Nest reliefs in a cryptically incubating shorebird are quick, but vocal

In species with biparental care, coordination of parental behaviour between pair members increases reproductive success. Coordination is difficult if opportunities to communicate are scarce, which might have led to the evolution of elaborate nest relief rituals in species facing a low predation risk. However, whether such conspicuous rituals also evolved in species that avoid predation by relying on crypsis remains unclear. Here, we used a continuous monitoring system to describe nest relief behaviour during incubation in an Arctic-breeding shorebird with passive nest defence, the Semipalmated Sandpiper Calidris pusilla. We also explored whether behaviour of exchanging parents informs about parental coordination and predicts incubation effort. We found that incubating parents vocalized twice as much before the arrival of their partner than during other times of incubation. In at least 75% of exchanges, the incubating parent left the nest only after its partner had returned and initiated the nest relief. In these cases, exchanges were quick (25 s, median) and shortened over the incubation period by 0.1-1.4 s/day (95% CI), suggesting that parents became more synchronized. However, nest reliefs were not cryptic. In 90% of exchanges, at least one parent vocalized, and in 20% of nest reliefs the incubating parent left the nest only after its returning partner called incessantly. In 27% of cases, the returning parent initiated the nest relief with a call; in 39% of these cases, the incubating partner replied. If the partner replied, its following off-nest bout was 1-4 h (95% CI) longer than when the partner did not reply, which corresponds to an 8-45% increase. Our results indicate that incubating Semipalmated Sandpipers, which rely on crypsis to avoid nest predation, have quick but acoustically conspicuous nest reliefs. Our results also suggest that vocalizations during nest reliefs may be important for the coordination and division of parental duties.