Assessing variation in survival in Acrocephalus warblers: a comparative study of the life-history strategies of two closely related species

The Spanish population of the vulnerable Moustached Warbler Acrocephalus melanopogon, which has undergone a continuous decline in recent decades, is the most important in Western Europe. It coexists with the widespread Reed Warbler Acrocephalus scirpaceus, whose population has increased. Differences in breeding productivity have been identified as a factor behind these diverging trends. We investigated these warblers’ apparent adult survival rates to assess if this demographic parameter might also influence these different trends. Additionally, we tested to see whether there were any sex-related differences in survival and whether the different migration strategies of these two species (short-distance vs. long-distance migrant) might influence survival. To do so, we ringed both species intensively at 10 ringing stations located in El Marjal de Pego-Oliva Natural Park (Eastern Spain) in 2011–2015. In total, we ringed 502 Moustached Warblers and 459 Reed Warblers and built sex-known encounter histories for 396 (223 males and 173 females) and 394 (255 males and 139 females), respectively. We found that transient male Reed Warblers represented 35% of the population and that apparent survival rates did not differ between these two warblers, being 0.486 ± 0.050 (mean ± SE) for both male and female Moustached Warblers, 0.475 ± 0.105 for male and 0.471 ± 0.171 for female Reed Warblers. Our results indicate that there were no significant sex-related differences in survival in either species, as expected in species that exhibit bi-parental care behaviour. This study should encourage conservation managers and bird ringers alike to study survival as a means of improving knowledge of avian population dynamics.