Differential reproductive success in a red-cockaded woodpecker population: Implications for hatch failure rates and translocation policy

Background/Question/Methods The Red-cockaded Woodpecker (RCW, Picoides borealis) is an endangered species inhabiting mature pine flatwoods in the southeastern USA. Egg hatch failure rates are known to be much higher than usual in many RCW populations in peninsular Florida, which can serve as a significant drain on productivity and hamper recovery. While the causes of hatch failure in RCWs are not fully understood, both environmental factors and inbreeding effects are possible causes. As a cooperative breeding species, the effective population size of RCWs may be considerably smaller than the actual number of adults. Reproductive success is also variable among groups, which can further limit genetic diversity. We analyzed reproductive data for the RCW population at Avon Park Air Force Range between 1994 and 2009 to determine the proportion of breeding birds that have produced offspring that went on to breed in turn.

Results/Conclusions 55.2% of breeding birds during this period (51.2% of males and 62.9% of females) were not observed to have produced offspring who went on to breed themselves. 55.1% of breeding birds of known parentage were sired by just 13.4% of the male breeders, with 5 individuals (3.9%) siring 22.8%.  61.3% of breeding birds of known parentage were the offspring of 15.3% of breeding females, with 6 individuals (4.8%) producing 26.6%. Birds have been introduced into this population through translocations periodically since 1998, which should increase genetic diversity. However, hatch failure rates have generally continued to increase during this period. Only 8.6% of breeding birds have been produced by translocated individuals, with over half of these coming from just 3 individual males. Male translocatees on average produced 4 times as many offspring who went on to breed than female translocatees, although the difference was not quite significant (p=0.098). There was no significant difference in the mean production of breeding offspring between translocated birds and birds born on site. While these results do not point to a definite cause for high hatch failure rates, the highly skewed nature of reproductive success in this population means that insufficient time may have passed since the beginning of translocation efforts for the increased gene flow to have an impact. Present policies governing inter-population RCW translocations require that these moves be stopped once the recipient population has grown to a target size. However, continued translocations should be considered for populations where genetic diversity is potentially limited, even if they have already grown past their recovery goals.