Wade Harrell’s research while affiliated with U.S. Fish and Wildlife Service and other places

Background The southeastern United States consists of diverse ecosystems, many of which are fire-dependent. Fires were common during pre-European times, and many were anthropogenic in origin. Understanding how prescribed burning practices in use today compare to historic fire regimes can provide perspective and context on the role of fire in critical ecosystems. On the Aransas National Wildlife Refuge (ANWR), prescribed burning is conducted to prevent live oak ( Quercus fusiformis ) encroachment and preserve the openness of the herbaceous wetlands and grasslands for endangered whooping cranes ( Grus americana ) and Aplomado falcons ( Falco femoralis ). This field note builds a digital fire atlas of recent prescribed burning on the refuge and compares it to the historical fire ecology of ANWR. Results Findings indicate that the refuge is maintaining fire-dependent ecosystems with an extensive burn program that includes a fire return interval between 2 and 10 years on a majority of the refuge, with some locations experiencing much longer intervals. These fire return intervals are much shorter than the historic burn regime according to LANDFIRE. Conclusions Following the fire return intervals projected by LANDFIRE, which project longer intervals than the prescribed fire program, would likely be detrimental to endangered species management by allowing increased woody plant encroachment and loss of open habitat important to whooping cranes and Aplomado falcons. Since prescribed fire is part of the management objectives on many national wildlife refuges in the United States, quantifying recent and historical fire ecology can provide useful insights into future management efforts, particularly in cases where endangered species are of special concern and management efforts may be counter to historical disturbance regimes.

The expansion of human infrastructure has contributed to novel risks and disturbance regimes in most ecosystems, leading to considerable uncertainty about how species will respond to altered landscapes. A recent assessment revealed that whooping cranes (Grus americana), an endangered migratory waterbird species, avoid wind-energy infrastructure during migration. However, uncertainties regarding collective impacts of other types of human infrastructure, such as power lines, variable drought conditions, and continued construction of wind energy infrastructure may compromise ongoing recovery efforts for whooping cranes. Droughts are increasing in frequency and severity throughout the whooping crane migration corridor, and the impacts of drought on stopover habitat use are largely unknown. Moreover, decision-based analyses are increasingly advocated to guide recovery planning for endangered species, yet applications remain rare. Using GPS locations from 57 whooping cranes from 2010 through 2016 in the United States Great Plains, we assessed habitat selection and avoidance of potential disturbances during migration relative to drought conditions, and we used these results in an optimization analysis to select potential sites for new wind energy developments that minimize relative habitat loss for whooping cranes and maximize wind energy potential. Drought occurrence and severity varied spatially and temporally across the migration corridor during our study period. Whooping cranes rarely used areas <5 km from human settlements and wind energy infrastructure under both drought and non-drought conditions, and <2 km from power lines during non-drought conditions, with the lowest likelihood of use near wind energy infrastructure. Whooping cranes differed in their selection of wetland and cropland land cover types depending on drought or non-drought conditions. We identified scenarios for wind energy expansion across the migration corridor and in select states, which are robust to uncertain drought conditions, where future loss of highly selected stopover habitats could be minimized under a common strategy. Our approach was to estimate functional habitat loss while integrating current disturbances, potential future disturbances, and uncertainty in drought conditions. Therefore, dynamic models describing potential costs associated with risk-averse behaviors resulting from future developments can inform proactive conservation before population impacts occur.

Whooping Crane Survey Results: Winter 2021-2022 543 Wild Whooping Cranes Estimated (95% CI = 426.5-781.8) The U.S. Fish and Wildlife Service estimated the abundance of whooping cranes in the Aransas-Wood Buffalo population for the winter of 2021-2022. Survey results indicated 543 whooping cranes (95% CI = 426.5-781.8; CV = 0.182) inhabited the primary survey area (Figure 1). This estimate included at least 31 juveniles (95% CI = 20.2-50.8; CV = 0.255) and 196 adult pairs (95% CI = 153.4-282.9; CV = 0.182). Recruitment of juveniles into the winter flock was 6.1 chicks (95% CI = 4.0-9.1; CV = 0.209) per 100 adults.

The challenge of conserving viable habitat while simultaneously predicting how land cover may geographically shift with future climate change has put pressure on ecologists and policy‐makers to develop near‐term (several years to a decade) ecological and geospatial predictions. This is particularly relevant for endangered species as ranges adjust to track a changing climate. The whooping crane is vulnerable to these changes, as the overwintering habitat of a small population is susceptible to climate impacts. This study mapped the historical spatial transformation of crane habitat in and around the Aransas National Wildlife Refuge. A time series of ecological niche models was developed to determine the biotic and abiotic factors correlated with crane presence and track how importance has changed over time. The results from the multitemporal models were used to predict areas along the US Gulf Coast where additional unoccupied habitat may be located for crane population expansion and model how the areas may degrade or change as sea levels rise through future climate change scenarios. Findings indicate that the percentage of emergent herbaceous wetland and water are the most important variables influencing crane presence. Sea level rise analysis indicates that potential habitat throughout the Texas–Louisiana Gulf Coast will be impacted considerably by climate change. The lack of large, continuous blocks of usable land cover could limit population expansion and future recovery efforts. However, the findings can help facilitate winter range expansion to accommodate the growing population by identifying additional areas to protect that could be used by the current wild population or experimental populations.

Electricity generation from renewable‐energy sources has increased dramatically worldwide in recent decades. Risks associated with wind‐energy infrastructure are not well understood for endangered Whooping Cranes (Grus americana) or other vulnerable Crane populations. From 2010 to 2016, we monitored 57 Whooping Cranes with remote‐telemetry devices in the United States Great Plains to determine potential changes in migration distribution (i.e., avoidance) caused by presence of wind‐energy infrastructure. During our study, the number of wind towers tripled in the Whooping Crane migration corridor and quadrupled in the corridor’s center. Median distance of Whooping Crane locations from nearest wind tower was 52.1 km, and 99% of locations were >4.3 km from wind towers. A habitat selection analysis revealed that Whooping Cranes used areas ≤5.0 km (95% confidence interval [CI] 4.8–5.4) from towers less than expected (i.e., zone of influence) and that Whooping Cranes were 20 times (95% CI 14–64) more likely to use areas outside compared to adjacent to towers. Eighty percent of Whooping Crane locations and 20% of wind towers were located in areas with the highest relative probability of Whooping Crane use based on our model, which comprised 20% of the study area. Whooping Cranes selected for these places, whereas developers constructed wind infrastructure at random relative to desirable Whooping Crane habitat. As of early 2020, 4.6% of the study area and 5.0% of the highest‐selected Whooping Crane habitat were within the collective zone of influence. The affected area equates to habitat loss ascribed to wind‐energy infrastructure; losses from other disturbances have not been quantified. Continued growth of the Whooping Crane population during this period of wind infrastructure construction suggests no immediate population‐level consequences. Chronic or lag effects of habitat loss are unknown but possible for long‐lived species. Preferentially constructing future wind infrastructure outside of the migration corridor or inside of the corridor at sites with low probability of Whooping Crane use would allow for continued wind‐energy development in the Great Plains with minimal additional risk to highly selected habitat that supports recovery of this endangered species.

Migratory birds use numerous strategies to successfully complete twice-annual movements between breeding and wintering sites. Context for conservation and management can be provided by characterizing these strategies. Variations in strategy among and within individuals support population persistence in response to changes in land use and climate. We used location data from 58 marked Whooping Cranes (Grus americana) from 2010 to 2016 to characterize migration strategies in the U.S. Great Plains and Canadian Prairies and southern boreal region, and to explore sources of heterogeneity in their migration strategy, including space use, timing, and performance. Whooping Cranes completed ~3,900-km migrations that averaged 29 days during spring and 45 days during autumn, while making 11–12 nighttime stops. At the scale of our analysis, individual Whooping Cranes showed little consistency in stopover sites used among migration seasons (i.e. low site fidelity). In contrast, individuals expressed a measure of consistency in timing, especially migration initiation dates. Whooping Cranes migrated at different times based on age and reproductive status, where adults with young initiated autumn migration after other birds, and adults with and without young initiated spring migration before subadult birds. Time spent at stopover sites was positively associated with migration bout length and negatively associated with time spent at previous stopover sites, indicating Whooping Cranes acquired energy resources at some stopover sites that they used to fuel migration. Whooping Cranes were faithful to a defined migration corridor but showed less fidelity in their selection of nighttime stopover sites; hence, spatial targeting of conservation actions may be better informed by associations with landscape and habitat features rather than documented past use at specific locations. The preservation of variation in migration strategies existing within this species that experienced a severe population bottleneck suggests that Whooping Cranes have maintained a capacity to adjust strategies when confronted with future changes in land use and climate.

Migration corridors for whooping cranes of the Aransas-Wood Buffalo population, delineating 95% core migration areas. Corridors were initially delineated using opportunistic sighting data only collected from 1942–2016 (A) and telemetry data collected 2010–2016 (B). A combined corridor was calculated as a weighted average of the two (C). (TIF)

Defining and identifying changes to seasonal ranges of migratory species is required for effective conservation. Historic sightings of migrating whooping cranes (Grus americana) have served as sole source of information to define a migration corridor in the Great Plains of North America (i.e., Canadian Prairies and United States Great Plains) for this endangered species. We updated this effort using past opportunistic sightings from 1942–2016 (n = 5,055) and more recent (2010–2016) location data from 58 telemetered birds (n = 4,423) to delineate migration corridors that included 50%, 75%, and 95% core areas. All migration corridors were well defined and relatively compact, with the 95% core corridor averaging 294 km wide, although it varied approximately ±40% in width from 170 km in central Texas to 407 km at the international border of the United States and Canada. Based on historic sightings and telemetry locations, we detected easterly movements in locations over time, primarily due to locations west of the median shifting east. This shift occurred from northern Oklahoma to central Saskatchewan at an average rate of 1.2 km/year (0.3–2.8 km/year). Associated with this directional shift was a decrease in distance of locations from the median in the same region averaging -0.7 km/year (-0.3–-1.3 km/year), suggesting a modest narrowing of the migration corridor. Changes in the corridor over the past 8 decades suggest that agencies and organizations interested in recovery of this species may need to modify where conservation and recovery actions occur. Whooping cranes showed apparent plasticity in their migratory behavior, which likely has been necessary for persistence of a wetland-dependent species migrating through the drought-prone Great Plains. Behavioral flexibility will be useful for whooping cranes to continue recovery in a future of uncertain climate and land use changes throughout their annual range.

Results of analyses to determine changes in characteristics of the whooping crane migration corridor, 1942–2016. (DOCX)

The U.S. Fish and Wildlife Service has completed aerial surveys of the primary survey area centered on Aransas National Wildlife Refuge to estimate the abundance of whooping cranes in the Aransas-Wood Buffalo population. Preliminary analyses of the survey data indicated 329 whooping cranes (95% CI = 293–371; CV = 0.073) inhabited the primary survey area (Figure 1). This estimate included 38 juveniles (95% CI = 33–43; CV = 0.078) and 122 adult pairs (95% CI = 108–137; CV = 0.071). Recruitment of juveniles into the winter flock was 13 chicks (95% CI = 12–14; CV = 0.036) per 100 adults, which is comparable to long-term average recruitment. The precision of this year's estimate achieved the target set in the whooping crane inventory and monitoring protocol (i.e., CV < 0.10).