Unexpected diversity in socially synchronized rhythms of shorebirds

Abstract

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.

Discriminating uniparental and biparental breeding strategies by monitoring nest temperature

“…nearly 50% of the day, as they share incubation duties equally; Bulla et al . ). Hence, in biparentally incubated nests, recesses can also indicate rapid shifts between the two incubating adults.…”

“…Indeed, as stated above (see also Bulla et al . ), using the −5 °C threshold we would miss some recesses when ambient temperature is above 30–35 °C. Using the −1 °C threshold would result in scoring false recesses when the recorded incubation temperature is not very stable (i.e.…”

“…, Bulla et al . ). As different incubation strategies result in different nest attendance patterns, monitoring nest temperatures should allow us to discriminate between these strategies, provided that the ambient temperature at ground level is sufficiently different (i.e.…”

“…, Bulla et al . ), shorebirds, which comprise some 320 species, are an ideal taxon for studying the evolution of breeding strategies. Within this group, the incubation strategy is most often presumed to be fixed within species, but some species can exhibit multiple patterns of care within local populations, ranging from biparental to uniparental incubation by males or females (Webb et al .…”

“…Fontaine & Martin , Bulla et al . ) have been proposed as selective pressures to explain incubation strategies.…”