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

In this project, we are investigating the factors and processes determining abundances of migratory songbird populations. Our focal species is the Black-throated Blue Warbler (Dendroica caerulescens) population in the Hubbard Brook Experimental Forest, N.H., which has remained relatively stable for the last 32 years (Holmes & Sherry 2001).

Black-throated Blue Warbler male feeding nestlings

Through long term monitoring, accompanied by experimental studies, we have demonstrated that the suitability of breeding habitat influences this species' (1) population density, (2) age structure, and (3) annual per capita reproductive output (Holmes et al. 1992, 1996, Sillett et al. 2000, Sillett 2000). Furthermore, long-term demographic data indicate that recruitment of yearling males into the breeding population is positively correlated with nesting success in the previous summer (Fig. 1; Sillett et al. 2000) and that reproductive output declines significantly as the density of birds increases (Fig. 2; Sillett 2000). Although the latter relationship is strong, evidence for regulation, the mechanism(s) underlying regulation in this population, as well as for those of any migratory songbird, remains elusive. For instance, one or more mechanisms could explain these patterns: (1) direct or indirect intra-specific interactions that increase as population size increases (a crowding mechanism), or (2) increased occupancy of poorer quality territories with increasing population size (a site dependent mechanism; see Rodenhouse et al. 1997, 1999, 2000, McPeek et al. 2001).

Recently, we tested for the effects of crowding and site dependence in our study population. To test for crowding effects, we experimentally reduced the neighborhood density experienced by focal pairs of warblers, and compared behavioral and demographic traits between two treatments: a normal-density control consisting of pairs randomly selected from those within a 150 ha area and a reduced density group which consisted of pairs whose neighbors had been experimentally removed.

This density reduction experiment yielded clear demographic and behavioral differences in the two treatments (Sillett 2000). Overall, mean number of young fledged annually per territory was significantly greater in the reduced-density treatment versus the control. Behavioral differences included, significantly larger territories in the reduced-density treatment than in controls. Adults made more food deliveries to nestlings per hour in the reduced-density treatment than in the control, and males spent proportionally more time foraging in the reduced-density treatment compared to control males. This experiment demonstrates that neighboring conspecifics can affect warbler reproductive success and behavior, and therefore that crowding is an important regulatory mechanism operating in this population.

Saddled Prominent (Heterocampa guttivita: Notodontidae), a defoliating caterpillar at Hubbard Brook

Our tests of the site dependent mechanism revealed that site suitability also affects reproductive performance and behavior. Site suitability was characterized by measuring vegetation density, food abundance, and nest predator abundance for 3 years on more than 50 randomly selected sites (territories) distributed among four study areas ranging in elevation from about 250 to 850 m. Sites differed markedly in all measures of suitability; these differences among sites were predictable, and BTBWs responded to these differences. For example, older (ASY) males occupied sites with significantly greater leaf density than did yearling (SY) males, and returning birds that shifted sites between breeding seasons moved to sites with greater leaf density than their previous territory. Sites that were occupied in all 3 years of the study, i.e., the presumed best sites, had greater leaf density in the shrub layer, higher food biomass, and fewer nest predators than those occupied for only one or two years.

These differences in site characteristics corresponded with significant differences in annual production of young. Poor sites, when occupied, produced only about half the number of young on average when compared to good sites. Thus, quality of individual sites (territories) affects reproductive performance in this species, which is a key condition for a site dependent regulatory mechanism (Rodenhouse et al 1997, McPeek et al. 2001).

These results, combined with demographic modeling, suggest that the negative feedback found in long term demographic data is strong enough to regulate the local population at the densities observed (Sillett & Holmes in press), and may be generated by one or more regulatory mechanisms, either acting singly or together (Rodenhouse et al. 1999). To date, therefore, the results from long-term demographic monitoring, the density reduction experiment, site suitability measures, and demographic models suggest that population regulation in our study species, and probably other songbirds, is regulated not by one but by multiple negative feedback mechanisms.

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

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

McPeek, M. A., Rodenhouse, N. L., Holmes, R. T. & Sherry, T. W. 2001 Site dependent population regulation: population-level regulation without individual-level interactions. Oikos 94, 417-424.

Rodenhouse, N. L., Sherry, T. W. & Holmes, R. T. 1997 Site-dependent regulation of population size: a new synthesis. Ecology 78, 2025-2042.

Sillett, T. S. 2000 Long-term demographic trends, population limitation, and the regulation of abundance in a migratory songbird. Dartmouth College: Dissertation.

Sillett, T. S. & Holmes, R.T. (in press). Long-term demographic trends limiting factors, and the strength of density dependence in a breeding population of a migratory bird. In Birds of Two Worlds. (eds Greenberg, R. S. & P.P. Marra) Smithsonian Institution Press.

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