Transmitter effects on growth and survival of Forster’s tern chicks

Radio‐telemetry is a commonly used scientific technique that allows researchers to collect detailed movement, habitat use, and survival data of animals; however, evidence indicates that using telemetry can affect behavior and survival. Using multiple breeding colonies and years, we investigated the effects of attached radio‐transmitters on growth and survival of Forster's tern (Sterna forsteri ) chicks in San Francisco Bay, California, USA, 2010–2011. We tested these potential effects at isolated islands that allowed for high re‐capture rates (typically >85%) in radio‐marked and banded‐only chicks. Modeled Gompertz growth curves suggested that transmitters had a small negative effect on some of the asymptotic growth parameters of tern chicks; tarsus (−1.5 ± 0.7% [SE]), culmen (−1.7 ± 1.2%), and wing (−4.9 ± 2.0%) lengths were shorter for radio‐marked chicks compared to banded‐only chicks. In contrast, there was no difference in asymptotic mass between radio‐marked chicks and banded‐only chicks. Survival from hatching to fledging was lower for radio‐marked chicks than banded‐only chicks during 2010 (banded‐only = 0.313 ± 0.162 vs. radio‐marked = 0.250 ± 0.165) and 2011 (0.193 ± 0.030 vs. 0.123 ± 0.027). Most of the transmitter effect occurred within the first week after hatching, rather than in older chicks. Notably, the effect of transmitters on chick survival was primarily additive, indicating that the effect of transmitters on radio‐marked chicks was not influenced by other ecological covariates. Given the effect radio‐transmitters had on survival did not change across temporal or ecological gradients, transmitters can still be used to evaluate ecological factors affecting survival and timing of mortality and radio‐marked birds can be used to make inferences to the general population.