Principal Investigators:
Nicholas L. Rodenhouse, Wellesley College (nrodenho@firstclass.wellesley.edu)
T. Scott Sillett, Smithsonian Institution (silletts@nzp.si.edu)
Richard T. Holmes, Dartmouth College (richard.t.holmes@dartmouth.edu)

Date Prepared: March 2003

Background
Climate patterns throughout the world are changing rapidly, as evidenced by increases in average global temperature and in the annual variability of weather conditions. The impacts of these changes on animal populations are not well understood (Rodenhouse 1992, Sillett et al. 2000). However, they can be assessed through long-term demographic studies that take advantage of temporal and spatial variability in weather across environmental gradients. In this project, we have extended our long-term studies of birds in New England forests (Holmes & Sherry 2001, Sillett & Holmes, in press; see Bird population and community studies at HBEF) in ways that allow us to identify and assess how weather, and ultimately, climate, in combination with biotic factors, affects the spatial distribution, abundance, and demography of bird species that breed in temperate forests.

Figure 1. Hypothesized direct and indirect relationships between weather and seasonal fecundity of birds nesting in northern hardwoods forests. Lines indicate causal relationships to be tested; dashed lines indicate potential relationships for which little data currently exists. Potential feedbacks between years, e.g., between breeding productivity in one year and bird abundance in the next are not shown. Each variable can be represented by different measures (e.g., weather by precipitation or temperature).

In 2002, we began to examine these processes through demographic studies of one focal species, the Black-throated Blue Warbler (Dendroica caerulescens) (Holmes 1994), and extensively by quantifying annual shifts in the spatial distribution and abundance of this focal and other bird species at a landscape scale -- that of the 3160-ha Hubbard Brook Experimental Forest in the White Mountains, New Hampshire (P.J. Doran & R.T. Holmes, unpubl. data). The 600 m altitudinal gradient at this site provides the breadth of environmental conditions needed to compare the effects of within- and between-year differences in local weather conditions and biotic factors (e.g., food supply, nest predator abundances) on bird population dynamics and reproductive performance (Rodenhouse et al., in review).

Food (caterpillar) abundance is quantified by visual searches of foliage

Our approach to the problem
Weather can influence bird populations directly or indirectly (Fig. 1). Extreme direct effects of weather include large-scale mortality of adults as a result of catastrophic conditions (see Rodenhouse 1992) and of eggs and young due to exposure, starvation or structural failure of nests (e.g., Rodenhouse & Holmes 1992). For migratory songbirds, mortality of adults due to the direct effects of weather is rare during the breeding season, and instead occurs mostly during migration or as a result of poor wintering conditions (Marra & Holmes 2001, Sillett & Holmes 2002). Even so, such non-breeding season mortality can influence the size and age-structure of the breeding population in subsequent summers (Sillett et al. 2000).

Weather can affect bird demography indirectly in a number of ways. For example, it can influence: (1) abundance and availability of food (i.e., arthropods), which influence nestling survival (Sherry & Holmes 1992), the frequency of double-brooding (Rodenhouse & Holmes 1992, Nagy & Holmes, in review), and hence seasonal fecundity (Sillett et al. 2000); (2) settlement patterns and subsequently the spatial distribution of individuals among sites that differ in suitability, (3) synchrony of settling which in turn affects local density and the intensity of density-dependent processes (Sillett 2000), and (4) the abundance and activity of nest predators (Rodenhouse & Holmes 1992). These indirect effects of weather all potentially influence bird breeding success and seasonal fecundity, which in turn influences subsequent recruitment and potentially population size (Sillett et al. 2000). Yet, the relative importance of weather/climate and biotic variables in determining seasonal fecundity has rarely been examined.

Study areas at different elevations within the Hubbard Brook valley

One way to determine the importance of weather and other factors is to examine avian demography along an environmental gradient where weather conditions vary in a predictable way. It is well known that bird species vary in their distributions and abundances over environmental gradients reflecting their responses to climate and associated vegetational changes along those gradients. Altitudinal gradients in particular are useful, because many aspects of avian environments that potentially influence breeding productivity vary with altitude, including temperature, precipitation, windiness, seasonality, vegetation composition and physiognomy, as well as arthropod species composition, richness and abundance. Yet, few studies of birds have examined the demographic consequences of such variation.

In this on-going investigation, we are testing hypotheses concerning the effects of abiotic conditions (e.g., weather) and biotic factors (e.g., population density, food, predators) on bird populations distributed across the 600 m environmental (altitudinal) gradient within the Hubbard Brook Experimental Forest. The research in progress will help to elucidate ecological mechanisms generating changes in bird distribution, abundance, and seasonal fecundity across an environmental gradient, and thus will contribute to an understanding of how weather in combination with biotic factors influences forest birds. Such information is key to assessing the potential local effects of climate change on bird populations, and ultimately to predicting how these populations and the biological communities of which they are a part will respond in the future to climate-caused environmental change.

Key References
(cited references not listed here can be found in the HBES Publication list)

Holmes, R. T., T. W. Sherry, P. P. Marra, & K. E. Petit. 1992. Multiple-brooding and annual productivity of a Neotropical migrant passerine, the Black-throated Blue Warbler (Dendroica caerulescens), in an unfragmented temperate forest. Auk 109: 321-333.

Holmes, R. T., P. P. Marra, & T. W. Sherry. 1996. Habitat-specific demography of breeding Black-throated Blue Warblers (Dendroica caerulescens): implications for population dynamics. Journal of Animal Ecology 65: 183-195.

Nagy, L.R., and R.T. Holmes. In review. Individual variation in reproductive effort: factors influencing the frequency of double brooding in a passerine bird.

Rodenhouse, N. L. 1992. Potential effects of climate change on a Neotropical migrant landbird. Conservation Biology 6: 263-272.

Rodenhouse, N. L., & R. T. Holmes. 1992. Results of experimental and natural food reductions for breeding Black-throated Blue Warblers. Ecology 73: 357-372.

Sherry, T. W., & R. T. Holmes. 1992. Population fluctuations in a long-distance Neotropical migrant: demographic evidence for the importance of breeding season events in the American Redstart. Pp. 431-442 in Ecology and Conservation of Neotropical Migrant Landbirds (J.M. Hagan and D. W. Johnston, eds.), Smithsonian Press, Wash. D.C.

Sillett, T. S., & R.T. Holmes. 2002. Variation in survivorship of a migratory songbird throughout its annual cycle. Journal of Animal Ecology 71, 296-308.

Sillett, T. S., R.T. Holmes, & T.W. Sherry. 2000. Impacts of a global climate change on the population dynamics of a migratory songbird. Science 288: 2040-2042.