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Keywords:

  • Acridotheres tristis;
  • Black Rats;
  • Common Mynas;
  • population recovery;
  • Rattus rattus;
  • seabird restoration

Abstract

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES

The population of Sooty Terns Onychoprion fuscata breeding on Ascension Island in the Atlantic Ocean was monitored over 17 years (1990–2007). This period spanned the programme of feral Domestic Cat Felis silvestris catus eradication from the island, which commenced in 2001 with the last Cat recorded in 2004. We report on the abundance of Sooty Terns and Black Rats Rattus rattus before and after Cat eradication. The Sooty Tern breeding population in the 1990s averaged 368 000 and Cats were killing Terns at an average rate of 33 adults per night. Following Cat eradication, adult Terns are no longer predated. However, egg predation by both Rats and Common Mynas Acridotheres tristis has continued with Mynas destroying more eggs than Rats. Unexpectedly, we observed a change in Rat predatory behaviour. Following Cat eradication, Rats have become a major predator of Sooty Tern chicks. Despite this change, the Tern population has shown a season-on-season increase since Cat eradication, 48.8% in 2005, 8.2% in 2006 and 6.1% in 2007, and the breeding population increased to 420 000 birds in 2007. Incubation success improved from 66.0 to 84.4% during Cat eradication, before dropping down again to 67.9% after Cats were eradicated and Rat control measures were introduced. Index traplines were set for Rats and Rat numbers fluctuated widely immediately after Cats were eradicated but there were no significant differences that could be attributed to changes in Cat numbers. Ascension Island Sooty Terns breed every 9.6 months and juveniles defer breeding for seven seasons. Hence 2008 is the first year in which an increase in the breeding Sooty Tern population directly attributable to Cat eradication is likely to be detected. We conclude that long-term monitoring is essential to guide conservation practice even in this relatively simple predator–prey system.

Worldwide, invasive alien species are one of the main causes of decline of seabird populations on islands (Moors & Atkinson 1984). Predation by feral Domestic Cats Felis silvestris catus (hereafter referred to as ‘Cats’ and referred to as Felis catus in Baker, et al. (page 86)) (Driscoll et al. 2007) has been considered responsible for a large percentage of global extinctions of birds and Cat eradication has often been an integral part of conservation policies aimed at population recovery (Nogales et al. 2004). However, the effects of Cat eradication on the target conservation species have rarely been monitored and the results may not be straightforward. Ground-nesting seabird populations were found to have responded positively to Cat eradication on Baker Island, Pacific Ocean, where Sooty Terns Onychoprion fuscata re-established themselves following the elimination of Cats (Moors & Atkinson 1984). However, such re-establishment is not necessarily indicative of population growth but may result from movement of birds between existing colonies. Furthermore, on Little Barrier Island, New Zealand, the eradication of Cats did not lead to an increase in passerine populations (Girardet et al. 2001). We report here on the abundance of Sooty Terns before and after the eradication of Cats on Ascension Island, Atlantic Ocean.

During the last 300 years three alien predators, Cats, Black Rats Rattus rattus (hereafter referred to as ‘Rats’) and Common Mynas Acridotheres tristis (hereafter referred to as ‘Mynas’), were introduced to Ascension Island. Feral populations of all three species were soon established, apparently leading to a decline in seabird populations (Stonehouse 1962). As a consequence the Ascension Island Government (AIG) produced a management plan to halt these seabird population declines (Pickup 1999). The objective was, ‘To re-establish by 2005 a breeding seabird colony on the Ascension mainland, containing amongst other species, Ascension Island Frigate Birds and Red-footed Boobies. To maintain the existing colonies at their present levels.’ To accomplish these objectives it was proposed that all feral Cats be eradicated from the island and Rat densities controlled.

Following the establishment of the three alien predators, eight seabird species occupying 22 sites ceased to breed on the main island of Ascension (Bourne et al. 2003). For example, in the 19th century large colonies of boobies nested on the desert plains (Bedford 1838) but these are now extinct. In 1942 the estimated Sooty Tern population was one million birds (Chapin 1954) and between 1957 and 1959 the populations of the four terns (Sternidae) breeding on Ascension were estimated to be: 750 000 Sooty Terns, 2000 White Terns Gygis alba, 75 000 Black Noddies Anous minutus and 1000 Brown Noddies Anous stolidus (Stonehouse 1962). Over the last half century there have been significant reductions in all tern populations: Sooty Terns by 51% (Ratcliffe et al. 1999), White Terns by 33% (Easterbrook 2004), Black Noddies by 73% (Ashmole et al. 1994) and Brown Noddies by 30% (White et al. 2002). The declines of all these species together, including those that nest safely away from predators on a few offshore islets, suggest that food availability could be involved. However, Ashmole et al. (1994) suggested that predatory pressures may predominantly be to blame for the abandonment of breeding sites on the main island.

Seabirds do not suffer human exploitation on Ascension and the habitat in these seabird colony areas is largely unchanged except for the rapid spread of Mexican thorn bush Prosopis juliflora, introduced in the 1980s (Pickup 1999), and the construction of a number of communication masts; both changes post-date the decline in seabirds.

In contrast, Rats that predate eggs and chicks are associated with the decline of many seabirds (Towns et al. 2006). Rats probably arrived on Ascension in 1701 when HMS Roebuck foundered 100 m offshore. However, when Rat abundance was measured in the main island Sooty Tern colony in 1992 no Rats were found (Merritt et al. 1992), but in 1995 Rats were present at low density (Bell & Ashmole 1995).

Darwin (1844) visited Ascension in 1836 and referred to the Cats introduced just 21 years earlier as a ‘great plague’ as they took adult seabirds and their chicks. Ashmole (1963) estimated that 3.5% of the Sooty Tern breeding population were killed each season. In 1995 the Cat population was estimated at 600–800 (Bell & Ashmole 1995). In the vicinity of the Sooty Tern colony Cats were found at a density of 6–20 per km (Bell & Boyle 2004). Mynas are egg predators (Feare & Craig 1998), but they have been regarded as less of a problem than Cats or Rats. Common Mynas were introduced to Ascension in 1879 and in 2006 the population was estimated at 800 birds (Hughes 2006).

Implementation of the Ascension Island Management Plan aimed at Cat eradication commenced in 2001 and the last feral Cat was recorded in 2004. However, > 100 neutered pet Cats remain on the island (Bell 2005). Rat control in the Tern colonies using bait boxes containing Rodenticide commenced in 2004, and during the Cat eradication programme many Rats were killed as a result of taking poison bait aimed at Cats (Bell 2005). The programme of Cat eradication and Rat control had some immediate successes with five species of seabirds starting to breed again on the main island sites (Sanders 2007). We have determined the breeding population and incubation success of Sooty Terns before and after Cat eradication. During the period 1990–1998 we completed three baseline surveys of the Sooty Tern breeding populations and Ratcliffe et al. (1999) completed a fourth. The mean population size was estimated at 368 000 birds (range 302 000–417 000, n = 4; Ratcliffe et al. 1999). Following Cat eradication, in this study we try to answer the following questions: (1) Has there been an end to the predation of Sooty Terns by Cats, bearing in mind that 100 neutered pet Cats remain on the island? (2) Has the rate of Rat predation on Sooty Terns changed? (3) Has the incubation success of Sooty Terns improved? (4) Has the size of the breeding population of Sooty Terns changed over time?

METHODS

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES

Study area

Ascension (07°57′S, 14°24′W; 97 km2) is one of the volcanic islands that make up the United Kingdom Overseas Territory of St Helena and is isolated in the tropical South Atlantic Ocean midway between South America and Africa. The territory is an Important Bird Area (Sanders 2006). More than half of its surface consists of cinder plains, ash cones and basaltic lava flows. The average annual rainfall is 144.0 mm (Anon 1998) and plant species richness on the plain is < 11 species per 2.6 km2 (Duffey 1964). The dry coastal plain is the traditional nesting site for seabirds. More than 99.9% of the Sooty Terns nest in the colonies on Waterside and Mars Bay (Fig. 1).

image

Figure 1. Map of Ascension Island showing locations of the Sooty Tern colonies at Mars Bay and at Waterside. Historically, seabird colonies were located on the plain that lies to the west of Travellers Hill and Two Boats Village. Boatswainbird Islet and the 14 offshore stacks support a significant proportion of the Ascension Island population of 11 seabird species.

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Data collection

Monitoring data were collected by 14 expeditions mounted by the Army Ornithological Society that visited the island from 1990 to 2007. Expedition dates were planned to coincide with the peak of the Sooty Tern breeding season. Nest losses were monitored on 10 expeditions and a Sooty Tern population census conducted on 11 occasions. Each season about half of the volunteers were visiting Ascension for the first time and the mean effort was 73 person-days (range 17–127 days). Data were gathered using standardized formats. Population surveys were completed 48 days (range 24–93 days, n = 11) after the first egg was laid. Ashmole (1963) estimated that the peak of the breeding season occurred 40–60 days after the first egg of the season was laid. Sooty Terns incubate their eggs for 29 days; egg-laying lasts more than 3 months and is synchronized in subcolonies. Birds are on their breeding grounds for 6 months (Ashmole 1963). The mean duration of the expeditions was 13 days (range 6–24 days, n = 14) during which 14% of the egg-laying phase and 7% of the full breeding season were monitored.

Sooty Tern breeding population

Breeding population surveys using very similar census techniques were conducted that spanned the period before (1990–2001), during (2002–2003) and after the Cat eradication programme (2004–2007). The total Sooty Tern population is so large that the censuses estimated mean nest density in sample quadrats and extrapolated these to the estimated area of the colony (Bibby et al. 1992). During each census, the breeding area was searched systematically for subcolonies. Sooty Terns nest in spatially separate subcolonies, typically c. 10 subcolonies occupied by breeding birds that are at different stages in the breeding cycle, some subcolonies having chicks while others are still laying (B.J. Hughes pers. obs.). The area of the subcolony was surveyed only when the subcolony had stopped expanding and when Terns were incubating.

The area of each subcolony was surveyed precisely in 1990, 1996 and 1998 using compass and tape ring-traverses. In 2000 and 2001 the colonies were surveyed twice, first using compass and tape ring-traverses and then using GPS. The two surveys provided virtually identical results. Only GPS surveys were used from 2002 onwards. Coordinates around the perimeter of each subcolony at intervals of about 20 m were plotted on 1-mm2 graph paper at a scale of 1 : 1000. The total area of the subcolony was deduced by counting the number of 1-mm squares in each subcolony plot. A curvimeter was then used to measure the length of periphery of each subcolony on the plot.

Typically the colony splits into subcolonies that range in size from 0.1 to 6 ha. Subcolonies occupy different parts of the breeding site from the previous season. Nest density was determined from > 100 randomly placed 10-m2 circular ‘quadrats’. Transects were placed at random through the larger subcolonies. At every 10 paces along a transect a pole with a 1.784-m-long string attached was used to describe a circle of area 10 m2. The number of clutches within each circle was counted by two observers. Clutches (rarely more than one egg) were used in the estimate of population size as nests without eggs are often not identifiable. Transects were not placed in subcolonies where eggs had already started hatching. The mean density combined with area was used to estimate the number of nests in these subcolonies. Nest densities differed among subcolonies but were grouped together to obtain a bigger sample if their density did not differ significantly. Variations in mean nest densities on the periphery and in the core of the subcolonies were tested using a z-test. Periphery refers to a 7-m-wide strip around the perimeter of the colony; the remainder of the colony was referred to as core.

The frequency distribution of nest density in quadrats in some subcolonies was non-normal with a skew towards lower levels. A boot-strapping program developed by the Royal Society for the Protection of Birds (RSPB) was used to produce a frequency distribution (Ratcliffe et al. 1999). The mean nest density and 2.5 and 97.5 percentiles were then calculated and applied to the total colony area to give a population estimate with confidence intervals. Spearman Rank Correlation was used to test for a trend in the Sooty Tern population from 1990 to 2007. To test if the objective of the management plan had been achieved we compared the baseline population (mean 1990–1998) with the average population size after Cat eradication.

Predation of nests and incubation success

Eggs were monitored by marking nests with numbered plastic tags and following their fate for the duration of the expedition. Predation levels on adult Sooty Terns were calculated by collecting and counting corpses. To measure the effectiveness of the Rat control measures we monitored nest survival rates using the Mayfield technique (Johnson & Shaffer 1990). This work commenced in 1998. Each season c. 100 nests were marked and then checked on every other day of the expedition. Nests that failed were categorized under the following headings: Myna predation; Rat predation; desertion and others. Eggs that had small holes (Fig. 2) were attributed to Myna predation and eggs missing from the nest were attributed to Rat predation. ‘Others’ may have included nests predated by Cats (Moors & Atkinson 1984). It is likely that a few failed nests that contained smashed eggs were wrongly categorized. Smashed eggs were recorded in the ‘others’ category unless Myna predation was observed in the vicinity, when the smashed egg was placed in the ‘Myna’ category. Incubation success over the full period was calculated from the daily survival rate and extrapolated. Nests were monitored at less than 7 m from the periphery and in the core of the colony. Nests were marked in sets of 10–20. Sets were positioned randomly generally in two well-established and two new subcolonies at either Mars Bay or Waterside. A chi-square test was used to determine whether more nests failed on the periphery than in the core of the colony. Each season we estimated the number of nests which were on the periphery and in the core of the subcolonies. Incubation success each season was determined by combining a single pooled estimate for nest survival in the core with the seasonal number of birds breeding in the core, plus a separate seasonal estimate for survival at the periphery multiplied by the seasonal number of birds breeding in the periphery divided by the total number of breeding pairs. This total seasonal incubation success rate does not take into account the fact that some birds will re-lay and fledge a chick.

image

Figure 2. Sooty Tern eggs on Ascension Island predated by Mynas. Note the puncture holes made by Mynas at the blunt end of each egg. Scale bar is c. 50 mm.

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We used the number of nest failures, the number of nests that survived and a chi-squared test to establish if nest survival had changed following the introduction of Rat control measures in 2004. Variations in the Rat population in the colony were monitored using a simple abundance index calculated as the number of Rats trapped in the Tern colony and expressed as captures per 100 corrected trap-nights (C/100TN) (Cunningham & Moors 1983). On each visit 50 break-back traps baited with peanut butter and cornflakes were set out in pairs along the edge of the colony. Trapping occurred over two consecutive nights. Traps were set in late afternoon, left overnight (14 h) and disabled early the following morning.

Predation of adults

Prior to the eradication of Cats, predation of adult Sooty Terns was monitored by first clearing the perimeter of the colony of all corpses and then revisiting the colony every second day for the duration of the expedition (c. 2 weeks) to collect and record the number of freshly killed birds. The survey team circled the two colonies over a distance of c. 3 km and visited known Cat larders searching for dead birds. The vast majority of dead birds found had been substantially eaten. The nightly average number of birds killed in the two colonies was then calculated and extrapolated across the first 100 days of the breeding season. Evidence that Cats had predated adult Terns at the start of the breeding season could be seen in the colony that appeared to conform to the extrapolation. Towards the end of the season Cats began to take large chicks in preference to adults. Cats may also have bred so the mortality data gathered over 2 weeks were not extrapolated into the future. The last 80 days of the breeding season were not monitored. Following Cat eradication the few adult birds found dead were checked for signs of predation. We used Spearman Rank Correlation to test the decline in adult Sooty Tern mortality in relation to Cat predation.

RESULTS

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES

Sooty Tern breeding population

Estimated numbers of breeding Sooty Terns over the 13 breeding seasons in which surveys were conducted between 1990 and 2007 are shown in Figure 3. In all seasons the birds nested at two colonies: Waterside (holding two-thirds of the population) and Mars Bay (holding the remaining third; Fig. 1). In our analysis, the Sooty Tern population varied between 150 000 and 420 000 adults but without any statistically significant trend (rs = 0.19, n = 13, P = 0.52; Fig. 3). The mean Sooty Tern breeding population in the seven seasons after Cats were eradicated was 394 000 (with year-on-year growth in: October 2005, 366 000 ± 20 000 birds; August 2006, 396 000 ± 48 000; and May 2007, 420 000 ± 14 000). This exceeds the baseline population size (mean of four seasons prior to 1999 of 368 000 birds) by 26 000 (7%) but this was not significant (χ2 = 2.24, P = 0.13). The area occupied by nesting Sooty Terns over the survey period varied between seasons from 3.63 to 11.36 ha and the egg density within the colony varied between 1.18 and 2.16 eggs/m2.

image

Figure 3. Estimated numbers (± 95% confidence limits) of Sooty Terns breeding on Ascension Island from 13 censuses that took place both before (grey bars) and after (open bars) the Cat eradication programme. Note that the subannual breeding cycle results in birds breeding twice in 2004. The number of quadrats (n) measured to obtain the mean density each season is placed at the base of each bar. The census in 1997 was completed by N. Ratcliffe (RSPB).

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Predation of nests and incubation success

We monitored 1158 nests across 10 seasons. Of the 820 nests (5706 nest days) monitored less than 7 m from the perimeter of the colony, 315 failed and, of the 338 nests (3527 nest days) in the core of the colony, only 16 nests failed. Thus, the proportion of nest failure at the periphery of the colony during the 29 days of incubation was significantly greater than in the core of the colony (mean whole egg-stage survival rate in core = 69.3 ± 32.6%, n = 9; periphery 17.7 ± 22.4%, n = 9; χ = 131.4, P = 0.01). Nests were not monitored on the periphery in 1998 and not monitored in the core in 2003. As so few nests were lost in the core of the subcolonies a single incubation success rate of 0.823 (mean of nine seasons) was used. On the periphery of the colony incubation success ranged from 0.021 to 0.853 and was calculated separately for each season.

Of the 233 nests monitored in the three seasons prior to Cat eradication and the introduction of Rat control measures, 107 failed. Of the 656 nests monitored in the five seasons following Cat eradication, 209 failed. Thus, there was a significant reduction in the total number of nests lost following the eradication of Cats and the introduction of Rat control measures (χ2 = 14.2, P < 0.0001). Prior to Cat eradication, 22 (9%) failed nests were due to Rat predation while after Cat eradication the corresponding failure was 42 (6%) nests. This suggests a reduction in the extent of nest failure that could be attributed to Rat predation, although this trend was not significant (χ2 = 1.94, P = 0.16). In our sample 331 nests failed during the 10 seasons of monitoring. We attributed 82 (25%) to Myna predation, 67 (20%) to Rat predation, 168 (51%) to desertion and 14 (4%) to unknown factors. Six (2%) of the failed nests contained smashed eggs. Mynas were not considered a threat in the management plan but we found that they are a major predator of Sooty Tern eggs and were the cause of 25% of egg losses.

Each season (1994–2007) Sooty Tern incubation success was measured for about half of the 29-day incubation period and incubation success for the whole egg stage was calculated using the Mayfield method. Incubation success for the whole egg stage varied between 59.1 and 86.7% (mean 70.6 ± 9.6%, n = 10; Fig. 4). The mean value for incubation success prior to Cat eradication was 66.0 ± 6.3% (n = 3), during Cat eradication it was 84.4 ± 3.2% (n = 2) and after Cat eradication (during the commencement of the phased introduction of Rat control measures) it was 67.9 ± 7.8% (n = 5).

image

Figure 4. Incubation success rate of Sooty Terns breeding before (grey bars) and after (open bars) Rat control measures were introduced on Ascension Island. Note that the subannual breeding cycle results in birds breeding twice in 2004. The number of nests marked with plastic numbered tags (n) each season is placed at the base of each bar. In total 1158 nests were marked.

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The mean Rat index pre-eradication was 3.5 ± 2.6 C/100TN (n = 4). A temporary increase in the Rat population took place immediately after Cats were eradicated in 2004 and before Rat control measures began around the Tern colony. The number of Rats trapped increased to 19 C/100TN at Waterside in 2004 and to 35 C/100TN at Mars Bay in 2005. A reduction, possibly due to enhanced Rat control measures, occurred in 2006 and 2007 when the mean Rat index post-eradication was 9.5 ± 11.5 C/100TN (n = 8). The increase in the Rat population following the Cat eradication programme was not significant (χ2 = 1.46, P = 0.10). These indexes were estimated in near-desert locations (at Sooty Tern colonies) and are not indicative of the Rat population across the whole island.

During 473 days of fieldwork prior to the eradication of Cats we observed no direct predation of Sooty Tern chicks by Rats. However, Rats were taking Sooty Tern chicks at Waterside in May 2007 and more than 100 chicks were taken in 2004. Severe predation from Rats was observed at Mars Bay in 2005 when during a 40-day period following the ringing of 200 chicks we recovered 93 (46%) from Rat predation events.

Predation of adults

The only predators of adult Sooty Terns recorded in the colony were Cats. During the seasons 1990–2000, before Cats were eradicated, > 4500 Sooty Tern adults were collected as victims of Cat kills during the pre-laying and incubation phases, and for 2 weeks after the first chicks had hatched (100 days in total; Fig. 5). Thereafter, Cats were killing large chicks as well as adults. Following the 2000 season the number of Cat kills declined and ceased after 2002. Nightly averages of Cat kills during a 2-week period immediately after chick hatching at Mars Bay and Waterside subcolonies showed similar declines from 1990 to 2002 (Fig. 5). Assuming that Cat predation continued at the same intensity in the second half of the season as it did in the first, the overall percentage of the adult population predated by Cats varied from 1.8% (5800 birds) in 1990 to 0.1% (340 birds) in June 2002 following the knock-down phase of the Cat eradication programme. Following Cat eradication, predation of adult Sooty Terns has declined significantly from 33 birds per night in the early 1990s to nil birds in 2003–07 (rs = –0.82, df = 9, P = 0.006; Fig. 5).

image

Figure 5. Mortality of adult breeding Sooty Terns attributed to Cat predation on Ascension Island. The total number of birds killed during the 100-day period (61% of the full breeding season) that included pre-laying, incubation and a 2-week period immediately after the first chicks hatched are shown as data points. Nightly average kills at each subcolony are shown as bars. Note: data for Waterside in April 1994 are estimated and the trendline between non-successive years is dashed.

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DISCUSSION

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES

Seven breeding seasons after Cat predation ended we estimated that 420 000 ± 14 000 Sooty Terns were breeding on Ascension Island in May 2007. The aim of the Cat eradication and Rat control programme of maintaining the Sooty Tern population at the 1990s level of 368 000 birds was achieved in 2005 on target. The population also increased in the following two seasons. A notable finding of our work is that seasonal variability in the proportion of mature birds that return to Ascension to breed makes accurate census and monitoring of population trends difficult. It is clear from our research that single surveys risk coinciding with periods of deferred breeding.

We have shown that predation by Cats on breeding Sooty Terns ended in 2002 despite the fact that pet Cats, some with feral origins, are found less than 5 km from the Tern colony. We found a significant improvement in incubation success following the eradication of Cats. During the period 1998–2007 we estimate that two-thirds of Sooty Tern nests survived until eggs had hatched. Incubation success improved appreciably during the Cat eradication programme in 2002–03 but post-eradication incubation success fell back but still remained significantly higher than the pre-eradication level.

Did Cats limit Sooty Tern numbers?

If Cats were limiting the Sooty Tern population then their elimination should result in an increase in the adult population. We estimated that Cats were taking more than 5000 birds or 1.8% of the adult population each breeding season. Ashmole (1963) suggested that adult mortality from Cats was as high as 3.5%. The average seasonal increase in the Sooty Tern population numbers pre-eradication (n = 5) and post-eradication (n = 7) using the median dates of September 1997 and December 2004 was 5000 birds (c. 2%). Three season-on-season increases in the adult population support these findings but, as this increase was not significant, a longer dataset is needed to overcome the effects of deferred breeding (Fig. 6).

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Figure 6. Seasonal changes in the percentage of Sooty Terns breeding on Ascension Island since Cats stopped preying upon Sooty Terns in 2002. Season-on-season growth or decline are shown as bars above or below the x-axis, respectively.

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Eradication of Cats could permit Sooty Terns to re-locate from other tern colonies in the Atlantic. The nearest large Sooty Tern colonies are Rocas Atoll, Brazil (2200 km away), and the Island of São Tomé, Gulf of Guinea (2600 km away). Our adult survival mark–capture programme has provided evidence of a single incident of re-location from Rocas Atoll. This suggests that some immigration may have occurred to Ascension Island but re-colonization, as it did on Baker Island following the eradication of Cats (Moors & Atkinson 1984), has not happened. On Little Barrier Island, 9 years after Cats were eradicated, Girardet et al. (2001) found no significant increase in the passerine populations that could be attributed to the change in Cat numbers. Similarly, we found no significant increase in the seabird population (95% of seabirds on Ascension are Sooty Terns) in seven breeding seasons following Cat eradication. However, Sooty Terns defer the start of breeding for seven seasons (B.J. Hughes unpubl. data); hence it is feasible the breeding population will increase in 2008 when first-season breeders that fledged in 2002 return.

What other factors may limit population numbers?

The Sooty Tern breeding population and subsequent breeding success can be subject to major fluctuations caused presumably by food shortages. Thus, more than half (59%) of the sexually mature population deferred breeding in 2000 (Fig. 3). A large number of chicks died of starvation in 1991 (N. Sylverwood-Browne pers. comm.). Similarly, in 1997 (Simmons & Prytherch 1998) and in 2002 (D. Boyle pers. comm.) starvation appeared to have resulted in high levels of chick mortality.

Each breeding season Mynas predate Sooty Tern eggs and they may cause breeding Terns to desert. We estimated that the two communication masts close to the Tern colony kill more than 900 Sooty Terns per season (B.J. Hughes unpubl. data). Periodically, the Terns on Ascension are infested with Pelican Ticks Carios capensis that cause them to be agitated and that may reduce their productivity (B.J. Hughes unpubl. data).

At present the top predators on Ascension are Rats. The data we have collected suggest that current Rat control measures in the Tern colony may be holding the Rat population close to the level prior to Cat eradication. Incubation success improved in 2002 and 2003 while Cats were culled and when poison bait which killed many Rats was widely distributed throughout the period in the Tern colonies (Bell & Boyle 2004). Although the Rat population has fluctuated widely following Cat eradication, there has been no significant increase in the Rat population. Similarly, on Little Barrier Island Rat numbers were not significantly different following Cat eradication but Rats continued to be viewed as a threat and were eradicated in 2003 (Towns et al. 2006). On Ascension there was no significant difference in predation by Rats on the eggs of Sooty Terns following the eradication of Cats but we did observe a change of behaviour and found Rats predating chicks.

Direction of future research

Cat eradication cost over £500 000 and took 2 years to complete. It was a necessary step in the restoration of the seabird population but ongoing successful Rat control measures are needed if predation of chicks is to be moderated. Our research appears to support the findings of Fitzgerald et al. (1991) who found that removal of Cats without simultaneous action to control Rats may do little to restore the seabird population. Mynas appear to predate more eggs than Rats and may stimulate birds to desert. Further research into Myna predation is needed. Sooty Tern populations have been monitored for seven seasons after Cats were eradicated. However, more surveys are still needed. This is because of the delayed breeding of Sooty Terns and the first recruits from the post-Cat eradication era have yet to recruit to the breeding population.

This paper could not have been produced without the enthusiasm, energy and sheer hard work that more than 50 members of the Army Ornithological Society contributed and to them we owe a deep debt of gratitude. B.J.H. is particularly grateful to his friend Colin Wearn from the Royal Air Force Ornithological Society for his tremendous support, particularly with ringing. We thank the Royal Society for the Protection of Birds for airline tickets to Ascension for some volunteers. We are grateful to Dr Richard Bradbury and to two anonymous referees for valuable comments on the manuscript.

REFERENCES

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES
  • Anon. 1998. 74 Years of Ascension Island Rainfall. Ascension Island: Meteorological Office.
  • Ashmole, N.P. 1963. The biology of the Wideawake or Sooty Tern Sterna fuscata on Ascension Island. Ibis 103: 297364.
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