The Heat Is Online

Warming Threatens Migrating Birds

El Nino hits birds north and south, study finds

Reuters News Service USA: June 19, 2000


WASHINGTON - El Nino, the Pacific current blamed for causing floods, droughts and other weather disasters, also may help kill off delicate migrating bird populations, researchers said on Thursday.


El Nino makes for drier weather in the winter in the Caribbean, the researchers said, leading in turn to thinner, less-fertile birds. Other experts said the study, published in Science, showed that even subtle changes linked to global warming have profound effects for animal populations.

T. Scott Sillett of Dartmouth College in New Hampshire and colleagues at Tulane University in New Orleans studied black-throated blue warblers, which breed in eastern North American forests and spend their winters in the Greater Antilles islands of the Caribbean.

Sillett's teams watched the warblers at their summer homes in New Hampshire and their winter nests in Jamaica.

They compared their population swings to the cycle of El Nino, the Pacific Ocean current that recurs every few years, bringing unusually warm water to a large area off the coast of Peru. It affects weather all over the world. They also watched a counter-cycle that has come to be called La Nina.

"Adult survival and fecundity (fertility) were lower in El Nino years and higher in La Nina years," they wrote in their report, published in the journal Science.

"During El Nino years in Jamaica, reduced rainfall probably leads to a decreased amount of food available for warblers in the winter dry season and, hence, to lower survival," they added. Thinner birds would not breed as well in summer.

"La Nina years, in contrast, tend to be wetter and thus would result in increased food availability and higher survival." They noted reports that show the El Nino cycle has affected populations of seabirds, raptors such as hawks, primates, rodents and other animals.

"Evidence is accumulating that bird populations are being affected by global warming associated with long-term climate change," they wrote.

"Global warming could also be increasing the severity of ENSO (El Nino Southern Oscillation) events. If this is true, we predict that variance in demographic rates of migratory bird populations will become amplified, leading to elevated extinction risk, especially for small populations."

Centuries ago El Nino occurred every two to 15 years, but recent research shows the pattern has become more frequent.

The researchers said the need to understand what affected bird populations was becoming pressing as migratory songbirds are disappearing at alarming rates.

Bernt-Erik Saether of the Norwegian University for Science and Technology said many studies had shown climate affects birds at their over-wintering points.

"The study of Sillett et al provides the first evidence that the demographics of a migrant bird, the black-throated blue warbler, may be strongly influenced by large-scale climate changes affecting not only survival in southern wintering grounds but also reproductive performance in northern breeding areas," he wrote in a commentary published in Science.
Story by Maggie Fox, Health and Science Correspondent
REUTERS NEWS SERVICE

Climate Change Could Wipe Out Migrating Songbirds

By Cat Lazaroff

HANOVER, New Hampshire, June 16, 2000 (ENS) - Global climate change could greatly reduce the number of songbirds migrating between North and South America. Researchers from Dartmouth College and Tulane University have shown that El Niño cycles, which could worsen due to global warming, reduce the birds. ability to survive and reproduce.

El Niño is the pattern in the Pacific Ocean in which warmer water triggers droughts, floods and other weather disasters, as well as reducing the food supply for many marine species. Many scientists believe the recent increase in the frequency and strength of El Niño cycles can be attributed to rising global temperatures.

In the Caribbean, where many so-called neotropical songbirds spend their winters, El Niño leads to drier winter weather. That, in turn, reduces the available seeds and insects to feed birds, which must work hard to store up enough fat to migrate back to nesting grounds in North America.

The thinner, weaker birds that do survive the migration are less fertile, and less likely to reproduce successfully.

"Our data show that the global climate cycle known as El Niño affects migratory birds both on their breeding grounds in North America and in their winter quarters in the tropics," said researcher T. Scott Sillett.

In a paper published in today. s issue of the journal "Science," Sillett and Richard Holmes, both of Dartmouth College, and Thomas Sherry of Tulane University analyzed 13 years of data on the black-throated blue warbler, a migratory songbird. They found that survival and reproduction rates for these birds were lower than average during El Niño years and higher during La Niña years when Pacific Ocean waters are cooler than average.

Neotropical migrants such as the black-throated blue warbler breed in North America and spend the winters in Latin America and the Caribbean. In recent years scientists have become alarmed by population declines in many neotropical migrant species.

"Most birds breeding in North American forests during the summer are neotropical migrants. Besides their aesthetic value, they eat huge quantities of insects, and research has revealed that migratory songbirds can actually enhance tree growth by consuming leaf eating caterpillars," Sillett said.

Part of the difficulty in determining the causes of songbird declines has been the inability of researchers to determine where particular birds went during migrations. Sillett, Holmes and Sherry were able to determine that their group of black-throated blue warblers always nests in New Hampshire and winters in Jamaica.

"Tracking individual songbirds throughout the year is very difficult, so being able to link summer and winter populations of a migratory species is a unique strength of our study," said Holmes.

The researchers found evidence that the climate changes associated with El Niño years diminished the birds' food supply, causing low reproductive success in their New Hampshire breeding grounds and low survival rates among adults wintering in Jamaica. In contrast, reproductive success and adult survival were much higher during La Niña years.

Poor reproductive success during El Niño summers resulted in fewer young birds arriving on the winter grounds in the fall.

"Adult survival and fecundity were lower in El Nino years and higher in La Nina years," the researchers wrote. "During El Nino years in Jamaica, reduced rainfall probably leads to a decreased amount of food available for warblers in the winter dry season and, hence, to lower survival."

"La Nina years, in contrast, tend to be wetter and thus would result in increased food availability and higher survival," the researchers reported.

Although the El Niño cycle is a natural phenomenon, many climatologists believe that global warming is escalating the cycle's frequency and severity. This could lead to stronger and more common El Niño and La Niñas cycles in the future.

"If the El Niño cycle becomes stronger, it could increase the chances of having years when warbler survival and reproduction rates reach extreme lows, perhaps even approaching zero," Sillett said. "More intense El Niño cycles could thus elevate the risk of extinction for neotropical migrant species with small population sizes."

Other species are affected as well, the researchers noted. The El Nino cycle has also impacted populations of seabirds, birds of prey such as hawks, monkeys and other primates, rodents and other animals. They pointed out that subtle changes linked to global warming, such as an alteration of the El Niño cycle, could have profound impacts on some species.

"Evidence is accumulating that bird populations are being affected by global warming associated with long term climate change," the researchers wrote.

Science Magazine, June 16, 2000

Impacts of a Global Climate Cycle on Population Dynamics of a Migratory Songbird

T. Scott Sillett, 1* Richard T. Holmes, 1 Thomas W. Sherry 2

Progress toward understanding factors that limit abundances of migratory birds, including climate change, has been difficult because these species move between diverse locations, often on different continents. For black-throated blue warblers (Dendroica caerulescens), demographic rates in both tropical winter quarters and north temperate breeding grounds varied with fluctuations in the El Niño Southern Oscillation. Adult survival and fecundity were lower in El Niño years and higher in La Niña years. Fecundity, in turn, was positively correlated with subsequent recruitment of new individuals into winter and breeding populations. These findings demonstrate that migratory birds can be affected by shifts in global climate patterns and emphasize the need to know how events throughout the annual cycle interact to determine population size.

1 Department of Biological Sciences, Dartmouth College, Hanover, NH 03755, USA.
2 Department of Ecology and Evolutionary Biology, Tulane University, New Orleans, LA 70118, USA.
* To whom correspondence should be addressed. E-mail:
t.scott.sillett@dartmouth.edu

The need to understand when and how bird populations are limited is made pressing by recent declines in the abundances of many species, especially migratory songbirds (1). Quantifying the effect and timing of limiting factors for migratory species, however, is difficult because the birds spend different parts of their annual cycle in different locations. Furthermore, events during one stage of the annual cycle are likely to influence populations in subsequent stages (2 ). Here we show through long-term demographic studies of a migratory songbird that the El Niño Southern Oscillation (ENSO) impacts demographic rates in both the breeding and nonbreeding seasons. Our findings also reveal links in the population dynamics of this species between stages of its annual cycle.

We measured the effect of ENSO on survival, fecundity, and recruitment of the black-throated blue warbler, a migratory songbird that breeds in forested regions of eastern North America and overwinters primarily in the Greater Antilles. This species is territorial, largely insectivorous, and exhibits strong site fidelity in both its breeding and wintering grounds (3). We quantified warbler demography from 1986 to 1998 at two locations during the annual cycle: the overwinter period at Copse Mountain, near Bethel Town in northwestern Jamaica, West Indies, and the breeding season at Hubbard Brook Experimental Forest, West Thornton, New Hampshire, USA. The species' habitat at both sites was mature mesic forest, relatively undisturbed by human activity. Demographic data were collected annually from the overwintering population in late October and from the breeding population in mid-May through August. For all analyses, we used annual mean monthly values of the standardized Southern Oscillation Index (SOI) to represent ENSO conditions for each calendar year (4). High, positive values of SOI indicate La Niña conditions and low, negative values indicate El Niño conditions (5 ).

Annual survival (6) of black-throated blue warblers in Jamaica (Fig. 1, A to B) was strongly associated with SOI: survival was low in El Niño years and high in La Niña years (Fig. 2). This result is best explained by the impact of ENSO on local climate and a concomitant change in food availability for overwintering birds. In the winter dry season, migrant songbirds are often food-limited (7) and can be in a state of physiological decline (8), especially in late winter before spring migration. During El Niño years in Jamaica, reduced rainfall (9 ) probably leads to a decreased amount of food available for warblers in the winter dry season and, hence, to lower survival. La Niña years, in contrast, tend to be wetter and thus would result in increased food availability and higher survival.

Annual survival of warblers breeding in New Hampshire (Fig. 1, C to D) was relatively constant and did not consistently fluctuate with changes in ENSO (Fig. 2). Breeding populations of black-throated blue warblers mix extensively on their Caribbean winter quarters (10), and these islands vary in the extent to which they are affected by ENSO (9, 11). Thus, the lack of association between annual survival in New Hampshire and SOI is probably due to many individuals in the breeding population overwintering on other islands, particularly Cuba (12), where the climatic effects of ENSO can be less severe compared with Jamaica (9, 13 ).

Because black-throated blue warbler fecundity is limited by food availability (18), we tested if variation in availability of lepidopteran larvae, the warbler's primary prey in summer, was related to ENSO. Food is most limited from mid-June to mid-July when adults are feeding nestlings and dependent juveniles. At the New Hampshire site, total larval biomass recorded annually on mid-June through mid-July censuses (19 ) was positively correlated with annual fecundity (r = 0.58, P < 0.05) and with SOI (r = 0.58, P < 0.04). Prey biomass was low during El Niño years and high during La Niña years. These results suggest that ENSO influences fecundity of black-throated blue warblers by affecting their food supply.

The effects of ENSO on warbler fecundity had consequences for demography in subsequent seasons (20). First, the number of juveniles at the Jamaica site each October (i.e., recruitment to the overwintering population; see arrow "1" in Fig. 1) was positively correlated with SOI (r = 0.71, P < 0.007) and with fecundity from the preceding summer in New Hampshire (Fig. 4A). Second, the number of yearling breeders each May in New Hampshire (i.e., recruitment of the preceding year's fledglings into the breeding population; see arrow "2" in Fig. 1) was positively correlated with both warbler fecundity (Fig. 4 B) and mean monthly SOI (r = 0.59, P 0.04) from the previous year. In both Jamaica and New Hampshire, low annual recruitment of both juveniles and yearlings was associated with El Niño conditions, whereas high recruitment was associated with La Niña events.

ENSO has been shown to impact demographic rates and food resources of many animal taxa, including seabirds (21), raptors (22), Pacific island passerines (23), primates and rodents (24), and arthropods (25). The results presented here provide evidence that ENSO, through its effect on food supply, limits survival, fecundity, and recruitment of a migratory songbird. Additionally, they illustrate an important interaction between summer and winter population dynamics that operates through a common link to the ENSO cycle. Populations of migratory birds are therefore susceptible over a range of spatial and temporal scales to shifts in global climate patterns.

Evidence is accumulating that bird populations are being affected by global warming associated with long-term climate change (26). Global warming could also be increasing the severity of ENSO events (27). If this is true, we predict that variance in demographic rates of migratory bird populations will become amplified, leading to elevated extinction risk, especially for small populations (28). Because many migratory bird species are declining in abundance and therefore of conservation concern, field research and demographic modeling efforts should focus on understanding how events throughout the annual cycle are interconnected and on how multiple limiting factors, both natural and human-related, determine population size.

REFERENCES AND NOTES

  1. C. S. Robbins, J. R. Sauer, R. Greenberg, S. Droege, Proc. Natl. Acad. Sci. U.S.A. 86, 7658 (1989) [ISI] [Medline] .
  2. P. P. Marra, K. A. Hobson, R. T. Holmes, Science 282, 1884 (1998) [ISI] [Abstract/Full Text] .
  3. R. T. Holmes, in The Birds of North America, No. 87, A. Poole and F. Gill, Eds. (Academy of Natural Sciences, Philadelphia, and American Ornithologists' Union, Washington, DC, 1994), pp. 1-22.
  4. Monthly SOI values can be found at www.cpc.ncep.noaa.gov/data/indices/ .
  5. G. N. Kiladis and H. F. Diaz, J. Clim. 2, 1069 (1989) .
  6. Survivorship data were based on resightings of color-banded individuals (n = 151 birds in Jamaica; n = 550 birds in New Hampshire). Populations at each site were censused annually, returning marked birds were considered "resighted," and unmarked individuals were captured in mist-nets and given a unique combination of colored leg bands. Warbler abundance at both sites was stable (R. T. Holmes and T. W. Sherry, unpublished data). Sex and age ratios at both locations were comparable; about 60% of birds were male and 57% of birds were 1 year old or older. Warblers banded at the New Hampshire site have never been resighted on winter quarters, nor have birds banded at the Jamaica site been resighted in breeding areas.
  7. A. M. Strong and T. W. Sherry, J. Anim. Ecol., in press.
  8. P. P. Marra and R. L. Holberton, Oecologia 116, 284 (1998) [ISI] .
  9. J. C. Rogers, J. Clim. 1, 172 (1988) .
  10. C. P. Chamberlain, et al., Oecologia 109, 132 (1997) [ISI] .