Improving the analysis of movement data from marked individuals through explicit estimation of observer heterogeneity

Abstract

Ring re‐encounter data, in particular ring recoveries, have made a large contribution to our understanding of bird movements. However, almost every study based on ring re‐encounter data has struggled with the bias caused by unequal observer distribution. Re‐encounter probabilities are strongly heterogeneous in space and over time. If this heterogeneity can be measured or at least controlled for, the enormous number of ring re‐encounter data collected can be used effectively to answer many questions. Here, we review four different approaches to account for heterogeneity in observer distribution in spatial analyses of ring re‐encounter data. The first approach is to measure re‐encounter probability directly. We suggest that variation in ring re‐encounter probability could be estimated by combining data whose re‐encounter probabilities are close to one (radio or satellite telemetry) with data whose re‐encounter probabilities are low (ring re‐encounter data). The second approach is to measure the spatial variation in re‐encounter probabilities using environmental covariates. It should be possible to identify powerful predictors for ring re‐encounter probabilities. A third approach consists of the comparison of the actual observations with all possible observations using randomization techniques. We encourage combining such randomisations with ring re‐encounter models that we discuss as a fourth approach. Ring re‐encounter models are based on the comparison of groups with equal re‐encounter probabilities. Together these four approaches could improve our understanding of bird movements considerably. We discuss their advantages and limitations and give directions for future research.

Are Dutch Skylarks Partial Migrants? Ring Recovery Data and Radio-Telemetry Suggest Local Coexistence of Contrasting Migration Strategies

“…suggesting that these longer-winged birds are coming from northern populations (Glutz von Blotzheim 1985). Our data do not allow us to make quantitative statements about the frequencies of the different migration strategies because we lack the crucial information on ringing efforts, recovery rates and reporting rates in The Netherlands relative to those in other countries (see Körner-Nievergelt et al 2010).…”

Migration pattern of Icelandic Lesser Black-backed Gulls Larus fuscus graellsii: indications of a leap-frog system

“…This well-known problem can be dealt with by different approaches. Here, we use the technique of comparing groups of birds assumed to have equal re-encounter probabilities (Korner-Nievergelt et al 2010). We compared resightings of Icelandic-ringed birds with two colour-ringing projects from the Netherlands.…”

Movements of Immature Eurasian SpoonbillsPlatalea leucorodiafrom the Breeding Grounds of the Eastern Metapopulation In the Pannonian Basin

“…Their advantage over the use of satellite and radio transmitters is their typically much larger sample size. On the other hand, all ringing data (including color-ringing data) are highly influenced by spatial and temporal patterns in observer effort, especially if sightings are collected over a wide area and for longer periods (Korner-Nievergelt et al 2010).…”