Raptors are making a comeback in cities around the world, thanks to the discontinuation of DDT. We’ve highlighted raptors including Red-tailed Hawks and Peregrine Falcons previously in our Urban Observation of the Week. Within cities, pigeon and rat populations provide ample food and researchers in many cities are investigating what resources and landscape variables dictate where raptors are found.
In New York City, the Parks Department is tracking where raptors are nesting. This work stems from the parks department desire to help the department of public health determine where it was safe to use rodenticide. Because raptors often consume rats, they are susceptible to secondary rodenticide poisoning. Katrina Toal presented on nest site characteristics of Red-tailed Hawks in NYC.
Toal used a species distribution model to investigate the fine-scale landscape variables that may contribute to nesting site preference. This particular model used presence-only data to compare where the nests are located and the site characteristics of that particular nest location. Toal found that Red-tailed Hawks prefer nest locations that contain tall trees surrounded by grass. Red-tailed Hawks also prefer nest locations to be close to “rat reservoirs”. However, the models revealed that the single most important variable for the hawks was proximity to a city park! The species distribution modeling has helped Toal and the NYC Parks Department identify areas that are more likely to have Red-tailed Hawk nests, thus increasing their ability to monitor and protect nesting raptors.
Ed Deal, from the Urban Raptor Conservancy, presented on Seattle’s adaptable urban Copper’s Hawks. Cooper’s Hawks started to colonize cities in the 1980s and 1990s, first in Milwaukee and now in many cities. Deal is investigating where all the Copper’s Hawk nests are to: 1)understand nest density, 2) verify nest fledglings to measure productivity, and 3) band birds to study longevity, mortality, and nest site reuse.
Deal found 46 nest-building pairs over 84 square miles of Seattle–a minimum hawk density of 1 pair / 1.8 square miles. Of these 46 nests, 30 of the nests were in Seattle parks and green areas, though nests were also found in private yards. On average each nest produced 3.63 fledglings. From year-to-year Deal found that 80% of nests were new nests and 20% of nests were reused from the previous year.
Clearly, raptors are using urban spaces. But big questions remain! Importantly, Deal doesn’t know if the population is increasing or if observers are getting better at finding them. It would also be interesting to compare landscape variables and nest density between urban and rural areas. Moreover, are urban raptors the descendants of a few bold individuals that moved into cities a few decades ago, or is migration from rural to urban areas common?
1400 miles south of Seattle in Ensenada, Baja California, México, Diego Toscano Medina is also studying urban Cooper’s Hawks, along with Osprey, Golden Eagle, American Kestrel, and other breeding raptors. Toscano is most interested in whether the stability of resources in cities outweigh the risks, which include collision with buildings, electric wires, and cars as well as egg poaching. After extensive surveys, Toscano found 19 nests from 6 species around the city. Road density pressure for the territory around the nests varied greatly. Coopers Hawks were under the most road pressure–perhaps they are the most urban adaptable?–while Golden Eagles selected nest sites away from urban areas. However, for all species, nest productivity and phenology were about the same in urban areas as observed elsewhere.
One of the challenges Toscano faced that was unfamiliar to the North American panelists was the need for secrecy in raptor research. Nest poachers are a significant problem, and Toscano had to be careful not to give the location of raptor nests away lest they be stolen. This challenge also precludes the use of community science, as was used by Toal in New York City.
The final two presenters both discussed Burrowing Owls–possibly the most adorable owl. Lois Balin, from Texas Parks and Wildlife, reported lessons learned from building an off-grid video monitoring system for burrowing owls. These terrestrial owls create underground burrow nests where they raise their young. Balin’s goal was to better understand owl nesting behavior, along with food and mortality sources. This is difficult due to lack of power at the park where the burrowing owls nest and the high cost and temporal limitations of video monitoring probes.
To work around these issues, Balin’s team built a DIY off-grid monitoring artificial nest system based on off the shelf components, including a home security system, a DVR, batteries, solar panels, PVC pipes, and Pelican boxes. Three cameras were installed in each artificial nest box–one above the nest, and one in each of two nest tunnels. Balin found this system is less disturbing to owls and less costly (approx USD 1800 compared with USD 6000 for a video probe). However, there was a lot of above ground infrastructure required, the solar panels were damaged by perching hawks, and the boxes housing the batteries and DVR became deformed due to the weight of soil.
But perhaps most importantly, Balin found that this system yielded a wealth of useful owl behavior data. Photos revealed that owls took many different prey species, including insects of all kinds, frogs, lizards, and snakes. Balin also recorded predation of three owlets from a nest by a skunk that somehow managed to get into the 4″ pipe opening. In later iterations, Balin tried using 3″ pipe openings for the nests, however, the owls wouldn’t use them…
Elizabeth White Rose, a PhD Candidate at the University of Florida’s School of Natural Resources and Environment, studies Florida burrowing owls. Once grasslands birds, they’re now found on cattle ranches, airports, golf courses, and urban open space. Previous research found that burrows were generally located on vacant lots, with the highest burrow density in moderate levels of development. White Rose wanted to better understand what aspects of urban spaces were attractive to owls.
White Rose placed GPS trackers on male owls with owlets in the nest for two weeks–these parameters ensured data about what was most important for reproduction. Using the spatial information from where these male owls went, White Rose was able to generate land use data for owl hunting areas along with random points from the same area.
By comparing the land uses that were used and those that were available to the owls, White Rose was able to confirm the nesting preference for vacant land. Additionally, owl density did increase with housing development, and there was a population level avoidance of vacant land for foraging behavior. There are sure to be important policy implications from this research–most importantly that conservation efforts can’t just focus on nesting areas but need to also focus on where the owls are getting resources like prey. Future research may try to identify a minimum threshold of vacant land needed for nesting and surveying people’s yards to determine prey abundance and the factors influencing that abundance.
- IUWC 2019: Inclusive Urban Ecosystems - June 10, 2019
- IUWC 2019: Community Science - June 7, 2019
- IUWC 2019: Urban Raptors - June 6, 2019