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Unequal Sex Ratio and Female-female Pairing in the Roseate Tern

Updated on April 4, 2011

Unequal Sex Ratio

Adult female roseate terns tend to have significantly higher survival rates than males (Spendelow et el 2005). As a result, there is a significant number of breeding age females that cannot find male mates, so the average breeding success of females is lower than that of males (Szcys et el 2001). The overall ratio of female to male birds of breeding age in the Northeastern Atlantic population is estimated at 1:27, but this ratio varies with colony site (Gochfield et el 1998). At the Buzzard’s Bay colony site, approximately 56 percent of the breeding age roseate terns are female (Spendelow et el 2005). Studies at the Bird Island colony have shown a sex ratio bias at hatching, with 55 percent of offspring being female. This bias is shown to be more significant in first-laid eggs. The sex ratio at fledging at Bird Island is more biased than at hatching, with 57 percent of the fledglings being female (Szcys et el 2001). In 1999, studies showed that as many as 20 percent of all breeding females at the colony site lacked male mates. At the Falkner Island colony site survival rates differ for adult males and females- males have an annual survival rate of approximately 71 percent whereas female survival rates are around 75 percent (Spendelow et el 2005). Male roseate terns appear to be the most limiting resource within the breeding population (Nisbet & Hatch 1999).

There is no evidence that suggests male chicks are more expensive to raise then female chicks, so the reasons for the unequal sex ratio are unclear (Szcys et el 2001). Lower male survival rates could be due to risks associated with child care, as the males do most of the foraging for chick and mate feeding. Males expend more energy on foraging trips than their female counterparts, and also take greater risks by leaving the safety of the nesting and colony sites (Spendelow et el 2005).

Female-female Pairing and Supernormal Clutches

The unequal sex ratio has resulted in female-female pairing during the mating season. The unequal sex ratio makes it difficult for young females to form a seasonally permanent pair-bond with a male, so some females turn to other female birds (Spendelow2 et el 2005). Female roseate terns that lack male mates tend to be of low phenotypic quality, and can exhibit late laying, small clutch sizes, and small eggs (Nisbet & Hatch 1999). Nests tended by male-female pairs are characterized by eggs laid two to four days apart whereas those nests tended by female-female pairs exhibit a shorter interval between egg laying, usually less than one day (Nisbet & Hatch 1999). Female-female nests are also characterized by supernormal clutches. Supernormal clutches are nests with three or more eggs, usually laid in a nest less than two days apart, and are usually indicative of female-female pairs (Shealer 1995). In addition to female-female pairing, some roseate nests are attended by female-female-male trios, and some by quartets of females as a result of the lack of available male mates (Nisbet& Hatch 1999).

Although larger in size than a normal brood, supernormal clutches often have much lower hatching success than those nests tended by male-female pairs (Szcys et el 2001). This is because female-female pairs are less likely to lay fertile eggs. At the Bird Island study site, the hatching success of two-egg clutches laid by female-female pairs was approximately 46 percent whereas the hatching success of two-egg clutches laid by male-female pairs averaged 98 percent (Nisbet & Hatch 1999). Females that cannot obtain male mates may achieve some reproductive success by acquiring fertilization through extra-pair copulations, but this is rare among roseates, and thus acts as an important constraint on their reproductive success. Female-female pairs are also less likely to successfully raise young from the eggs that do hatch (Nisbet& Hatch 1999). This is probably due to a multitude of factors, including the lack of male parent feeding and the time at which most supernormal clutches are laid. Supernormal clutches are usually laid late in the breeding season at a time when food supply is typically decreasing (Nisbet & Hatch 1999). Nisbet and Hatch noted that A chicks (first born chicks) fledged from supernormal clutches had lower average body mass than A chicks hatched from normal clutches, suggesting that they suffer from the lack of male parent feeding (1999). At the Bird Island study site the overall productivity of females attending supernormal clutches was 0.34 fledglings per female whereas productivity of females attending normal clutches was approximately 1.13 fledglings per female (Nisbet & Hatch 1999). At Bird Island, 30 percent of the supernormal clutches hatched three chicks, but none of the C chicks survived (Nisbet & Hatch 1999).

At the Falkner Island colony site approximately ten percent of all nesting attempts are made by female-female pairs (Shealer 1995).Of all the clutches laid at the Bird Island breeding colony, anywhere from one to seven percent of them are supernormal clutches comprised of more than two eggs (Nisbet& Hatch 1999). It also appears that the frequency of supernormal clutches is increasing, suggesting that the unequal sex ratio is becoming more skewed over time, a fact that does not bode well for the roseate tern population (Nisbet & Hatch 1999).

A Case Study in Supernormal Clutch Success

At Falkner Island, there is one recorded case in which a male-female pair successfully fledged a third chick, however the C chick had developed slower and fledged later than its siblings. In this case, the male parent delivered food to the nest at an exceptionally rapid rate, which could have contributed as to why the C chick fledged successfully. The C chick was also fed by researchers, and it is unknown as to whether or not it would have survived without this supplementation (Spendelow et el 1997).

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