Plunging floater survival causes cryptic population decline in the Common Loon

Abstract

Populations of many vertebrates are declining and geographic ranges contracting, largely as a consequence of anthropogenic threats. Many reports of such decline, however, lack the breadth and detail to narrow down its causes. Here we describe population decline in the Common Loon (Gavia immer), a charismatic aquatic bird, based on systematic resighting and measurement of a marked population. During our 27-year investigation, age-adjusted chick mass has fallen by 11%, mortality among young and old chicks has increased by 31% and 82%, respectively, and fledging success has declined by 26%. Meanwhile, the return rate of marked nonbreeders (“floaters”) has plunged by 53%, and the adult population overall has declined by 22%. Consistent with the thinning ranks of floaters, the rate of territory eviction has decreased by 52% during the study. Despite the decline in floaters, territory occupancy remains unchanged. However, a matrix model, updated with recent estimates for breeding success, juvenile survival, and senescence, yields a recalculated deterministic population growth rate (λ) of 0.94 for our study population, which suggests that declines in vital rates could lead to a loss of 52% of the current population and a decline of 37% in territory occupancy by 2031. Lack of data on floaters in other upper Midwest and New England loon populations leaves their status in doubt.

Interrelated impacts of climate and land‐use change on a widespread waterbird

“…Although our study population of common loons appears to be stable over the long term, recent declines in vital rates (and thus population size since 2010), coupled with growing threats under global change projections, suggests that the species' status along the southern edge of its range should be reassessed (Piper et al, 2020).…”

Early evidence of natal‐habitat preference: Juvenile loons feed on natal‐like lakes after fledging

“…This result highlights the need to incorporate an understanding of multiple life stages in ongoing policy considerations regarding loon conservation and fisheries management. Loon populations have declined significantly at our study site over the last three decades, with juveniles exhibiting reduced survival and a decline in mass (Piper et al., 2020). As temperate habitats experience climate change, clarifying the habitat‐life stage interactions of this iconic northern species might help us mitigate future shifts in lake ecology and habitat quality.…”

Migration patterns and wintering distribution of common loons breeding in the Upper Midwest

“…Despite the potential contribution of transmitters on loon mortality, we believe that radiomarked individuals provided useful and important information on the timing and contributing causes of mortality. Piper et al (2020) reports a 2.28% annual decline in juvenile survival from 2000 through 2019 among common loons in a Wisconsin population. Increased morality among juvenile and young adult ‘floaters' during migration and/or wintering may have contributed to recent population declines observed in Wisconsin (Piper et al 2020).…”

“…Piper et al (2020) reports a 2.28% annual decline in juvenile survival from 2000 through 2019 among common loons in a Wisconsin population. Increased morality among juvenile and young adult ‘floaters' during migration and/or wintering may have contributed to recent population declines observed in Wisconsin (Piper et al 2020). The high rate of juvenile loons in our study suggests that this may be the case.…”

“…Survival rates of loons during their first few years of life are much lower (0.53 for survival to age of 3 years; Piper et al 2012) than that of adults (0.92 annually (Mitro et al 2008) which equates to about 0.78 over a 3‐year period). Piper et al (2020) provided evidence that juvenile survival rates within a Wisconsin study area have declined in recent years. If widespread throughout the region, reduced juvenile recruitment could have population‐level impacts.…”