The causes of dispersal and the cost of carryover effects for an endangered bird

Background/Question/Methods  

The decision to disperse or remain philopatric between breeding seasons has important implications for both ecology and evolution. While emigration decisions are often based on reproductive output through the use of either personal or public information, our understanding of the underlying mechanisms that reduce reproduction and motivate emigration decisions are poorly understood. Reproductive failure is often caused by poor environmental conditions leading to the possibility that these conditions may indirectly stimulate breeding dispersal in many systems. Breeding dispersal can be beneficial to populations by promoting gene flow although it can be costly to individuals such as search costs that can result in late breeding. Delayed breeding can lead to phenological mismatches with environmental conditions, potentially leading to reproductive failure and feedback loops of recurring dispersal. Furthermore, dispersal can result in spatial carryover effects with the possibility that local reproduction could carryover to affect dispersers’ reproduction in distant locations, although there is currently little empirical evidence for spatial connectivity of vital rates. We examined potential direct (i.e. water depth used as a cue for emigration decisions) and indirect (i.e. water depth effects on reproduction leading to personal and public information that is used as a cue for emigration decisions) effects of hydrology on emigration decisions and both current and future reproductive success of an endangered, wetland raptor, the snail kite (Rostrhamus sociabilis plumbeus), by coupling structural equation modeling with 20 years of mark-recapture and nesting data across the geographic range of this species.

Results/Conclusions

We found that water depth affected emigration via an indirect path through the use of personal, not public, information. Importantly, we found that these dispersers tended to initiate nests later the following breeding season. This led to a phenological mismatch of hydrology whereby immigrants nested later, later nests experienced lower water depths, lower water depths led to higher nest failure, and higher nest failure led to another cycle of breeding dispersal. These results reveal a substantial benefit of philopatry--earlier initiation of reproduction--which allows philopatric individuals to better coincide with environmental conditions that are beneficial for successful reproduction. Our results also highlight reproductive carryover effects whereby local environmental conditions have implications not only for local demography but also for future demography at distant sites connected through dispersal.