Reproductive success and demography of the Orange-bellied Parrot Neophema chrysogaster
thesisposted on 2023-05-26, 08:00 authored by Holdsworth, MC
The Orange-bellied Parrot is one of only two obligate migratory parrots in the world. The species is listed nationally as endangered and has been the subject of intensive study and conservation activities over the past 25 years. Reproductive and demographic data collected over this period from the wild population form the basis of this thesis. Remote breeding sites in southwestern Tasmania at Melaleuca and Birchs Inlet were used to study this species in the wild. Through deployment of up to 52 artificial nest boxes and observations of natural nests at Melaleuca it was possible to collect information on a range of reproductive success parameters over a long period, including 12 consecutive breeding seasons. In addition, the provision of up to 33 nest boxes over seven consecutive years at Birchs Inlet provided a comparison with the use of nest boxes by several competitors at Melaleuca. The use of colour-bands to identify 760 individuals from 16 different cohorts provided the means to assess a range of behavioural and demographic parameters of the species. This study confirmed the Orange-bellied Parrot has a regular migratory pattern with birds beginning to return to the breeding area on the 2nd October (plus or minus 5.1 days s.d) in each year. The first birds to return are those in their second year of life or older, while first-year adult birds begin to arrive 13 days later. The median arrival date for birds in their second year or older was the 23rd October compared with 9th November for firstyear adult birds. There was no difference between the sexes in arrival date. The mean date of last departure from Melaleuca was 5th April (plus or minus 11.1 days s.d). A total of 190 nests with known contents were studied in the wild and, of these 185 nests contained eggs and five nests contained no eggs. This study found the earliest laying date was on the 29th November and the latest was the 19th January with eggs (95%) laid during December (n = 101 eggs). Clutch size ranged from 2-6 eggs with over half of the clutches having 5 eggs and 95.3% of all clutches 4-6 eggs in size. The mean clutch size was 4.7 eggs per active nest across all years and there was no evidence the species can produce second clutches in the wild. The mean incubation period for Orange-bellied Parrot eggs was 21.4 plus or minus 0.8 days (n = 49 observed incubations). The mean dimensions of unhatched eggs was 22.9 plus or minus 0.98 mm by 18.5 plus or minus 0.67 mm (n = 99 eggs). The 185 nests with eggs studied by this work contained a total of 874 eggs. Of these 695 eggs hatched and 179 eggs failed to hatch. Of the 179 failures, 107 eggs (69.7%) were infertile, 48 eggs (26.8%) were fertile and 24 eggs (13.4%) were of unknown fertility. Of the 48 unhatched fertile eggs, 23 eggs (47.9%) were early-term failures, 15 eggs (31.2%) were mid-term failures and 10 eggs (20.8%) were late-term failures. The mean egg fertility rate for the species was calculated to be 85.6% (plus or minus 2.91 s.e). Hatching success from all eggs laid was 79.5% (i.e. 695 nestlings hatched from 874 eggs laid) with the mean hatching success across all years being 80.2%. A total of 89 nestlings died prior to fledging. Early stage deaths represented 44.9% (n = 40) of all mortalities and late stage deaths 55.1% (n = 49). The annual egg failure and nestling mortality varied across years. Of a total of 268 egg and nestling failures across all years, 66.8% (n = 179 eggs) were attributable to hatch failure and 33.2% (n = 89 nestlings) to mortality. Unhatched infertile eggs represented most (39.9%, n = 107) of all failures. Of the 190 nesting attempts, only 27 failed to produce any young. The most common cause of total nest failure was attributed to failure to hatch (44.4%, n = 12) followed by nestling deaths (37%, n = 10) and no eggs laid (18.5%, n = 5). Average brood size was 4.0 nestlings plus or minus 0.09 s.e (range = 1-6) from 173 nests with 65.9% of nests producing four (33.5%) or five (32.4%) nestlings. The majority of nests produced four fledglings with a mean fledgling brood size of 3.7 plus or minus 0.09 s.e (range = 1-6) from the 163 successful nests. Only 4.3% of successful nests produced the maximum of six fledglings. Of the 190 Orange-bellied Parrot nests studied, 85.8% (n = 163) produced fledglings. The distribution of nest productivity is presented and discussed in detail. The number of fledglings produced per breeding attempt varied between zero and six. A total of 69% of all nests produced 3-5 fledglings whereas 33% of all nests produced four fledglings. The fledging success for 12 consecutive breeding seasons was 87.2% (606 fledglings from 695 nestlings) and the mean fledging success across all years was 86.9% (plus or minus 2.47 s.e). The overall breeding success for the Orange-bellied Parrot was 69.3% (606 fledglings from 874 eggs laid). The overall reproductive output of the species was 3.3 fledglings per nest (606 fledglings from 185 nests) from an investment of 4.7 eggs laid. Egg fertility, nestling survival and fledgling survival of Orange-bellied Parrots in the wild is noticeably higher than for the captive population, and is equal to or exceeds many other Psittacidae. The reproductive success results reported here are comparable with the more common Turquoise Parrot Neophema pulchella of mainland Australia. Although the Orange-bellied Parrot has a healthy mean fecundity rate of 1.62 females/egg laying female, there was some variability between years, with a low of 0.87 females/egg laying female in 1998/99. The mean lifespan of the Orange-bellied Parrot was calculated to be 2.22 years (plus or minus 0.074 s.e, range = 0.37-11.70, N = 693) with no significant difference between male and females. Males lived on average for 2.75 years (plus or minus 0.127 s.e, range = 0.43-11.70, n = 240) and females lived on average for 2.67 years (plus or minus 0.141 s.e, range = 0.18-10.41, n = 189). The oldest male recorded was 11.70 years of age and the oldest female recorded was 10.41 years of age. This study was not able to compare the reproductive lifespan of wild Orange-bellied Parrots with captive-bred birds due to database problems or with other Psittacidae due to lack of comparable studies. The capacity to compare the wild population with captive-bred birds and other Psittacidae will greatly enhance our knowledge of the species. This work suggests the Orange-bellied Parrot does not have a strong fidelity to mates, nest site or nesting zone. This finding is contrary to previous assumptions made about the species. This study did not measure hollow availability; however, a comparison of the use of nest boxes between Birchs Inlet and Melaleuca indicates competition from introduced species may be limiting the breeding range and reproductive success. Survivorship rates of juveniles to first breeding (c. one year old), adults and both sexes were determined. Mean survivorship of juveniles over the study was 55% (plus or minus 3.2 s.e) and is within expected limits when compared to other Psittacidae. Mean survivorship of adults was 63.6% (plus or minus 2.0 s.e). There was a decreasing trend in survival rates across all cohorts from 1999 onward with average annual survival declining markedly thereafter. The reason for this decline is unclear. There was no difference in survival rate of each sex over the study. This study has significantly increased our understanding of the reproductive success and demography of the Orange-bellied Parrot. This information will reduce the level of uncertainty in the Population Viability Analysis model for the species and, in turn, increase the power of such models to assess the species status and test the effectiveness of conservation measures. Some of the results of this study have important implications for future research and conservation of the species. These are discussed, and include management of nest boxes, refinement of mark-recapture studies, population viability analysis and influence of introduced nest competitors.