Unexpected diversity in socially synchronized rhythms of shorebirds
Authors: Bulla Martin , Valcu Mihai , Dokter Adriaan M. , Dondua Alexei G. , Kosztolányi András , Rutten Anne , Helm Barbara , Sandercock Brett K. , Casler Bruce , Ens Bruno J. , Spiegel Caleb S. , Hassell Chris J. , Küpper Clemens , Minton Clive , Burgas Daniel , Lank David B. , Payer David C. , Loktinov Egor Y. , Nol Erica , Kwon Eunbi , Smith Fletcher , Gates H. River , Vitnerová Hana , Prüter Hanna , Johnson James A. , St Clair James J. H. , Lamarre Jean-François , Rausch Jennie , Reneerkens Jeroen , Conklin Jesse R. , Burger Joana , Liebezeit Joe , Bêty Joël , Coleman Jonathan T. , Figuerola Jordi , Hooijmeijer Jos C. E. W. , Alves José A. , Smith Joseph A. M. , Weidinger Karel , Koivula Kari , Gosbell Ken , Exo Klaus-Michael , Niles Larry , Koloski Laura , McKinnon Laura , Praus Libor , Klaassen Marcel , Giroux Marie-Andrée , Sládeček Martin , Boldenow Megan L. , Goldstein Michael I. , šálek Miroslav , Senner Nathan , Rönkä Nelli , Lecomte Nicolas , Gilg Olivier , Vincze Orsolya , Johnson Oscar W. , Smith Paul A. , Woodard Paul F. , Tomkovich Pavel S. , Battley Phil F. , Bentzen Rebecca , Lanctot Richard B. , Porter Ron , Saalfeld Sarah T. , Freeman Scott , Brown Stephen C. , Yezerinac Stephen , Székely Tamás , Montalvo Tomás , Piersma Theunis , Loverti Vanessa , Pakanen Veli-Matti , Tijsen Wim , Kempenaers Bart 
Published: November 2016

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 synchronization12where 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.