Investigating the possible change in breeding strategy of African black oystercatchers

Avian clutch size varies considerably among species.Clutch size ranges between the typically larger plastic clutches produced by indeterminate layers, and the fixed modal clutch sizes of determinate layers.The small clutch sizes of determinate laying waders are not well understood with most theories focussed around rearing costs, predation, and prey limitations.However this does not explain the short term changes (decadal) which may occur within wader species, such as the change in clutch size of the Eurasian Oystercatcher (Haematopus ostralegus) whereby larger than modal clutches were produced through a change in mating habits resulting in polygamy and egg dumping.African Black Oystercatchers (Haematopus moquini) have a modal clutch size of two eggs but occasionally lay one or three eggs.It has been noted that an increase in frequency of larger threeand even four-egg clutches has occurred over the last few decades.By analysing extensive historical nest records dating back five decades I verified the occurrence of this increase in three-egg clutches over many of the sites within their breeding range.As African Black Oystercatchers are very territorial, co-operative polygyny was rejected as a sole cause (through observations).As eggs were found to be significantly similar (through intra-clutch egg shape comparisons) within clutches egg-dumping as a cause was also rejected.It was thus established that the increase in three-egg clutch frequency was indicating an increase in breeding effort.On further analysis of this increase, it was discovered that three-egg clutches offer no benefit to species fitness or breeding success as they do not result in an increased fledgling output to those of the smaller two-egg clutches.This is primarily due to inefficient incubation as a result of the extra egg, and the inability to maintain and feed a larger brood size.This study validates previous assertions that three-egg clutch frequency is increasing for African Black Oystercatchers.This increase is a substantial change in life history.In understanding what the primary influencing factors are, the information can further aid in the understanding of all wader clutch sizes.The methods adopted within this study can also be used to establish if clutch size increase is occurring within other species and test for factors such as polygyny and egg dumping.These changes in breeding strategy are uncommon, and if they occur it is important to understand if the increase is beneficial for the species.African Black Oystercatchers exhibiting this increase do not produce more fledglings or improve their breeding success compared with individuals laying two-egg clutches.With experience they may be able to brood three-egg clutches effectively and improve population growth.It is thus important to continue observing this species.Chapter 2: Temporal and spatial differences in three-egg clutch frequency of the African Black Oystercatcher Abstract Historically African Black Oystercatchers naturally produce a modal clutch size of two, rarely laying three-eggs.By reviewing over 4000 nest records from across their breeding range (dating back to the early 1960s), I found that three-egg clutches have become more frequent and in recent years make up over 6% of clutches.Three-egg clutches are currently site specific and found predominantly at south-western sites, with the most change in frequency occurring within the Western Cape.They are produced earlier in the season compared to smaller clutch sizes and rarely occur after January.Due to the site fidelity it is possible that a localised change in mating habit is occurring or a heritable trait is linked to this increase.However, further research is needed to identify the cause of this phenomenon and evaluate how this change may affect the breeding success of this species. Chapter 3: Are three-egg clutches due to an increase in breeding effort or a change in breeding strategy by the African Black Oystercatcher? AbstractOver the last few decades three-egg clutch frequency has increased for the African Black Oystercatcher.As this species has been a focus of study for over three decades it offers an opportunity to attempt to understand the factors increasing wader clutch size.Breeding effort in the Western Cape, South Africa was monitored over the 2011-2013 breeding seasons and combined with similar data records from across their breeding range.Behavioural observations and egg-shape comparisons suggest that polygyny and egg-dumping unlikely account for the increase in three-egg clutch frequency.This implies that an increased breeding effort by those predisposed to laying larger clutches is most likely the cause of three-egg clutches.Correlations were identified by modelling egg mass, repeat clutches and parental biometrics to clutch size and found that large clutches were related to males and females with a greater mass.Repeat clutches and total egg investment within the season have also increased for those laying three-egg clutches as producing three-egg clutches had no negative effect on initial egg mass or repeat clutch size.Further studies are needed to assess the dispersal of these pairs responsible for the increased frequency of three-egg clutches and identify changes in breeding success. Chapter 4 -Does an increase in clutch size translate into an increase in reproductive performance in the African Black Oystercatcher? AbstractThe recent increase in the frequency of three-egg clutches produced by African Black Oystercatchers is the result of increased breeding effort by monogamous pairs.It is important to assess whether this increased effort results in a larger number of fledglings.Although a few pairs that laid three-egg clutches were able to rear three chicks, this is rare (2% of three-egg clutches).Overall chick production from three-egg clutches (0.31 ± 0.68 chicks fledged per breeding attempt) is not significantly greater than production from two-egg clutches (0.22 ± 0.52 chicks).Both strategies are more successful than single-egg clutches (0.03 ± 0.18 chicks fledged per breeding attempt).Factors reducing the success of three-egg clutches include inefficient incubation of three eggs, the increased risk of predation linked to the longer incubation period, and low fledging success due to the high costs of feeding and protecting an extra chick.Three-egg clutches are possibly a deleterious genetic trait being maintained due to low competition and increased resource availability at some mainland sites.