Between‐patch natal dispersal declines with increasing natal patch size and distance to other patches in the endangered Southern Dunlin Calidris alpina schinzii

Abstract

Sweden 5 Ljungv€ agen 3, V€ ar€ obacka, 430 22, Sweden Natal dispersal has profound consequences for populations through the movement of individuals and genes. Habitat fragmentation reduces structural connectivity by decreasing patch size and increasing isolation, but understanding of how this impacts dispersal and the functional connectivity of landscapes is limited because many studies are constrained by the size of the study areas or sample sizes to accurately capture natal dispersal. We quantified natal dispersal probability and natal dispersal distances in a small migratory shorebird, the Southern Dunlin Calidris alpina schinzii, with data from two extensively monitored endangered metapopulations breeding in Sweden and Finland. In both metapopulations philopatry was strong, with individuals returning to or close to their natal patches more often than expected by chance, consistent with the patchy distribution of their breeding habitat. Dispersal probabilities were lower and dispersal distances were shorter in Sweden. These results provide a plausible explanation for the observed inbreeding and population decline of the Swedish population. The differences between Sweden and Finland were explained by patch-specific differences. Betweenpatch dispersal decreased with increasing natal patch size and distance to other patches. Our results suggest that reduced connectivity reduces movements of the philopatric Dunlin, making it vulnerable to the effects of inbreeding. Increasing connectivity between patches should thus be one of the main goals when planning future management. This may be facilitated by creating a network of suitably sized patches (20-100 ha), no more than 20 km apart from each other, from existing active patches that may work as stepping stones for movement, and by increasing nest success and prefledging survival in small patches.

Migration strategies of the Baltic dunlin: rapid jump migration in the autumn but slower skipping type spring migration

“…Each year fieldwork began in late April with searching for territories and resighting of birds, and continued until mid-July with searching for nests and broods. Fieldwork enabled us to effectively record the majority of adults and to follow their reproductive success (Pakanen et al 2011(Pakanen et al , 2017a. Adults were captured with mist-nets or cage traps while brooding chicks or incubating eggs.…”

Survival during the pre-fledging period rather than during post-fledging drives variation in local recruitment of an endangered migratory shorebird, the Southern Dunlin Calidris alpina schinzii

“…1 Location of the breeding sites of Southern Dunlin on the west coast of Sweden (1 = Ödsmåls kile, 2 = Torkelstorp, 3 = Tjolöholm, 4 = Ölmevalla, 5 = Båtafjorden, 6 = Klosterfjorden, 7 = Getterön). See Pakanen et al (2017) for further details on the sites; note that the breeding sites Klosterfjorden and Getterön were excluded from the analyses presented here due to insufficient data on brood survival…”

“…The study population is isolated from other populations with the nearest breeding population being about 300 km away from our study population in the province of Scania, SW Sweden (Blomqvist et al 2010). Due to the isolation from other populations, and because all known breeding sites are monitored regularly, and dispersal between the sites is rare and controlled for, we were able to reliably estimate local recruitment (Pakanen et al 2017).…”

Survival probability in a small shorebird decreases with the time an individual carries a tracking device

“…This could be the kind of difficult environmental conditions in which birds carrying geolocators incur more costs. Permanent emigration outside our study area is an unlikely explanation because there is little breeding habitat elsewhere within the Bothnian Bay area (Pakanen et al 2017), which is one of the advantages of studying survival in this population.…”

“…Due to lack of suitable breeding habitats, this population is confined to these clearly defined habitat patches and the closest populations are 400 km away. Therefore, extensive studies of this colour-ringed population allow detailed monitoring of movement and, by recording annual resightings, reliable examination of survival (see Pakanen et al 2016Pakanen et al , 2017 for more details on field methods).…”