We established a research and educational partnership to document Wedge-tailed Shearwater fallout and road mortality along the southeastern shore of O'ahu, involving four partners:

DOFAW - HPU Pelagicos Lab - U.S. Fish & Wildlife Service - Oikonos

The Wedge-tailed Shearwater ('Ua'u kani, Puffinus pacificus), a seabird protected by both Federal and State statutes, breeds on O'ahu, primarily on the offshore islets along the windward coast. Fledging shearwaters are grounded due to collisions with utility wires, light posts, and other structures. Shearwaters grounded on roadways are often hit by vehicles.

We surveyed a 16-km stretch of the Kalaniana'ole Hwy., spanning from Hawai'i Kai to Olomana Golf Course, (Figure 1), repeatedly during the wedge-tailed shearwater fledging season (Nov. - Dec.) over five years (2011 - 2015) and documented year-to-year changes in the timing and magnitude of shearwater fallout.

In 2011, we documented 128 downed shearwaters during the fledging season (Figure 2). These results highlight the potential for high fallout along the southeast coast of O'ahu, due to the presence of shearwater breeding sites upwind from a seaside highway and urban center (Waimanalo town). These surveys also highlighted the negative relationship between shearwater fallout and moon phase: with significantly more grounded shearwaters during nights with lower moon illumination (Pearson correlation, r = -0.61, n = 17 surveys, p < 0.005).

Figure 2. Wedge-tailed shearwater fallout (number of downed birds) encountered during standardized surveys of the study area. The open circles denote the survey dates and the blue line depicts the degree of lunar illumination.

 

While fallout was widely distributed along the coastline down-wind from Manana Island in 2011, downed shearwaters were concentrated in the vicinity of the Waimanalo Beach Park (Figure 3a), an area characterized by high light post densities (Figure 3b).

Figure 3a. In 2011, we documented 128 grounded shearwaters, with a hotspot of concentration at Waimanalo Beach Park.	Figure 3b. Map of light densities along the study area, showing the "hotspot" associated with the Waimanalo Beach Park.

Results

Road Surveys:  Throughout the four years (2011 – 2015) of this ongoing study, we have completed 68 surveys of a 16-km section of the Kalaniana'ole Highway, from Hawai'i Kai to Olomana Golf Course, and have documented 231 shearwater carcasses (Table 1).   

Because the number of shearwater carcasses documented during standardized surveys varied widely (from 0 to 30), we log-transformed the data as follows: y’ = log (y + 1). We analyzed the log-transformed data using analysis of variance (ANOVA), with one fixed factor (year) and two covariates (Julian date, and percent of the lunar disk that was illuminated). The ANOVA test revealed that shearwater fallout varied significantly from year to year, and in relation to the lunar cycle; yet, there was no significant date effect (Table 2).

An exploration of yearly patterns of fallout reinforced the important influence of the lunar cycle, as evidenced by the multi-modal distributions, with peak fallout during low moon periods and no fallout during high moon periods (Figure 4). Three of the four years (2011, 2012, 2014, 2015) had bimodal fallout distributions, with a primary and a secondary peak separated by a period of no deposition, when moon light approached 100% of the lunar disk. The fifth year (2013) was characterized by a single peak, preceding the full moon.    

Carcass Loss Study :  Live and dead shearwaters were documented using a standardized protocol, and a randomly-selected subset of carcasses were tagged and left on the road, as part of the scavenger removal trial. Over the three years of this study, we marked a randomly–selected subset of 65 shearwater carcasses with an aluminum tag attached to the feet with color string, and left them where they were first encountered.  Carcasses were resighted daily, starting in day 1 and until they reached day 9, when they were removed.

The resighting data collected during a four-year study (2012 – 2015) revealed that the loss rate varied as a function of the amount of time the carcass had been left on the road (number of days since first marked), from 23.1% for fresh carcasses (day 1), to 0.0% for old carcasses (day 6 and day 9).  Altogether, these results suggest that predators (cats, rats, mongoose) scavenge fresh carcasses, but are less attracted to them after they have been on the road for 3 days (Figure 5).  

We synthesized the carcass loss data for the entire study (2012-2015) using a Weibull model, with an additional variable to account for the non-linear influence of time on the resighting probability (Figure 6). The best-fit model had a significant intercept (4.619 +/- 0.059 S.E.), a significant slope (-0.491 +/- 0.068 S.E.) and a significant power exponent (0.788 +/- 0.135 S.E.). Moreover, because the power exponent was different from 1 (t = 5.8270, p = 0.0011), it revealed that the age of a carcass (number of days since marking) had a non-linear influence on the probability of resighting that carcass. Namely, newer and older carcasses were lost at higher and lower rates, respectively.