Climate variation has been linked to historical and predicted future distributions and dynamics of wildlife populations. However, demographic mechanisms underlying these changes remain poorly understood. Here, we assessed variation and trends in climate (annual snowfall and spring temperature anomalies) and avian demographic variables from mist‐netting data (breeding phenology and productivity) at six sites along an elevation gradient spanning the montane zone of Yosemite National Park between 1993 and 2017. We implemented multi‐species hierarchical models to relate demographic responses to elevation and climate covariates. Annual variation in climate and avian demographic variables was high. Snowfall declined (10 mm/year at the highest site, 2 mm at the lowest site), while spring temperature increased (0.045°C/year) over the study period. Breeding phenology (mean first capture date of juvenile birds) advanced by 0.2 day/year (5 days); and productivity (probability of capturing a juvenile bird) increased by 0.8%/year. Breeding phenology was 12 days earlier at the lowest compared to highest site, 18 days earlier in years with lowest compared to highest snowfall anomalies, and 6 d earlier in relatively warm springs (after controlling for snowfall effects). Productivity was positively related to elevation. However, elevation–productivity responses varied among species; species with higher productivity at higher compared to lower elevations tended to be species with documented range retractions during the past century. Productivity tended to be negatively related to snowfall and was positively related to spring temperature. Overall, our results suggest that birds have tracked the variable climatic conditions in this system and have benefited from a trend toward warmer, drier springs. However, we caution that continued warming and multi‐year drought or extreme weather years may alter these relationships in the future. Multi‐species demographic modeling, such as implemented here, can provide an important tool for guiding conservation of species assemblages under global change.
Black‐headed Grosbeaks (Pheucticus melanocephalus) have been observed to undergo prebasic molt during fall in the North American Monsoon region of the southwestern United States and northwestern Mexico, but it is unknown whether molt migration is pervasive across populations of the species. During the 2014 breeding season, we GPS‐tagged (where GPS is global positioning system) nine adult Black‐headed Grosbeaks in Yosemite National Park with archival GPS tags to determine specific locations where grosbeaks breeding in Yosemite spent portions of the non‐breeding season, and to assess whether those locations were consistent with molt migration. On 2 June 2015, one of these birds, a male GPS‐tagged on 19 June 2014, was recaptured with its GPS unit still attached. Data downloaded from the unit revealed that, by 20 August 2014, the bird had moved 1300 km from Yosemite National Park to Sonora, Mexico, where it remained until at least 15 October 2014. By 24 November 2014, the grosbeak had moved >1300 m from Sonora to the Michoacán‐Jalisco border region, where it remained until the last GPS‐determined location was obtained on 24 March 2015. The seasonal timing of these movements and the length of stay in Sonora are consistent with the expected behavior of a molt‐migrating bird. Remote‐sensed enhanced vegetation index (EVI) data indicated that the grosbeak arrived in the monsoon region near the area's annual peak in EVI, and then, as the index was declining sharply, departed for the Michoacán‐Jalisco region, where the index also declined during the same period, but substantially less so than in Sonora. Climate change in the coming decades is expected to delay the annual onset of the monsoon while also accelerating the initiation of arid, summer‐like conditions throughout much of western North America, possibly yielding a temporal mismatch between fall migration and the monsoon‐driven conditions that may be critical for molt‐migrating birds.
Remote tropical oceanic islands are of high conservation priority, and they are exemplified by range-restricted species with small global populations. Spatial and temporal patterns in rainfall and plant productivity may be important in driving dynamics of these species. Yet, little is known about environmental influences on population dynamics for most islands and species. Here we leveraged avian capture-recapture, rainfall, and remote-sensed habitat data (enhanced vegetation index [EVI]) to assess relationships between rainfall, vegetation greenness, and demographic rates (productivity, adult apparent survival) of three native bird species on Saipan, Northern Mariana Islands: rufous fantail (Rhipidura rufifrons), bridled white-eye (Zosterops conspicillatus), and golden white-eye (Cleptornis marchei). Rainfall was positively related to vegetation greenness at all but the highest rainfall levels. Temporal variation in greenness affected the productivity of each bird species in unique ways. Predicted productivity of rufous fantail was highest when dry and wet season greenness values were high relative to site-specific 5-year seasonal mean values (i.e., relative greenness); while the white-eye species had highest predicted productivity when relative greenness contrasted between wet and dry seasons. Survival of rufous fantail and bridled white eye was positively related to relative dry-season greenness and negatively related to relative wet-season greenness. Bridled white-eye survival also showed evidence of a positive response to overall greenness. Our results highlight the potentially important role of rainfall regimes in affecting population dynamics of species on oceanic tropical islands. Understanding linkages between rainfall, vegetation, and animal population dynamics will be critical for developing effective conservation strategies in this and other regions where the seasonal timing, extent, and variability of rainfall is expected to change in the coming decades.
The avifauna of Saipan, Northern Marianas Islands, includes a variety of endemic and range-restricted species of conservation concern. Monitoring population dynamics, trends, and vital rates of these species can help inform their conservation. Here we apply reverse symmetry models to 11 years (2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018) of capture-recapture data from 6 mist-netting stations on Saipan to estimate adult apparent survival probabilities, recruitment rates, population growth rate, and demographic contributions to population growth rate for five bird species: Mariana Kingfisher (Todiramphus albicilla albicilla), Rufous Fantail (Rhipidura rufifrons saipanensis), Bridled White-eye (Zosterops conspicillatus saypani), Golden White-eye (Cleptornis marchei), and Micronesian Myzomela (Myzomela rubratra saffordi). Population growth rate estimates suggested declines for all species except Golden White-eye. Survival was the largest contributor to annual population growth rate for all species except Micronesian Myzomela, for which population change was largely determined by recruitment (including 1-yo recruits and immigrating adults). Hierarchical models applied to the two species with the most captures and recaptures, Rufous Fantail and Golden White-eye, showed greater temporal than spatial variation in vital rates. Annual variation in vital rates confirmed the importance of survival in explaining population change, but also showed recruitment to be especially important in driving population growth in years of population increase. Our results provide new insights into drivers of population change in these species and highlight information gaps, such as a need for understanding seasonal habitat use and the scale and timing of movements.
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