Using geolocator tracking data and ringing archives to validate citizen-science based seasonal predictions of bird distribution in a data-poor region

Complex postbreeding molt strategies in a songbird migrating along the East Asian Flyway, the Pallas’s Grasshopper WarblerLocustella certhiola

“…Based on one geolocator‐tracked individual that left its breeding site on 2 September (and arrived in its wintering area in Thailand already on 25 September; Heim et al., 2020), adult birds of the population in the Russian Far East likely depart in late August or the beginning of September. Only two of the 16 birds examined in detail in the Russian Far East were judged to have interrupted their wing molt, arresting after growing P4–P9 and P3–P9, respectively.…”

“…Only two of the 16 birds examined in detail in the Russian Far East were judged to have interrupted their wing molt, arresting after growing P4–P9 and P3–P9, respectively. Thus, it cannot be ruled out that most individuals molt continuously during active migration, maybe encouraged by continuous suitable environmental conditions en route to their South‐East Asian wintering areas (Heim et al., 2020; Yong et al., 2015). Similar observations of molt bridging autumn migration have been made in other songbirds (Elkins & Etheridge, 1977; Herremans, 1990; Kiat et al., 2018; Nisbet & Medway, 1971; Schaub & Jenni, 2000b).…”

“…Although comparatively more nonmolting birds had a maximum muscle score, flight muscle scores on average remained moderate throughout the season in both groups, indicating a low protein catabolism within molting birds, presumably because the demand for protein was easily met by dietary intake (Jenni‐Eiermann & Jenni, 1996; Murphy & King, 1992; Podlaszczuk et al., 2017). The Pallas's Grasshopper Warblers are migrating largely over land without facing any major ecological barriers (Heim et al., 2020; Kennerley & Pearson, 2010). According to optimality models (Alerstam, 2011; Alerstam & Lindström, 1990), this would make longer nonstop flights redundant and would instead allow a nocturnal flight strategy with low fuel loads (Figure 3) and daily refueling in suitable habitats.…”

“…Study site was included as random effect. We built two sets of candidate models, one including all birds trapped during the autumn migration period (15 July–25 September, based on the bird tracked by Heim et al., 2020) and one including only individuals trapped at stopover sites (excluding the likely wintering site in Thailand). We graphically examined if model assumptions were met using residual plots and evaluated model fit with the conditional R 2 and marginal R 2 (Nakagawa & Schielzeth, 2013).…”

“…One of the latest arriving species breeding in Siberia is the Pallas´s Grasshopper Warbler Locustella certhiola , which stays only around two months on the breeding grounds (Bozo et al., 2019; Kennerley & Pearson, 2010; Sleptsov, 2018). Geolocator tracking revealed a very rapid migration in this species: A breeding bird from the Russian Far East covered a distance of almost 5,000 km in less than one month during autumn migration (Heim et al., 2020). These extreme time constraints might have led to the evolution of a peculiar molt strategy, but little information is available so far.…”