Title
Comparing reproductive success of a colonial seabird, the Magellanic Penguin, estimated by coarse- and fine-scale temporal sampling
Author(s)
Rebstock, Ginger A.;Boersma, P. Dee
Published
2017
Publisher
The Condor
Abstract
Fecundity, an important demographic variable for wildlife populations, is time consuming and expensive to measure. For seabirds, reproductive success is often estimated from 2 surveys of the colony, 1 during incubation and 1 during chick rearing. Using 33 yr of data on Magellanic Penguins (Spheniscus magellanicus), we compared reproductive success estimated from 2 surveys per season, from infrequent nest checks, and from daily nest checks to answer 4 questions: (1) Is reproductive success estimated from 2 surveys per season accurate enough for population models (within 15% of the ?true' value)? We found that the answer was yes, if surveys were done on optimal dates and chick counts were adjusted by the percentage of chicks that were heavy enough to fledge. Optimal dates depended on the synchrony and phenology of the species. If penguins bred particularly late or early in a given year, estimates were wrong even if counts were done on the average optimal date. (2) Can counts of occupied nests be adjusted based solely on date? We determined no, not without additional information on egg dates and egg and chick losses. (3) Can long-term trends in reproductive success be detected from 2 surveys per season? Our answer was yes; even using biased data from counts on suboptimal dates, trends with similar slopes were detected using both methods (2 surveys vs. nest checks). (4) How often must nests be checked to get reproductive success estimates comparable with those calculated from daily nest checks? We discovered that checking nests every 2 days was as good as doing daily checks. Reproductive success was overestimated by <5% when checks were conducted every 3?8 days and by <15% (in 90% of years) when checks were done every 9?30 days. The degree of overestimation depended on the timing of nest checks relative to egg laying, loss, and hatching. We conclude that reproductive success can be estimated from 2 surveys, but only if timing, synchrony, and variability of breeding are also known. Fecundity, an important demographic variable in wildlife populations, is time consuming and costly to measure. For seabirds, reproductive success is usually estimated from two samples of the colony, one during incubation and one during chick growth. Using 33 years of Spheniscus magellanicus data, we compared reproductive success estimated from two surveys per season, using infrequent nest checks and daily nest checks, to answer four questions. 1) Is reproductive success estimated from two surveys per season sufficiently accurate for population models (within 15% of the 'true' value)? Yes, if surveys were conducted on optimal dates and chick counts were adjusted for the percentage of chicks heavy enough to leave the colony. Optimal dates depend on the synchrony and phenology of the species. If nesting was particularly late or early, estimates were in error even if counts were made on the average optimal date. 2) Can counts of occupied nests be adjusted based on date alone? No, additional information on egg date and egg and chick losses is required. 3) Can long-term trends in reproductive success be detected from two surveys per season? Yes, even using biased data from counts made on suboptimal dates, trends with similar slopes were detected using both methods. 4) How often should nests be checked to obtain reproductive success comparable to the results of daily nest checks? Checking nests every other day was as good as daily nest checks. Reproductive success was overestimated by <5% for checks made every 3-8 days and by <15% (in 90% of years) for checks made every 9-30 days. The degree of overestimation depended on the timing of nest monitoring, in relation to laying, egg loss and hatching. We conclude that reproductive success can be estimated from two surveys, but only if the timing, synchrony and variability of reproduction are also known.
Keywords
fecundity;Magellanic Penguin;reproductive success;Spheniscus magellanicus;sampling frequency;survey;reproductive success;fecundity;sampling frequency;sampling;Magellanic Penguin

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PUB22500