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Ardeola 52(2), 2005, 341-345

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Ardeola 52(2), 2005, 341-345 Ardeola 52(2), 2005, 341-345 FLEDGLING SEX RATIO VARIATION IN COLONIES OF GULL-BILLED TERN GELOCHELIDON NILOTICA PATRONES DE VARIACIÓN EN LA RAZÓN DE SEXOS AL VUELO EN COLONIAS DE PAGAZA PICONEGRA GELOCHELIDON NILOTICA A. VILLEGAS*1, J. M. SÁNCHEZ-GUZMÁN*, R. MORÁN* & C. CORBACHO* Fisher (1930) predicted that natural selec- tality (Szczys et al., 2001; Fletcher & Hamer, tion should favour equal parental investment 2004) and body mass growth (Becker & Wink, in male and female offspring, and that popula- 2003), and have reported biases in hatching tion sex ratios at the end of the period of and fledgling sex ratio (Szczys et al., 2001; parental care should be inversely related to the Fletcher & Hamer, 2004), suggesting that, even cost of producing individuals of each sex if fit- in species without strong sexual dimorphism, ness returns from investment in sons and daugh- sons might be more expensive to rear than ters are similar. However, when the relation- daughters or more vulnerable to adverse con- ship between fitness return and environmental ditions during growth. conditions differs between sexes, natural se- This paper reports the patterns of fledgling lection would favour facultative adjustments sex ratio variation in different colonies of Gull- of offspring sex ratio in response to varying billed Tern Gelochelidon nilotica, a monoga- environmental conditions at the time of breed- mous tern with adult males approximately 4% ing (Trivers & Willard, 1973; Charnov, larger than females (Sánchez & Fasola, 2002). 1982). Data were collected during 1997, 1999, and There is growing empirical evidence that 2000 in six colonies of Gull-billed Tern in Ex- birds are able to manipulate the sex of their off- tremadura (SW Spain). The species has bred spring and that this manipulation can be adap- regularly in the region during the last decade on tive (Sheldon, 1998). Offspring sex ratios have islets located in the large reservoirs construct- been found to vary with parental attributes, ed to provide irrigation for agriculture (Sánchez such as timing of breeding, sequence in the & Sánchez, 1991). The total breeding popula- clutch, maternal age, and body condition tion of the species in the area is currently esti- (e.g. Tella et al., 1996; Nager et al., 2000; mated at 1100 pairs (Sánchez, 2003). Colonies Szczys et al., 2001; Velando et al., 2002, Gen- included in this study were located in three ovart et al., 2003), and with environmental con- different reservoirs, Los Canchales (1997, 1999), ditions, such as food availability (Cooch et al., Sierra Brava (1997, 2000), and Orellana (1997, 1997; Torres & Drummond, 1999). 2000). Regular surveys of the colonies were per- In Terns, a family of birds with negligible formed during the breeding season in order to sexual size dimorphism, recent studies have study the numbers and reproductive perform- found sex-specific differences in nestling mor- ance of the birds. Nests and eggs were marked, * Grupo de Investigación en Conservación, Área de Zoología, Universidad de Extremadura. Avda. de Elvas s/n, E-06071 Badajoz, Spain 1 Corresponding author: [email protected] 342 VILLEGAS, A., SÁNCHEZ-GUZMÁN, J. M., MORÁN, R. & CORBACHO, C. and the clutch size and laying date were record- the mean laying date between the beginning of ed for most nests in each colony. Newly hatched May and the middle of June. Both variables chicks were identified with individually num- showed significant differences between bered plastic bands when first found, and were colonies (clutch size: Kruskal-Wallis test: χ2 later ringed with numbered metallic rings (Di- = 76.55, df = 5, P < 0.001; ANOVA: laying rección General de Conservación de la Natu- date: F5,1049 = 92.32, P < 0.001). Birds raleza, Ministerio de Medio Ambiente, Spain). breeding in Los Canchales and Sierra Brava in Chicks were searched for in each visit to the 1997 laid earlier and had larger clutches than colony until they disappeared or fledged. those breeding in the other colonies (Table The mean clutch size of the colonies was 1). used as an index of the parental or environmen- A total of 460 fledglings were sexed in the tal situation of the colonies during the breed- six Gull-billed Tern colonies. The proportion ing season. Data recording began some time of fledgling males varied significantly between after the start of incubation in some colonies. colonies (G = 11.12, df = 5, P = 0.048), and Thus the original clutch size of some nests two of them, Los Canchales and Sierra Brava might have been underestimated if there were in 1997, showed a significant bias towards losses by predation before the start of monitor- males, with sex ratios of 62% and 69%, respec- ing. Egg predation rates in the colonies stud- tively (Table 1). The logistic regression indi- ied were low (0.7 - 2.5%), with no statistical- cated no significant dependence of sex ratio ly significant differences between them (χ2 = on colony site, year, or mean laying date (P > 8.51, df = 5, P = 0.13), and therefore the bias, 0.05). Only mean clutch size had a signifi- if any, would be expected to be small. cant effect on the proportion of fledgling males A sample of chicks was blood-sampled in (Wald: χ2 = 6.81, df = 1, P = 0.009). each colony each year when they were approx- Breeding parameters, such as laying date, imately 20 days old. Blood (ca. 50 µl) was tak- clutch and egg size, or hatching, and fledg- en from the brachial vein, transferred into a ling success can reflect the environmental or tube with 1 ml of 100% ethanol and stored at parental situation at the time of breeding (e.g. 4°C until analysis. Sex was determined using Sydeman et al., 1991; Suddaby & Ratcliffe, polymerase chain reaction (PCR) amplifica- 1997; Oro et al., 1999), and therefore com- tions of the CHD genes (Ellegren, 1996), with parisons of these variables between colonies the modified protocols of Fridolfsson & Elle- may signal differential breeding conditions gren (1999). in them. Since reproduction is a costly process Differences in laying dates and clutch sizes that involves an investment of effort that can between colonies were tested with one-way impair parental condition and survival, par- ANOVA and Kruskal-Wallis tests, respective- ents should adapt the number of chicks to ly, with subsequent Tukey and Nemenyi post- their rearing capacities under the specific hoc tests for multiple comparisons (Zar, 1999). breeding conditions (Stearns, 1992), and the The sex ratio was defined as the proportion first way to do this is by modifying the clutch of males in the sample of sexed chicks. A G- size (Murphy, 2000). A relationship between test was used to examine departures from 1:1 clutch size and food availability, female age, in the fledgling sex ratio, and a logistic regres- or female body condition has been found in sion analysis to study the effect of colony site, several tern species (e.g. Safina et al., year, mean laying date, and mean clutch size 1988; Suddaby & Ratcliffe, 1997; Fletcher on the fledgling sex ratio of the colonies. & Hamer, 2004). Thus, the inter-colony dif- The mean clutch size of the colonies stud- ferences in the mean clutch size of Gull-billed ied ranged from 2.17 to 2.73 eggs per nest and Tern in the present study suggest differences Ardeola 52(2), 2005, 341-345 FLEDGLING SEX RATIO VARIATION IN COLONIES OF GULL-BILLED TERN GELOCHELIDON NILOTICA 343 TABLE 1 Fledgling sex ratios and breeding parameters in the Gull-billed Tern colonies studied. [Razón de sexos al vuelo y parámetros reproductores en las colonias de Pagaza Piconegra estudiadas.] Clutch size Laying date(2) Fledgling sex ratio Year Site(1) [Tamaño de puesta] [Fecha de puesta] [Razón de sexos de volantones] Mean ±SD n Mean ±SD n % Males n G(3) p 1997 CA 2.73 ± 0.58A(4) 158 5.9 ± 1.55A(4) 157 62 127 7.63 0.006 1997 OR 2.28 ± 0.73B 144 8.1 ± 2.07B 141 50 26 0.00 1.00 1997 SB 2.63 ± 0.74A 78 6.35 ± 1.60A 78 70 59 9.19 0.002 1999 CA 2.23 ± 0.69B 303 7.91 ± 1.30B 300 43 60 1.05 0.30 2000 OR 2.17 ± 0.69B 56 11.84 ± 0.99C 32 48 25 0.00 1.000 2000 SB 2.27 ± 0.79B 410 8.75 ± 2.31B 342 56 163 2.70 0.10 1 CA: Los Canchales; OR: Orellana; SB: Sierra Brava. 2 Weeks since 1 April. 3 G-test; df = 1 in all test, Yates’correc- tion for continuity was used. 4 Different letters within a column indicate significant differences between colonies ac- cording to post-hoc tests. in either the parental quality or the environ- quence (Velando et al., 2002; Genovart et al., mental conditions experienced by the indi- 2003; Fletcher & Hamer, 2004), and as a con- viduals in them. sequence colonies with different clutch sizes The fledgling sex ratio showed a clear re- have different sex ratios; (3) there is a season- lationship with colony breeding conditions, a al decrease in the proportion of males per brood greater proportion of males being produced in (Tella et al., 1996; Velando et al., 2002; Gen- the colonies with greater mean clutch size. Bi- ovart et al., 2003), and since early clutches are ases in fledgling sex ratio may be due to either larger than late ones (Eyler et al., 1999), ear- of two processes - the adaptive adjustment by lier colonies have higher mean clutch sizes and the parents of the offspring sex ratio in favour sex ratios than later ones. Further studies that of the sex with the higher fitness return (Trivers would include data on hatchling sex ratios and & Willard, 1973), or the differential pre-fledg- patterns of sex-linked pre-fledging mortality ling survival of the two sexes (Torres & Drum- are needed to fully understand how the varia- mond, 1999; Nager et al., 2000).
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