Brood success of northern bobwhite is biased by incomplete detectability during flush-counts

Abstract

Flush-count surveys of game bird broods are a common method of deriving estimates of brood survival, but detection of chicks during surveys is < 1 due to factors such as vegetation obscurity, adult brooding behavior and variation by observer. Radio-telemetry is an alternative method for estimating survival that circumvents such factors and allow for higher detection. However, this practice is costly and labor-intensive and therefore not readily adopted. We sought to estimate detection probability of chicks during flush-counts as a function of vegetation height and adult brooding behavior. Secondly, we evaluated compromises in detection by comparing estimates of brood survival derived from flush-counts and radiotelemetry. Lastly, we compared counts between two observers to discern whether an additional observer could increase accuracy of counts. We radiomarked 247 northern bobwhite Colinus virginianus chicks at 10–12 days of age and conducted 46 flush-counts at 21 days of age. Vegetation height substantially decreased detection (β = –1.18; 95 CrI: –1.68 to –0.73); mean detection probability was 0.30 (95 CrI: 0.22–0.40). Observers failed to detect radio-marked chicks when adults exhibited running behavior (n = 16 chicks, n = 6 surveys), orphaning occurred (n = 11 chicks, n = 5 surveys) or brooding adults died or had transmitter failures (n = 4 chicks, n = 3 surveys). An additional observer did not affect counts with a mean difference of –0.6 chicks (95 CrI: –4.0 to 2.7) counted between observers. Chicks were not detected during 47% of surveys when ≥ 1 radio-marked individuals were known to be alive. Brood survival was 0.83 (95 CrI: 0.70–0.92) and 0.48 (95 CrI: 0.34–0.62) for radiotelemetry and naïve flush-counts, respectively. Because of low detectability of chicks during flush-counts, alternative methods should be considered by future researchers.