Breeding success of the masked booby (Sula dactylatra dactylatra, Pelecaniformes:Sulidae) at Middle Cay, Pedro Bank, Jamaica
Determinación de éxito reproductivo del alcatraz enmascarado (Sula dactylatra dactylatra, Pelecaniformes:Sulidae) en Cayo Middle, Banco de Pedro Jamaica
The masked booby (Sula dactylatra dactylatra) colony at Middle Cay, Pedro Bank, 60 miles south of Jamaica, has been affected by challenges resulting from anthropogenic ]]>
Key words: masked booby, Middle Cay, Pedro Bank, Jamaica, breeding success, sea bird, Mayfield Method.
Resumen
La colonis del alcatraz enmascarado (Sula dactylatra dactylatra) en Cayo Middle, Banco de Pedro, 60 millas al sur de Jamaica, ha sido afectada por una serie de desafíos resultantes de disturbios antropogénicos a su hábitat. A pesar de la degradación excesiva del hábitat, la colonia muestra resilencia al permanecer en el cayo. Entre junio de 2008 y junio de ]]>
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Palabras clave: enmascarados Piquero, medio Cay, Pedro Bank, Jamaica, cría éxito, aves marinas, método Mayfield
The Caribbean with its tropical ]]>
Sula ]]>
(Lesson, 1931); the masked booby (Atlantic). This seabird species is extant in separate colonies on two of three inhabitable cays located at Pedro Bank Jamaica. The cays with established masked booby colonies are known as Middle and Bird Cay. Though historical reports have indicated that the third cay known as Top Cay also served as a masked booby habitat, there has been no recent observation to substantiate the reports. ]]>
By serving as a habitat for more than five seabird species, Pedro Bank’s Cays (Pedro Cays) are of importance to Jamaica’s seabird diversity and also to Neotropical migrants that stopover to rest during their yearly southward journey. The Bank’s submerged environment also provides good habitat for a variety of organisms and represents one of Jamaica’s last remaining healthy marine ecosystems, supporting coral reefs, sea grass beds and ]]>
(Levy & Koenig, 2009). This healthy marine ecosystem makes Pedro Bank an ideal location for subsistence fishing which is the main economic activity practiced at the location. Fishermen from mainland Jamaica and other Caribbean islands have been reported to reside on Top Cay and Middle Cay instead of returning to their mainland home between fishing bouts (Allen & Webber, 2013). The presence of fisher folk on Top and Middle Cay has resulted in severe disturbances to those cays’ environments thus subjecting native species to direct and indirect anthropogenic influences which may affect their fecundity.
This study was designed to provide information on the Middle Cay masked booby colony’s health or resilience within its disturbed habitat through the determination of its breeding success and comparisons between the colony’s observed success and that of healthy colonies in relatively undisturbed habitats.
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Materials & Methods
Study Area: The Pedro Bank is a seamount which rises abruptly from about 500m and extends more than 160.93km east to west. Located 80km south of Jamaica, with its area of 8 040km2, the bank is one of the largest offshore banks in the ]]>
The Pedro Cays are a ]]>
Middle Cay located at 17˚1’N and 77˚46’W houses a base for the Jamaica Defense Force Coast Guard and is normally inhabited by hundreds of Jamaican fishermen (Espeut, 2006). ]]>
m2 (Zans, 1958), has been reported to experience a rapid increase in human inhabitants since the 1990s (Hay, 2006). Middle Cay is densely populated and consists mainly of corrugated zinc and wooden shacks (Zenny, 2005). More than 50% of the surface area of the cay is occupied by squatter housings (Hay, 2006).
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Sampling: The site was sampled at least once per month between June 2008 and June 2009 with and exception of May 2009 during which no sampling was conducted. Two types of sampling visits were done: day visits and weeklong visits.
Nests were selected for monitoring upon the detection of a laid egg in a nest. Once a nest was selected ]]>
Based on the size of the colony (approximately 227 birds) and a formula from Zar (1999) a sample size of 16 breeding pairs was determined to be a sufficient to obtain statistically sound breeding success values for the colony. Nonetheless twenty-seven nests were monitored through to the offspring’s fledgling ]]>
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The hatching and fledgling success of a colony can provide valuable information about that colony’s health within its breeding habitat. Hatching success indicates the proportion of that colony with the ability to hatch viable offspring from a set of eggs laid. While on the other hand, the fledgling success of a colony indicates the proportion of that colony with the ability to raise chicks to the point at which they achieve flight. The product of the hatching and fledgling successes of a colony will provide the breeding success of that colony (Priddel, Hutton, Olson & Wheeler, 2005). ]]>
Two methods were ]]>
1. The Mayfield Method (Mayfield, 1961; Johnson, 1979)
2. A “Naive ]]>
The results obtained from the application of the above methods to the data for hatching success were compared to assess the significance of any difference between the results. The same procedure was then followed for the determination of fledgling success. The result obtained from the Mayfield Method for the hatching success was then compared to the range of hatching successes ]]>
The breeding success (the product of the hatching success and fledgling success, Priddel et al., 2005) of ]]>
In Mayfield (1961) analysis of Kirtland’s Warbler (Dendroica kirtlandii) he encountered various discrepancies in his data that could not be rectified using customary ]]>
For the Mayfield Method the following information must be known: The sample size (number of nests being monitored).
The stage of development of the offspring within each nest.
The time taken for the species to hatch, fledge or successfully breed must be known.
The fate of each monitored offspring. ]]>
Days between visits to sample site.
Exposure time of offspring in each monitored nest (Following Johnson (1979) that when days between visits are > 15 days, 40% of exposure time must be used instead of 50%)
Total exposure time of monitored offspring within the sample.
The method incorporates the exposure time of sample nests in its determination of success and thus is believed to be more accurate than other methods. To use this method the daily mortality (dmr) rate of the colony must be determined. This is done by dividing the number of nests to fail at hatching (fh) by the total exposure time (tet) of the sample nests within the colony (summation of the exposure time of all sample nests). The daily survival rate (dsr) may then be determined by subtracting the dmr from 1. The Mayfield Method allows for the calculation of the amount of variance contained in the data with the use of the following formula:
v = (1)/(((tet)3)/((tet-fh)fh))
Where “v” is the variance. The standard error (SE) many now be determined by finding the square root of the variance: SE=√v
In the context of this research, hatching or fledgling success (S) of the colony may now be determined by raising the daily survival rate to the power of the total time taken by the species to hatch or fledge respectively, so S=(tth): dsrtth. The resulting success (S) may be reported as a percentage ranging from 0% to 100%. The upper and lower limits of success may also be calculated from the results and reported with the proportion of observed success. This is done for the upper limit by using the formula: (dsr + (2SE))tth. And for the lower limit using the formula: (dsr - (2SE))tth.
“Naïve Estimator”: This method was dubbed the “Naïve estimator” by Rivera (pers comm., 2009). This was done because unlike the Mayfield Method that used exposure time to calculate its results thus making them more accurate, the “Naïve Estimator” does not. Due to this the results obtained by the “Naïve Estimator” may be considered as less accurate since it does not consider variation in its calculation. The information needed for the use of the “Naïve estimator” is as follows:
The number of nests in the sample
The number of failed nests.
Using the “Naïve Estimator” hatching mortality (hm) would be calculated by dividing the number of nests that failed to hatch (fn) by the number of monitored nests (n), as shown in the formula: hm=fn/n. The hatching success (hs) could then be calculated by subtracting the hatching mortality from 1, as shown in the formula: hs=1-hm. Because the results of the “Naïve Estimator” are not corrected for errors, its results were only used in comparisons with the results of the Mayfield method and not to draw conclusions from this study.
Results
The results of the one year field assessment are reported as observations, in tables and as calculated hatching success, fledgling success and breeding success. Population counts revealed an estimate of 227 total boobies forming the colony from which 44 nests were monitored to provide the below results.
Hatching success: Two nests within the colony received eggs during the June 2008 to August 2008 period. These two nests were monitored and labelled sample Nests 1 and 2. However both nests failed to produce a hatchling. A third nest (Nest 3) received eggs in August 2008 and was monitored (Table 1). Sample Nest 3 also failed to produce a hatchling. In September of the same year there was a 100% increase in the number of eggs laid in nests over previous nest monitoring months; three nests became active in September 2008. Of the three nests two were successful in producing hatchlings. Most colony nests received eggs in October 2008 (Table 1). Twenty-one nests with eggs were marked and monitored from October 2008. These nests were monitored to their offspring’s fledgling developmental stage, at which point the fate of a monitored offspring could be determined. Of the twenty-one nests, nine failed to produce a hatchling while 12 were successful. In April 2009 a second mass laying of eggs at the colony was observed. Seventeen of the nests from the second colony breeding attempt were monitored. These seventeen nests were monitored to determine their hatching success. Of the seventeen nests two successfully produced hatchlings; Nests 28 and 39 (Table 1). Therefore, of the forty-four monitored nests, sixteen successfully produced hatchlings. Despite the size of a nests’ clutch only one chick was raised by the adults of successful nests. No dead chicks were observed in failed monitored nests where un-hatched eggs were absent.
The results obtained by using the “Naïve Estimator” and the Mayfield Method are presented in Tables 2 and 3 respectively. While the “Naïve Estimator” produced a proportion hatchling success of 0.36/36%; corrected to 2 decimal places (Table 2) the more reliable Mayfield Method produced a hatchling success of 0.4/40% (Table 3). The Mayfield Method also provided lower and upper 95% confidence interval spread of 0.29/29% at minimum and 0.57/57% at maximum.
Fledgling success: Data were collected from twenty-seven nests (Nests 1 to 27) for the determination of the colony’s fledgling success. Of the 14 nests that successfully produced a hatchling, 13 were successful at producing a fledgling (Table 1). According to the “Naïve Estimator” the fledgling success was found to be 0.93 (Table 4); corrected to 2 decimal places, which was the same as the result produced by the Mayfield Method (Table 5). The lower 95% confidence interval spread was found to be 0.79 while the upper 95% confidence interval spread was 1.0. The daily survival rate was found to be 0.99936 thus indicating that almost no fledgling mortality occurred on a day to day basis. ]]>
Breeding success: The breeding successes obtained from the two methods employed (Table 6). to determine the breeding success of Middle Cay’s masked booby colony the product of the observed hatching and fledgling successes was found. The colony’s breeding success was determined to be 37.20% according to the Mayfield Method and 33.81% according to the “Naïve Estimator”. ]]>
Discussion
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Ultimately, breeding success is the product of hatching and fledgling successes and these are calculated estimates which account for a researcher’s observations at the nests as well as a researcher’s time away from the nests with no observations. The three successes therefore need to be considered in the final analysis.
Hatching success is a potentially ]]>
Table 2 shows that successful sample nests produced only one offspring despite the observation that a maximum of two eggs were laid in nests. This observation is characteristic of the species and is due to the persecution of the younger chick by its older sibling; a process known as siblicide (Nelson, 1978). Consistent with the biology of the ]]>
According to Anderson (1990) the ]]>
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The fledging success of Middle Cay’s masked booby colony was determined to be 93% by both the Mayfield Method and “Naïve Estimator”. Kelper (1969) observed a fledging success of 79% from a study done on 43 boobies in 1964 and a success of 83% from a study done on 30 boobies the following year (1965) at Green Island, Kure. The results from Kelper’s 1969 study were used in the analysis because the Green Island colony was not exposed to adverse disturbances such as that of the Middle Cay colony. Using the average ]]>
The acceptable breeding success for ]]>
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Greatest error in this calculation of breeding success may be attributable to the hatching success calculation but confidence in the data surrounding fledging success and observations of activities in the colony especially over the 24 hour period support the finding of a colony whose breeding success is less than acceptable for this species. Conservation interventions if considered should surround the improvement of the hatching success by reduction of the environmental pressures observed at Pedro Cays but ]]>
Acknowledgments
The completion of this research ]]>
References
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1. Centre for Marine Sciences, Department of Life Sciences, University of the West Indies, Mona. Kingston 7, Jamaica; dale.webber@uwimona.edu.jm
Received 17-X-2013 Corrected 13-III-2014 Accepted 24-III-2014