Combining feather <em>δ</em>²H, mist netting, and eBird data to investigate post-breeding migration of neotropical passerines on the Guanahacabibes Peninsula, Cuba

The Gulf of Mexico hosts major migration pathways linking birds breeding in the United States and Canada to nonbreeding areas in Central and South America, but little is known about the role of western Cuba within these pathways. Long-term migration monitoring has been missing from western Cuba, but since 2015, a post-breeding migration banding station has operated annually at the tip of the Guanahacabibes Peninsula. Our objective was to investigate patterns of post-breeding migration in western Cuba, combining multiple sources of data to examine connectivity and timing of migration for 15 passerine species. We used feather stable-hydrogen isotopes (δ2Hf) to estimate breeding origins and used constant-effort mist netting and eBird Status and Trends relative abundance estimates to examine post-breeding migration and arrival timing in Cuba and the surrounding regions. The majority of species had southeastern origins (e.g., Setophaga ruticilla [American Redstart]), which is consistent with the breeding distribution of the most frequently encountered species in Cuba during the stationary nonbreeding period. Origins among boreal breeding species were more varied than expected, assuming parallel migration, with an unexpectedly high representation of northwestern breeding populations in species like Parkesia novaboracensis (Northern Waterthrush) and broad continent-wide origins in species like Catharus ustulatus (Swainson’s Thrush). Comparing the timing of migration in Cuba to the surrounding regions, we found evidence supporting the movement of migrants through western Cuba to the Yucatán Peninsula/ Central America for nine species. Although these patterns are speculative, and eBird Status and Trends data were highly variable, these results indicate that western Cuba, and the Guanahacabibes Peninsula in particular, is a region used by several species of Neotropical migrant passerines during post-breeding migration. More research is needed on stopover ecology in Cuba and targeted VHF radio tagging within the Gulf of Mexico to better understand how migratory birds use this important area during post-breeding migration. Methods Our study was part of a larger initiative to document the post-breeding migration of Neotropical migrant passerines through western Cuba. This program was organized by ECOVIDA (Centro de Investigaciones y Servicios Ambientales / Environmental Research and Services Center) and involved regular mist-netting operations at Cabo de San Antonio (21.865ºN, 84.949ºW) on the western tip of the Guanahacabibes Peninsula. This banding station consisted of 12 mist nets, open daily for five hours from 0700 to 1200, in semideciduous forest, open transitional areas, and coastal vegetation. Constant effort mist netting was performed in 2019 (September 11– November 4), 2022 (October 5 – 24), and 2023 (September 22 – October 28). Our study species were Vireo griseus (White-eyed Vireo), V. olivaceus (Red-eyed Vireo), Dumetella carolinensis (Gray Catbird), Catharus minimus (Gray-cheeked Thrush), C. ustulatus (Swainson’s Thrush), Seiurus aurocapilla (Ovenbird), Parkesia novaboracensis (Northern Waterthrush), Mniotilta varia (Black-and-White Warbler), Limnothylipis swainsonii (Swainson’s Warbler), Leiothlypis peregrina (Tennessee Warbler), Setophaga americana (Northern Parula), S. citrina (Hooded Warbler), S. magnolia (Magnolia Warbler), S. ruticilla (American Redstart), and Passerina cyanea (Indigo Bunting). These 15 species were chosen to reflect the most common species captured at the Cabo de San Antonio banding station (i.e., those captured in all three years) that also had geographically distinct breeding ranges (e.g., boreal forest vs. eastern temperate forest), and those with nonbreeding grounds at least partially south/southwest of Cuba. Some additional exploratory sampling occurred on warbler species where fewer feathers were collected (n = 7 Helmitheros vermivorum [Worm-eating Warbler], n = 1 Protonotaria citrea [Prothonotary Warbler], n = 8 Geothlypis trichas [Common Yellowthroat], n = 3 Setophaga caerulescens [Black-throated Blue Warbler], n = 4 Setophaga pensylvanica [Chestnut-sided Warbler], n = 1 Setophaga tigrine [Cape May Warbler]). We have includedthise data here to encourage future use of these data. Feathers were individually stored in paper coin envelopes prior to isotope analysis. In the laboratory, feather samples were soaked in 2:1 chloroform: methanol overnight, rinsed, and dried in a fume hood at ambient temperature. Feather barbs from the distal end of the feather were analyzed for stable hydrogen isotopes by weighing subsamples (0.35 mg) into silver capsules (Mettler Toledo XP9 Excellence Plus XP Micro Balance, Greifensee, Switzerland). Samples were assayed for~ δ2Hf~ at the Laboratory for Stable Isotope Science-Advanced Facility for Avian Research stable isotope facility at the University of Western Ontario. The sample carousel (Uni-Prep, Eurovector, Milan, Italy) was heated to 60ºC, evcuated and flushed with dry helium and then held under helium flow for the duration of an analytical run. The Uni-Prep was coupled with a Eurovector elemental analyzer and combusted at 1350ºC on a glassy carbon reactor. The resultant H2 gas was analyzed on a coupled Thermo Delta V Plus (Thermo Scientific, Bremen, Germany) continuous-flow isotope ratio mass spectrometer via a Conflo device (Thermo Scientific, Bremen, Germany). Sample results were expressed in the standard delta (δ) notation in parts per thousand ‰(‰ deviation from in-house keratin standards (CBS: -197‰; KHS: -54.1‰). The δ2Hf value of the non-exchangeable H fraction was derived according to the comparative equilibration approach (Wassenaar and Hobson 2003). Based on within-run (n = 5 each) keratin standards, measurement error was estimated to be ±2‰. All δ2Hf values were expressed relative to Vienna Standard Mean Ocean Water (VSMOW).