×21 moving window) of NDVI (index of vegetation greenness) from Landsat 8 imagery. Darker green areas indicate regions with higher habitat heterogeneity.

Ecological processes influence patterns of species diversity at multiple scales, and landscape grain strongly affects habitat niches and thus biodiversity potential. Vertebrate species in particular tend to select habitat based on parameters acting at multiple scales. For example, several bird species might strongly prefer large patches of primary forests at broader scales, but at a finer scale habitat selection might be strongly influenced by the amount of heterogeneity within habitat patches. Habitat heterogeneity can also influence species diversity patterns as a result of specialization by certain species on different habitat types. And in general, high heterogeneity increases opportunities for species coexistence. It has been hypothesized that avian diversity is strongly influenced by local scale ecosystem patterns. Vegetation structure is one example of a local scale characteristic that many birds seem to key in on, particularly for nest site selection. But collecting these types of data on the ground is logistically difficult and time consuming. If we can characterize habitat heterogeneity using remotely sensed images, this can potentially be a powerful tool for biodiversity science, allowing rapid classification of vegetation, as well as inference about habitat quality and ecological niches.

A set of indices collectively called image texture holds promise for meeting this need. These indices characterize the amount and pattern of contrast in the tonal values of adjacent pixels, a product of the unique spectral signature of different plant species and combinations within the area covered by the pixels. First-order image texture measures differences in spectral values within a defined neighborhood (e.g., a 3×3 window) surrounding each pixel. More advanced image texture analysis involves 2nd-order texture measures based on a spectral value co-occurrence matrix (GLCM) or local indicators of spatial autocorrelation. It has previously been shown that image texture measures are powerful predictors of avian species richness, in an upper Midwestern U.S. grassland-savanna-woodland system, and in a desert- ecosystem. Building on what has been learned in previous studies, Laura will calculate two 1st-order textures (range and standard deviation), two 2nd-order textures (contrast and angular second moment), plus one local indicator of spatial autocorrelation (the G* statistic).

Map of North American Breeding Bird Survey (BBS) route locations in the conterminous United States. Each breeding season, approximately 4,000 BBS routes are surveyed across the study area. Laura will compare texture measures of habitat heterogeneity with BBS data, with the goal of mapping patterns of avian biodiversity across the conterminous U.S.
Map of North American Breeding Bird Survey (BBS) route locations in the conterminous United States. Each breeding season, approximately 4,000 BBS routes are surveyed across the study area. Laura will compare texture measures of habitat heterogeneity with BBS data, with the goal of mapping patterns of avian biodiversity across the conterminous U.S.

A strength of Laura’s project is the use of Landsat data at 30 m resolution as the basis of texture measures, as this resolution is relevant to many animal species. Laura will characterize image texture across the entire conterminous U.S. She will calculate texture of two different Landsat products- NDVI (which indicates vegetation greenness) and the SWIR band (which highlights leaf and soil moisture content). She will also calculate texture of the cumulative Dynamic Habitat Index currently being derived by PhD student Elena Razenkova, which characterizes plant productivity.

Here to stay: Endangered Kirtland’s warblers call red pine dominated plantations in Wisconsin home

Figure 2 Male Kirtland's warbler. (c) Ashley Olah 2016
Figure 2 Male Kirtland's warbler. (c) Ashley Olah 2016
Male Kirtland’s warbler. (c) Ashley Olah 2016

After coming close to extinction by the early 1970s, nearly twenty years ago Kirtland’s Warblers (Setophaga kirtlandii) achieved the population recovery goal set by the U.S. Fish and Wildlife Service and since then have exceeded that goal; stabilizing at nearly twice the population size set as the goal when they were first listed as endangered. The current population estimate is approximately 2300 pairs. Therefore the US Fish and Wildlife Service is in the process of removing Kirtland’s warblers from the Endangered Species List. While reaching this goal is a huge step forward, there may be negative implications of delisting this extremely specialized species, as it will always rely on habitat management for its persistence.

Kirtland’s Warblers are habitat specialists, meaning that they have evolved to use only very specific habitats. Traditionally, Kirtland’s warblers bred in patches of wildfire-regenerated jack pine forest, on sandy soils, and they only used these areas while the trees were young and approximately five to 20 feet tall. Their stronghold has been lower Michigan, where approximately 99% of the population exists. Fire suppression and deforestation associated with European settlement in the Great Lakes region led to major decreases of young jack pine forests, the essential habitat necessary for foraging and nesting by Kirtland’s warblers. While Kirtland’s warblers don’t nest in the jack pine trees themselves, they do make use of the lower branches of the adolescent trees to camouflage and protect the nests, which are located on the ground, often close to the base of a tree. As the jack pines age they shed these low hanging branches and nests not hidden from view are exposed and vulnerable to predation. These low live branches available only in young stands of jack pine seem to be an in important element females use to select the placement of nests.

The decline in young jack pine forests together with nest parasitism by Brown-headed cowbirds led to the decline in Kirtland’s warblers, and their listing on the Endangered Species List. In subsequent management aimed at recovering the species, habitat had to be created by harvesting and planting jack pine in patterns that mimicked the way jack pine grew after wildfires – patches of dense pines with interspersed openings. Management, also including cowbird control, was successful, warbler population size increased, and eventually all suitable habitat in lower Michigan was saturated by breeding Kirtland’s warblers, and some birds began to prospect for new breeding areas outside of their core range.

Around 2007, Kirtland’s warblers started to colonize a non-traditionally used habitat, commercially owned red pine plantations in central Wisconsin. Kirtland’s warbler have been breeding in those plantations of young red pine ever since.

Figure 1 Ecological Regions where Kirtland's warblers breed in Wisconsin. In the Northwest Sands (Bayfield County; red) and the Northeast Sands (Marinette County; green), Kirtland’s warblers breed in jack pine forests. In the Central Sands (Adams County; blue), Kirtland’s warblers breed in in red pine dominated plantations.
Figure 1 Ecological Regions where Kirtland’s warblers breed in Wisconsin. In the Northwest Sands (Bayfield County; red) and the Northeast Sands (Marinette County; green), Kirtland’s warblers breed in jack pine forests. In the Central Sands (Adams County; blue), Kirtland’s warblers breed in in red pine dominated plantations.

The population recovery in Michigan, subsequent prospecting for new breeding areas, and colonization of habitat in Wisconsin is exciting, and is promising news for the long term persistence of the Kirtland’s warbler. When members of a species are spread across different geographic areas, if a disaster strikes one area, the individuals in other areas can still carry on and perhaps serve as a source to repopulate the area where the disaster occurred. While the Wisconsin population is not large enough to buffer the species from a large catastrophic event in the core range in Michigan, this range expansion is a step in the right direction, and researchers and managers in Wisconsin are working to increase the small number here by ensuring that suitable habitat is available.

What is it about red pine plantations that attract Kirtland’s warblers though? Ashley, in collaboration with scientists from the Wisconsin Department of Natural Resources and the U.S. Fish and Wildlife Service, set out to observe the warblers and collect data about the Wisconsin red pine dominated plantations they breed in to try to find the answer.
Ashley has found that a major factor in Kirtland’s warbler habitat selection is the age of the red pines, which parallels their use of jack pines in Michigan. Red pine saplings are planted at relatively high density, similar to the density of naturally regenerated jack pines that Kirtland’s warblers rely on. Red pines also tend to retain their lowest branches longer than jack pines. This trait of red pines may mean that they can hide nests on the ground in a way that Kirtland’s warblers find suitable, for more years than jack pine do. Additionally red pines thrive on the sandy soils that support jack pine, and that quickly drains after rain events, keeping Kirtland’s warblers’ ground nests dry. A final factor that Kirtland’s warblers seem sensitive to when deciding where to place their nest is litter depth.

Figure 3 Red pine dominated plantations used by breeding Kirtland's warblers in Adams County, Wisconsin. (c) Ashley Olah 2014
Figure 3 Red pine dominated plantations used by breeding Kirtland’s warblers in Adams County, Wisconsin. (c) Ashley Olah 2014

Ashley has also found that nest success of Kirtland’s warblers in red pine plantations is comparable to nest success in jack pine.

Ashley continues to study Kirtland’s warblers’ use of red pine dominated forests in Wisconsin. Using data collected from nest cameras and radio transmitters she plans to identify nest predators, learn more about Kirtland’s warblers’ nesting behavior, and to document survival rates of juveniles after they leave the nest at her Adams County, Wisconsin study site.