CONSERVATION OF AUDUBON’S SHEARWATER IN THE BAHAMAS: STATUS, THREATS, AND PRACTICAL SOLUTIONS William A. Mackin Biology Department, Elon University 2625 Campus Box Elon, NC 27244 ABSTRACT 2000 years (Lee, 2000; Steadman et al.1984). These long-lived seabirds lay a single egg per Populations of Audubon‟s Shearwater nesting attempt and have protracted juvenile pe- (Puffinus lherminieri) are a fraction of previous riods of five or more years. Survival in adults of levels and continue to decline. In this paper, I dis- this species and other closely related populations cuss the status, threats, and solutions to the de- has been estimated at over 90% per season cline. Measurements of survival of adults in the (Mackin, 2004; Lee and Haney, 1996). Shearwa- field range from 79.9% by mark and recapture to ters show high site-fidelity, and formerly large 89%-94% using the rate of replacement of mates populations have gone extinct or been decimated in breeding pairs. Using estimates of reproductive in the last century. Documented cases include success of 30%-50%, standard demographic mod- those at Bermuda, Mona Island off Puerto Rico, els indicate that adult survival needs to be around and Tintamarre island near St. Martin. This spe- 95% to achieve population growth (net reproduc- cies is sensitive to the presence of introduced rats tive rates (R) > 1). There are 39 islands in the Ba- (Rattus rattus and Rattus norveigicus; Lee 2000) hamas where shearwaters have been reported to and other mammals. breed, but 7 are known to have introduced preda- Traditional methods of building life-tables tors and 20 others have not been surveyed. I are difficult for shearwaters. Tracking cohorts trapped small mammals at 16 seabird colonies in throughout life requires decades, and there are no the Exumas. Rats (Rattus rattus and Rattus Nor- morphological traits (such as annual growth rings) veigicus) were detected on 5 cays and mice (Mus that provide information about age. Thus, each of Musculans) on one. I also measured death rates of the life history variables must be estimated indivi- shearwaters on the cay that has mice, Allan‟s Cay, dually to approximate the demographic trajectory and another without mice, Long Cay. Death rates of a population. Such methods are the only way to were 0.008 birds /m2 on Allan‟s Cay and 0.004 determine whether seemingly healthy colonies at birds /m2 on Long Cay. The increased mortality protected locations are replacing themselves or appears to be from Barn Owls that are attracted to slowly dwindling due to external population pres- the mice. Four practical measures for conservation sures. include surveying and formally protecting the re- The largest known shearwater colony re- maining islands, eradicating introduced mammals maining in the West Indies is at Long Cay, Exuma where possible, immediately controlling intro- Cays Land and Sea Park, The Bahamas. Between duced mammals by trapping near breeding sites, 800 and 1200 pairs breed there (Mackin, 2004), and constructing additional nest sites at predator- which represents as much at 40% of the remaining free locations breeding pairs in the West Indies (Lee, 2000) There are at least 10 other sites where shearwaters INTRODUCTION nest within the Park and the northern Exuma Cays, making it the last known area where this Populations of Audubon‟s Shearwater formerly widespread species occurs in abundance. (Puffinus lherminieri) have crashed since humans The second largest known colony in the reached the islands of the West Indies in the last Bahamas was Allan‟s Cay, to the north of the Ex- 1 The 11th Symposium on the Natural History of the Bahamas uma Park. This island is about one-third smaller Birds were banded and nests were marked than Long Cay and had a similar density of birds at Long Cay between 1999 and 2006 (see descrip- (1.5-2.5 pairs / 100 m2) to that on Long Cay as tion in Mackin, 2004). Survivorship of adults was recently as the year 2000. That population, how- estimated in two ways. First, pairs in which both ever, has been experiencing mysterious die-offs of members were banded were monitored in succes- birds with hundreds reported dead in some sea- sive years to see if either member of the pair was sons. In May 2003, I saw a house mouse (Mus replaced. Second, the survival of the overall popu- musculus) and found many dead shearwaters with lation was estimated using the Jolly-Seber method barn owl feathers near the carcasses. While the (Greenwood, 1996). The model estimates popula- mice probably do not cause shearwater nests to tion size and survival by tracking whether indi- fail often, they appear to be attracting Barn Owls viduals were new captures or recaptures and in- to the cay. cludes information about the last time the individ- I have visited approximately half of the ual was captured. One important assumption of remaining shearwater colonies in the Bahamas the model is that individuals that have been cap- and searched for undiscovered populations tured do not leave the area or become better at throughout the Exumas. In all cases, the birds avoiding capture in future occasions. have been found in rocky shore habitat where The reproductive success of active breed- there are abundant, natural cavities in the limes- ing pairs was taken from Mackin (2004), where tone. Wherever I have found rocky shore with nests were monitored in May and June and it was Morning Glory (Ipomea indica) and the capertree assumed that large, healthy chicks left at the end (Capparis flexuosa), I have invariably found of the field study each year would successfully shearwaters nesting. This habitat is restricted to fledge. Since recent work (Mackin, unpublished the coastal areas of cays, and, consistent with that data) indicates that Barn Owls take a heavy toll on fact, most of the important shearwater cays are young shearwaters before fledging, I decreased long, thin islands that have large amounts of coas- the estimate by one-half as a best guess of the re- tline. If it can be determined that an island is free productive success until quantitative estimates can of introduced mammals, it should be possible to be made. get an estimate of how many shearwater pairs can From these estimates, I built a spreadsheet nest at a colony simply by the amount of coastal that modeled the population given the parameters habitat that exists there. using demographic methods. I found the sum of In this paper, I analyze 7 years of banding survival and reproduction at each age class (Σlxbx) data to estimate the survival and reproduction of to find the Net Reproductive Rate (R0) and gener- Audubon‟s Shearwater in The Bahamas and esti- ation time (T = Σ(xlxbx)/R0). From this basic ta- mate survivorship curves and reproductive rates ble, I substituted different parameters into the for this population. Using this model and the 39 model to determine how sensitive the model is to locations where Shearwaters are reported to breed changes in reproduction and survival. in The Bahamas, I quantify the remaining habitat, assess the threats posed to the population by in- Area and Coastline of Remaining Colonies troduced mammals and Barn Owls (Tyto alba), and explore how eradicating introduced mammals To identify the remaining habitat for the and building artificial nest sites could halt the de- species in the Bahamas, I used the West Indies cline of this species. Seabird Geographic Information System (Mackin, 2007) to find all locations where Audubon‟s Shearwaters have been reported to breed in the METHODS modern times. I then used Google Earth software to locate and measure the area and coastline of Estimates of Survival and Reproduction each cay. 2 The 11th Symposium on the Natural History of the Bahamas Presence of Rats and Mice ported them back to Elon University. With the help of C. Campbell and J. Goshen, I inspected I assessed the presence of introduced them for juvenile characteristics including dark, mammals on cays in the Exumas by setting sher- unworn plumage, wing chords less than 190 mm man style traps (3” x 3.75” x 9” or 4” x 4.5” x in length, or remaining down. 12”) baited with peanut butter or rodent bait. Work was assisted by C. Cambell and Z. Ewart Additional Nest Sites from Elon Univesity. Focal cays included Allan‟s, Long, Warderick Wells, Narrow Water, Tessa Shearwaters nest in any cavity that is large Roberts, Radar, and Hog. Traps were placed in enough for two adult birds to sit comfortably and randomly aligned transects stratified across the protected from heat and rain. New sites were long axis of each cay or the breeding area of larg- created at 3 small colonies near the Exuma Park er cays (Warderick Wells). Each transect con- Headquarters to see if new populations can be es- tained 6 traps spaced at 10 m intervals. tablished or enhanced at these protected sites. I also trapped other islands with fewer Nests were constructed by moving rocks to create seabirds by quickly placing a single transect with covered openings with similar properties to 6 to 8 traps in accessible areas. These islands in- shearwater nests at Long Cay. Cement was used cluded Norman‟s, Shroud, Elbow, Hawksbill, Lit- to hold rocks in place wherever necessary. tle Cistern, Hall‟s Pond, O‟Briens, Cambridge, North Rocky Dundas, and South Rocky Dundas RESULTS Cays. When mammals were captured, they were Estimates of Survival and Reproductive Success killed individually by transferring to a pillow case and administering a quick blow to the head or by In nests that were monitored in multiple drowning within the trap.