Nest relief in the cryptically-incubating semipalmated sandpiper is quick but vocal

Biparental care requires coordination between parents. Such coordination might prove difficult if opportunities to communicate are scarce, which might have led to the evolution of elaborate and noisy nest relief rituals in species facing a low risk of predation. 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 behavior during incubation in an Arctic-breeding shorebird with passive nest defense, the semipalmated sandpiper (Calidris pusilla). We then explored whether nest relief behavior provides information about parental cooperation 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 75% of nest reliefs, the incubating parent left the nest only after its partner had returned and initiated the nest relief. In these cases, exchanges were quick (25s, median) and shortened over the incubation period by 0.1 - 1.4s per 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 instantaneously. In 30% 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, the next off-nest bout was 1 - 4hr (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 division of parental duties. LAY SUMMARYO_LIBiparental care requires coordination between parents. In species where both parents incubate and nests are visible, parents often perform complex nest relief rituals. Is this also the case in species where nests are cryptic? C_LIO_LIWe video-recorded nest relief behavior at nests of cryptically incubating semipalmated sandpipers. C_LIO_LIParents relieved each other quickly, but they vocalized frequently. In 20% of nest reliefs the incubating parent left only after its returning partner called instantaneously. In 30% of cases the returning parent initiated the nest relief with a call. If the partner replied, its next off-nest bout was 1 - 4hr longer than when the partner did not reply, which corresponds to an 8 - 45% increase. C_LIO_LIOur results suggest that vocalizations during nest relief may be important for the division of parental duties. Further work in sandpipers and other taxa is needed to elucidate the role of parental vocalization in coordinating and synchronizing parental duties. C_LI