Date of Degree
Eastern screech owls (Megascops asio) are one of the few raptor species that permanently reside in New York City (NYC). To better inform management of this charismatic species in urban parks, I sought to determine the present status and future viability of existing screech owl populations in NYC and identify potential landscape characteristics that affect park occupancy by screech owls. Using captive, non-releasable owls and isolated free-living owls, I developed a method of identifying individual screech owls via vocalization analysis. Using call-broadcast surveys and subsequent recording of owl responses, I gathered capture-recapture histories of urban owls in three NYC parks and one comparison nature preserve, the Mianus River Gorge Preserve (MRGP), a rural/suburban nature preserve in Bedford, NY. These histories were used to estimate abundance and survival rate in each of the parks. I then projected simulated populations using my estimated adult survival and previously published yearling survival and reproductive rates. I built my projection model to include density dependence based on park area, realistic sex ratio fluctuations, and periodic drops in survival rate due to hypothetical environmental events. Survival in urban parks was 0.98 - 1.0, much greater than in the MRGP, 0.57 ± 0.15. Despite the high adult survival, populations in parks could be extirpated within 10 - 20 years by severe drops in survival rate (e.g., extreme winters, storms, or secondary poisoning) if such conditions occurred more often than once every ten years or if parks were smaller than 1.0km2.
In 2008 and 2009, my colleagues and I conducted a citizen science-based study on screech owl occupancy patterns across three counties adjacent to NYC: Westchester and Putnam, NY, and Fairfield, CT. Volunteers conducted call-broadcast surveys on their own properties and sent my colleagues and me the results in 2008 and 2009. Occupancy and detection were modeled as functions of the amount of forest cover and impervious surface cover at each survey point. These models were validated against an independent dataset collected by myself and other trained scientists. Validated models indicated a negative association between occupancy and percent forest cover or, similarly, a positive association with percent impervious cover. Both the citizen science and the systematic datasets supported similar owl-habitat patterns of higher occupancy probabilities in developed areas compared to rural.
The above patterns described eastern screech owl habitat use in rural and suburban areas outside of NYC. I hypothesized that at some point urbanization would become too intense for owls to tolerate, and they would be primarily relegated to protected greenspaces in extremely urbanized cities, as I observed in NYC. In 2010, I surveyed 13 additional parks in NYC and the more urban southern sections of Westchester to characterize occupancy patterns in highly urban areas. I used similar landscape measurements as in the citizen science study, only these measurements were taken across the entire park. Occupancy appeared to decline sharply if the percent impervious cover surrounding a park rose above 50 - 60%. I interpreted this pattern as evidence that high urbanization around a park acts primarily as a barrier to immigration. It is also possible that high urbanization around a park leads to higher mortality from vehicles.
In terms of management, my work has indicated that in large parks (e.g., >1km2), extinction probability is relatively low, but parks larger than 3km2 may be less suitable if big parks allow the establishment of larger raptors such as barred (Strix varia) or great horned owls (Bubo virginianus). Managers can enhance population persistence by increasing the amount of available habitat via habitat restoration and reforesting or re-meadowing developed but unused parks (e.g., large lawns and paved areas) and increasing over-winter survival and reproductive rates by installing nest boxes. However, frequent chance events can extirpate any park-bound population, and thus managers and city planners should also look to enhance the probability of dispersal and recolonization via corridors or reducing the general level of urbanization around protected parks.
Nagy, Christopher M., "Population Dynamics and Occupancy Patterns of Eastern Screech Owls (Megascops Asio) in New York City Parks and Adjacent Suburbs" (2012). CUNY Academic Works.