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BackgroundGeolocators are useful for tracking movements of long-distance migrants, but potential negative effects on birds have not been well studied. We tested for effects of geolocators (0.8–2.0 g total, representing 0.1–3.9 % of mean body mass) on 16 species of migratory shorebirds, including five species with 2–4 subspecies each for a total of 23 study taxa. Study species spanned a range of body sizes (26–1091 g) and eight genera, and were tagged at 23 breeding and eight nonbreeding sites. We compared breeding performance and return rates of birds with geolocators to control groups while controlling for potential confounding variables.ResultsWe detected negative effects of tags for three small-bodied species. Geolocators reduced annual return rates for two of 23 taxa: by 63 % for semipalmated sandpipers and by 43 % for the arcticola subspecies of dunlin. High resighting effort for geolocator birds could have masked additional negative effects. Geolocators were more likely to negatively affect return rates if the total mass of geolocators and color markers was 2.5–5.8 % of body mass than if tags were 0.3–2.3 % of body mass. Carrying a geolocator reduced nest success by 42 % for semipalmated sandpipers and tripled the probability of partial clutch failure in semipalmated and western sandpipers. Geolocators mounted perpendicular to the leg on a flag had stronger negative effects on nest success than geolocators mounted parallel to the leg on a band. However, parallel-band geolocators were more likely to reduce return rates and cause injuries to the leg. No effects of geolocators were found on breeding movements or changes in body mass. Among-site variation in geolocator effect size was high, suggesting that local factors were important.ConclusionsNegative effects of geolocators occurred only for three of the smallest species in our dataset, but were substantial when present. Future studies could mitigate impacts of tags by reducing protruding parts and minimizing use of additional markers. Investigators could maximize recovery of tags by strategically deploying geolocators on males, previously marked individuals, and successful breeders, though targeting subsets of a population could bias the resulting migratory movement data in some species.Electronic supplementary materialThe online version of this article (doi:10.1186/s40462-016-0077-6) contains supplementary material, which is available to authorized users.

The behavioural rhythms of organisms are thought to be under strong selection, influenced by the rhythmicity of the environment1–4. Such behavioural rhythms are well studied in isolated individuals under laboratory conditions1,5, but free-living individuals have to temporally synchronize their activities with those of others, including potential mates, competitors, prey and predators6–10. Individuals can temporally segregate their daily activities (e.g. prey avoiding predators, subordinates avoiding dominants) or synchronize their activities (e.g. group foraging, communal defence, pairs reproducing or caring for offspring)6–9,11. The behavioural rhythms that emerge from such social synchronization and the underlying evolutionary and ecological drivers that shape them remain poorly understood5–7,9. Here, we address this in the context of biparental care, a particularly sensitive phase of social synchronization12 where pair members potentially compromise their individual rhythms. Using data from 729 nests of 91 populations of 32 biparentally-incubating shorebird species, where parents synchronize to achieve continuous coverage of developing eggs, we report remarkable within– and between-species diversity in incubation rhythms. Between species, the median length of one parent’s incubation bout varied from 1 – 19 hours, while period length–the time in which a parent’s probability to incubate cycles once between its highest and lowest value – varied from 6 – 43 hours. The length of incubation bouts was unrelated to variables reflecting energetic demands, but species relying on crypsis (the ability to avoid detection by other animals) had longer incubation bouts than those that are readily visible or actively protect their nest against predators. Rhythms entrainable to the 24-h light-dark cycle were less prevalent at high latitudes and absent in 18 species. Our results indicate that even under similar environmental conditions and despite 24-h environmental cues, social synchronization can generate far more diverse behavioural rhythms than expected from studies of individuals in captivity5–7,9. The risk of predation, not the risk of starvation, may be a key factor underlying the diversity in these rhythms.

1Small solar-powered satellite transmitters and GPS data loggers enable continuous, multi-2 year, and global tracking of birds. What is lacking, however, are reliable methods to attach 3 these tracking devices to small migratory birds so that (1) flight performance is not 4 impacted and (2) tags are retained during periods of substantial mass change associated 5 with long distance migration. We developed a full-body harness to attach tags to Red Knots 6 (Calidris canutus), a medium-sized shorebird (average mass of 124 g) that undertakes long-7 distance migrations. First we deployed dummy tags on captive birds and monitored them 8 over a complete migratory fattening cycle (February-July 2013) during which time they 9 gained and lost 31-110 g and underwent a pre-alternate moult of body feathers. Using each 10 individual's previous year fattening and moult data in captivity as controls, we compared 11 individual mass and moult differences between years between the tagged and reference 12 group, and concluded the attachment did not impact mass and moult cycles. However, some 13 birds shed feathers under the tags and under the polyester harness line commonly used in 14 avian harnesses. Feather shedding was alleviated by switching to smoothed-bottom tags and 15 monofilament harness lines. To field-trial this design we deployed 5 g-satellite transmitters 16 on ten Red Knots released on 3 October 2013 in the Dutch Wadden Sea. Bird movements 17 and tag performance appeared normal. However, nine tags stopped transmitting 11-170 18 days post-release which was earlier than expected. We attribute this to bird mortality rather 19 than failure of the attachments or transmitters and suggest that the extra weight and drag 20 caused by the tag and its feather-blocking shield increased the chance of depredation by the 21 locally common Peregrine Falcons (Falco peregrinus). Our results demonstrate that 22 species-and place-specific contexts can strongly determine tagging success. While captive 23 trials are an important first step in developing an attachment method, field trials are 24 essential to fully assess attachment designs. 25 26Chan 2