Throughout much of the 20thcentury and beyond, migratory birds have faced rapid declines in North America. The breeding bird survey (monitoring avian population trends since the mid-1960s) and concurrent land use changes implicate habitat degradation and fragmentation as underlying causes of these declines. The Swainson's Thrush (Catharus usutulatus) is a Nearctic-Neotropical migrant that has suffered declines of about 0.7% per year over the last 45 years and is subject to greater declines as human populations expand and suitable habitat at both the breeding and wintering grounds become altered. The breeding ground ecology of the Swainson's Thrush (SWTH) has been extensively studied over much of the last several decades, however, the wintering ground ecology lacks similar effort. No study to date, to our knowledge, has looked at the wintering habitat use and spatial occupancy as they relate to demographics and migratory connectivity of a local wintering population of SWTH. Therefore we initiated a study that incorporated data from the winters of 2013 and 2014 on SWTH winter ecology by using point counts, vegetation surveys, radio telemetry, molecular sexing and isotopic analysis techniques at Wildsumaco Wildlife Sanctuary, Ecuador.
Point counts were set up between two forest types, primary and secondary, and they statistically differed with primary sites having larger DBH (t = -2.78, df = 22, p = 0.013), higher cover of woody plants >5 m in height (t = -4.56, df = 22, p < 0.001), and higher canopies (t = -3.76, df = 22, p = 0.001). The total detections of SWTH was significantly higher in secondary forest sites (W = 534, df = 22, p < 0.001), with the highest detection rates in 5 of the 12 secondary stations. These five stations statistically differed from other secondary stations by exhibiting lower DBH (t = -3.49, df = 10, p = 0.007), lower canopy heights(t = -2.12, df = 10, p = 0.036), and higher tree cover (t = 1.98, df = 10, p = 0.044). SWTH densities significantly decreased over the course of the seven week surveying period (F = 6.54, df = 1, p = 0.027). Of the 86 birds caught between the 2013 and 2014 seasons, 85 of them sexed as male and 83 of them aged as young of the year (SY). An additional 27 blood samples donated to us by Kristen Ruegg from UCLA taken across a broader range of Ecuador sexed 20 of them as male. This almost completely uniform population of young males seemed to prefer secondary forest with higher amounts of small trees with low canopy heights. Since birds within this population decreased in density over the course of the field season, we suggest that this population is itinerant in behavior and possibly tracking seasonal fruit.
Radio telemetry revealed that this population of young males exhibited an extreme degree of overlap when tracked simultaneously. Up to eight birds were tracked at one time and they all occupied similar habitat space. Indeed we observed several individuals foraging in the same tree with little aggression towards conspecifics. These tracked birds also occupied relatively large areas (x̅= 2.36 ± 0.92 hectares)and had a short residency time (x̅= 17.1 ± 2.68 days). These findings corroborate the point count results, suggesting itinerant and non-territorial behavior in this wintering population. Intratropical movements are not uncommon in migratory species suggesting this non-territorial population at Wildsumaco either covers large areas to exploit seasonal resources or undergoes a mid-winter intratropical migration.
Migratory connectivity was assessed by analyzing stable hydrogen isotopes (2H) in feathers of SWTH. Precipitation 2H values correlate to the values in bird feathers; therefore this analysis is auseful approach to link the breeding and wintering grounds in some migratory birds. Feather 2H values were extremely variable indicating a broad breeding distribution among this wintering population. When these values were correlated with precipitation values in North America, we found that our birds could breed in three distinct areas stretching across the entire breeding range. This indicates weak migratory connectivity within the boreal subspecies of SWTH. Thus, habitat alteration in a local area on the wintering grounds can affect populations from across the breeding grounds, but these losses may be more difficult to detect.
The majority of future conservation efforts rely in part on researchers' ability to convey useful information from their studies to governmental and public stakeholders. By creating awareness within the local communities, partnerships can originate that might generate necessary support and funding for future projects. These projects should focus on obtaining as much information on the full annual cycle of declining migratory birds, concentrating on the wintering grounds. Efforts directed at this phase of the annual cycle will likely have the greatest conservation impact for migratory birds, especially given the proportional total area of the wintering ranges compared to the breeding ranges of most Nearctic-Neotropical migratory species.