Stochastic population models hindcast population trajectory and breeding history of an endangered parrot

Figures & data

Table 1. Life-history parameters used for stochastic modelling of the Norfolk Island Green Parrot population. EV represents environmental variation. NA indicates where a value is not applicable.

Life-history parameter	Variable or Fixed	Value(s)	Justification (Refer to Table S1)
number of iterations	fixed	1,000	NA
inbreeding depression	fixed	6.29 (50% due to recessive lethal alleles)	1
EV correlation (reproduction & survival)	fixed	0.5	2
EV correlation (between populations)	fixed	1	2
age of dispersal	fixed	1–21	3
dispersing sexes	fixed	both	3
survival of dispersers	fixed	100%	3
lifespan	fixed	21	4
dispersal rate	variable	Formula based on managed nests	3
number of managed nests	fixed	21	5
mating system	fixed	monogamy	6
age of reproduction	fixed	2–21	7
maximum broods per year	fixed	1	8
maximum progeny per year	fixed	10	8
sex ratio at birth	fixed	50:50	9
% of adult females breeding	variable	Formula based on managed and natural nests	10
SD in % of females breeding due to EV	fixed	10%	10
number of natural nests	variable	50, 100 or 200	11
number of broods per year	fixed	1	8
no. offspring per nest per year	variable	2.74 ± 2.12 S.D. or 2.06 ± 1.59 S.D.	8
yearly mortality rate (consistent across all age groups)	variable	15–25%	7
SD in mortality rate (consistent across all age groups)	fixed	5%	7
catastrophes	fixed	nil	NA
% of adult males in breeding pool	fixed	100%	12
initial population size	variable	44, 68 or 92	13
carrying capacity (per population)	fixed	5,000 ± 500 S.D.	4
harvest	fixed	nil	NA
supplementation	fixed	nil	NA

Figure 1. Process used to convert Norfolk Island Green Parrot managed population and natural population estimates produced by VORTEX into marked and unmarked population estimates.

Table 2. Results of the single population models showing Norfolk Island Green Parrot population estimates for combinations of mortality rates and initial population size.

	Model Inputs		Model Output
Model ID	2013 population size	Mortality rate	2021 population estimate ± S.D.
S1	44	25%	158 ± 26.5
S2	44	20%	193 ± 34.8
S3	44	15%	243 ± 47.8
S4	68	25%	164 ± 29.9
S5	68	20%	206 ± 38.5
S6	68	15%	256 ± 51.3
S7	92	25%	166 ± 30.9
S8	92	20%	209 ± 39.9
S9	92	15%	266 ± 58.7

Table 3. Metapopulation models leading to a percentage of Norfolk Island Green Parrots with bands within 5% of the estimate for 2021 (17.65%) with 25% band loss. All instances for which natural breeding females have a different mortality rate to the rest of the population also incorporate a 25% reduction in reproductive output from natural nest sites.

		Model Inputs				Model Outputs
Model ID	2013 pop.	Natural sites	Mortality rate	Natural breeding female mortality	Metapopulation ± S.D.	Percentage marked (25% band attrition)
M1	44	200	25%	25%	399 ± 88	19.85%
M2	44	200	20%	25%	435 ± 78.1	22.46%
M3	44	200	20%	20%	669 ± 138.3	15.00%
M4	44	200	15%	20%	692 ± 120.3	17.48%
M6	44	100	25%	25%	395 ± 83.5	20.05%
M7	44	100	20%	25%	430 ± 78.6	22.65%
M8	44	100	20%	20%	627 ± 98.2	15.81%
M9	44	100	15%	20%	642 ± 82.7	18.76%
M10	44	100	15%	15%	874 ± 120.5	14.18%
M11	44	50	25%	25%	360 ± 56.9	21.97%
M13	44	50	20%	20%	484 ± 69.8	20.66%
M15	44	50	15%	15%	622 ± 92.3	19.99%
M16	68	200	25%	25%	635 ± 130.6	13.65%
M17	68	200	20%	25%	635 ± 113.3	16.64%
M19	68	200	15%	20%	992 ± 149.5	13.12%
M21	68	100	25%	25%	597 ± 98.5	14.68%
M22	68	100	20%	25%	591 ± 79.8	17.86%
M23	68	100	20%	20%	820 ± 114.2	13.21%
M24	68	100	15%	20%	789 ± 102.5	16.47%
M25	68	100	15%	15%	1041 ± 152.7	12.99%
M26	68	50	25%	25%	444 ± 61.5	19.69%
M28	68	50	20%	20%	569 ± 86.3	18.98%
M30	68	50	15%	15%	704 ± 110.8	18.98%
M32	92	200	20%	25%	809 ± 138.5	13.43%
M36	92	100	25%	25%	703 ± 96.8	12.90%
M37	92	100	20%	25%	673 ± 87.7	16.14%
M39	92	100	15%	20%	858 ± 121.2	15.84%
M41	92	50	25%	25%	481 ± 71.8	18.66%
M43	92	50	20%	20%	601 ± 93.9	18.58%
M44	92	50	15%	20%	596 ± 90.1	22.71%
M45	92	50	15%	15%	749 ± 124.5	18.83%

Figure 2. Yearly Norfolk Island Green Parrot population estimates produced by all single population models and metapopulation models with a comparable proportion of marked birds in the population (±5%) to that observed in the wild population in 2021 (17.65%). Ribbons indicate standard deviation of the estimates.