Detection probabilities of bird carcasses along sandy beaches and marsh edges in the northern Gulf of Mexico

Abstract

We estimated detection probabilities of bird carcasses along sandy beaches and in marsh edge habitats in the northern Gulf of Mexico to help inform models of bird mortality associated with the Deepwater Horizon oil spill. We also explored factors that may influence detection probability, such as carcass size, amount of scavenging, location on the beach, habitat type, and distance into the marsh. Detection probability for medium-sized carcasses (200-500 g) ranged from 0.82 (SE = 0.09) to 0.93 (SE = 0.04) along sandy beaches. Within sandy beaches, we found that intact/ slightly scavenged carcasses were easier to detect than heavily scavenged ones and did not find strong effects of location on the beach on detection probability. We estimated detection rate for each combination of scavenging state, carcass size, and position along sandy beaches. In marsh edge habitats, detection ranged from 0.04 (SE = 0.04) to 0.86 (SE = 0.10), with detection rates rapidly increasing from small (< 200 g) to medium carcass sizes and leveling off between medium and extra-large (> 1000 g) carcasses regardless of vegetation type (Spartina or Phragmites). Carcasses of all sizes were generally harder to locate in Spartina-dominated marshes than in Phragmites-dominated ones. A subset of the data for which we could adequately assess the effect of distance into the marsh indicated that detection rates generally declined the farther a carcass was into marsh vegetation. Based on power analyses, our ability to identify predictors that influence detection rates would be higher with larger numbers of carcasses, greater numbers of search trials per carcass, or more balanced sampling distributions across predictor values.

Avian injury quantification using the Shoreline Deposition Model and model sensitivities

“…Recall that these probabilities are affected by several site-specific factors, including beach type, weather, tidal activity, scavenger activity, and carcass size, etc. (Byrd et al 2009;Varela and Zimmerman 2016;Zimmerman and Varela 2016).…”

“…However, several studies have found that these parameters can vary due to a range of site-specific factors, including beach type, weather, tidal activity, scavenger activity, and carcass size, etc. (Byrd et al 2009;Varela and Zimmerman 2016;Zimmerman and Varela 2016). In the case of DWH, site-specific field studies were conducted to estimate searcher efficiency and carcass persistence values that were specific to the northern Gulf of Mexico.…”

Persistence of avian carcasses on sandy beaches and marsh edges in the northern Gulf of Mexico

“…Specifically, Phragmites-dominated marshes may have more open space between stems than Spartina-dominated ones, allowing scavengers to more easily move through the vegetation and locate carcasses. The field investigators did not collect vegetation density data during the marsh edge persistence study, but inference can be made from the marsh edge searcher efficiency study results (Zimmerman and Varela 2020), in which it was easier to visually detect carcasses in Phragmites than in Spartina, presumably because of the lesser density of vegetation in Phragmites. In both marsh types, vegetation density generally increased with distance from the marsh edge into the marsh.…”

Avian injury assessment for the Deepwater Horizon oil spill

“…Himes Boor and Ford (2019) and Martin et al (2019) provide two methodologies to estimate the probability of whether birds dying on the water would be deposited on shorelines of the northern Gulf of Mexico. Zimmerman et al (2019) describe and provide the results of the Searcher Efficiency Study. Varela and Zimmerman (2019) describe and provide the results of the Carcass Persistence Study.…”