Experimentally Reducing Neighbor Density Affects Reproduction and Behavior of a Migratory Songbird

Abstract

Because populations of territorial birds are relatively stable compared to those of other animal taxa, they are often considered to be tightly regulated. However, the mechanisms that produce density-dependent feedbacks on demographic rates and thus regulate these populations are poorly understood, particularly for migratory species. We conducted a three-year density-reduction experiment to investigate the behavioral mechanisms that regulate the abundance of a Nearctic-Neotropical migrant passerine, the Black-throated Blue Warbler (Dendroica caerulescens), during the breeding season. We found that the number of young fledged per territory, territory size, and the proportion of time males spent foraging were significantly greater on territories around which neighbor density was experimentally reduced compared to control territories. Territory quality, proportion of nests depredated per territory, and male countersinging rates were not statistically different between treatments. These results indicate that individuals with more neighbors (i.e., in neighborhoods with greater conspecific density) have reduced breeding productivity. The results also suggest that a crowding mechanism that mediates interactions among territory-holders could generate the density dependence needed to regulate local abundance, at least in areas of homogeneous, high-quality habitat. The effect of the neighbor-density reduction on warbler fecundity and behavior varied with annual fluctuations in weather and food availability, and was strongest in 1997, an El Niñ o year, when conditions for breeding were least favorable. This variation in our experimental results among years implies that density dependence due to crowding may have its strongest impact on local abundance when environmental conditions are relatively poor.

Experimental food supplementation reveals habitat-dependent male reproductive investment in a migratory bird

“…Significant terms are in italic. than 95% of the study population each year and we have no evidence of floaters [28]. Furthermore, our previous results showed that food-supplemented males had above-average body mass during their social mate's fertile stage relative to controls [25].…”

Causes and Consequences of Tree Swallow (Tachycineta Bicolor) Dispersal in Saskatchewan

“…On the basis of theoretical and empirical investigations of avian dispersal, we predicted that (1) (Steele 1993, Holmes et al 1996; higher understory leaf density is correlated with a greater abundance of lepidopteran larvae (Holmes et al 1996). To quantify territory quality, vegetation measurements were taken late in the breeding season, after leaves fully emerged, using the methods in Sillett et al (2004). Briefly, from 1997 to 2001, vegetation measurements were taken within 11.2-m-radius circles placed at a random subset of plot grid intersections (see above); vegetation survey locations were ≥50 m apart.…”

Non-breeding season habitat quality mediates the strength of density-dependence for a migratory bird

“…Much of this work has investigated the relationship between density and reproductive success [8,17]. Studies designed to examine density-dependent effects on adult or juvenile survival have typically been conducted with relatively sedentary, small or recovering populations [18][19][20].…”