Effect of geolocators on migration and subsequent breeding performance of a long-distance passerine migrant

Abstract

Geolocators are small light-weight data loggers used to track individual migratory routes, and their use has increased exponentially in birds. However, the effects of geolocators on individual performance are still poorly known. We studied geolocator effects on a long-distance migrating passerine bird, the northern wheatear (Oenanthe oenanthe L.). We asked the general question of whether geolocators affect migratory behaviour and subsequent reproductive performance of small passerines by comparing arrival time, breeding time, breeding success and survival of geolocator versus control birds of known identity and breeding history. During two years geolocator birds (n=37) displayed a lower apparent survival (30%) as compared to controls (45%, n=164). Furthermore, returning geolocator birds (n=12) arrived on average 3.5 days later, started laying eggs 6.3 days later, and had lower nest success (25%) than control birds (78%). Our results suggest that geolocators affect migratory performance with carry-over effects to the timing of breeding and reproductive success in the subsequent breeding season. We discuss the implications of such geolocator effects for the study of migratory strategies of small passerines in general and suggest how to identify and investigate such effects in the future.

Figure 1. Return rates of control vs. geolocator birds.

Predicted apparent survival (return rate) probability (median with 50% and 95% CI) of control (n=164) vs. geolocator (n=37) birds as predicted from Bayesian GLM with covariates as specified in the Methods.

Figure 2. Arrival dates of control vs. geolocator birds.

Model predictions (median with 50% and 95% CI) for standardised arrival dates of control vs. geolocator birds in the deployment (open symbols; n_ctr=78, n_geo=34) and post-deployment year (filled symbols; n_ctr=34, n_geo=10) from Bayesian GLM with covariates as specified in the Methods.

Figure 3. Egg laying dates of control vs. geolocator birds.

Model predictions (median with 50% and 95% CI) for standardised egg laying dates of control vs. geolocator birds in the deployment (open symbols; n_ctr=143, n_geo=36) and post-deployment year (filled symbols; n_ctr=54, n_geo=9) from Bayesian GLM with covariates as specified in the Methods.