Christopher M. Rogers

Stability, regulation, and the determination of abundance in an insular song sparrow population

The population dynamics of the Song Sparrow, Melospiza melodia, were studied for 15 yr on Mandarte Island, in southwestern British Columbia, Canada. This population exhibited a high median density (7.8 females/ha) and fluctuated strongly (10—fold or more) from year to year. The population received few successful immigrants, even at low densities, and its dynamics were thus driven by local events. Two strong density—dependent regulating mechanisms were detected. First, reproductive output was strongly depressed at high densities because of an increased rate of nest failure and a decline in mean clutch size. Greater nest failure at high density was due to increased predation on eggs and nestlings. Nest failure increased with the rate of nest parasitism by Brown—headed Cowbirds (Molothrus ater) and was lowest when cowbirds were absent from Mandarte, suggesting that cowbirds either cause or facilitate nest failure. Second, the rate of juvenile recruitment was inversely related to the density of adults, becaus...

Predation Risk and Fasting Capacity: Do Wintering Birds Maintain Optimal Body Mass?

This paper tests the optimal body mass (OBM) model developed by Lima (1986) for small birds overwintering in a temperate climate. The OBM model views winter body mass as reflecting a trade-off between predation risk and the risk of starvation during periods of inclement weather. Specifically, stochastic computer simulations of winter sea- sons showed that costs (e.g., risk of being preyed upon) and benefits (e.g., fasting capacity) of fat associated with an intermediate mass combine to maximize the probability of over- winter survivorship (Lima 1986). The OBM model predicts that when benefits but not costs of extensive fattening are eliminated by predictable resources, the optimal solution will be for bird species to avoid costs by remaining leaner than species utilizing unpre- dictable resources. In the latter environment, benefits of fattening are important to surviving suddenly imposed fasting spells. Thus if mass and therefore winter survivorship are op- timized among environments, size of the fat reserve should vary inversely with resource predictability. As a test of this prediction, this study compares visible subcutaneous fat among three avian foraging guilds that occupy a gradient of winter resouce predictability. Strictly tree- foraging species utilizing tree-borne resources were considered to occupy a predictable environment relative to strictly ground-foraging species. In the latter foraging guild, re- sources may be unexpectedly and temporarily eliminated by moderate to heavy snowfall. Species with a partial dependence on each of these resource environments were assumed to occupy a resource environment of intermediate predictability. In four separate data sets involving small birds from southern Michigan, central Tennessee, northern Michigan, and southern Indiana, visible fat class varied inversely with resource predictability, as predicted. This trend was also observed in a comparison of species assumed alike in physiological factors that may independently influence size of fat reserve (body size, hypothermia, thermal conductance, basal metabolic rate, nocturnal roost microenvironment). In further con- cordance with theory, a comparison of dawn fat class between the Dark-eyed Junco and Tree Sparrow (from an unpredictable resource environment) and the Downy Woodpecker and Tufted Titmouse (from a predictable resource environment), which factored out all but a slight effect of basal metabolic rate, indicated significantly greater reserves in the two ground-foraging species. I conclude that aside from possible influences of physiological factors, resource pre- dictability likely affects winter body mass in small birds. This influence takes the form of guild-specific trade-offs between predation risk and fasting capacity (see also Stuebe and Ketterson 1982, Nolan and Ketterson 1983). These results support the OBM model (and similar models), and along with previous studies suggest the widespread importance of predation risk as a factor structuring biological systems.

Life‐History Theory in the Nonbreeding Period: Trade‐Offs in Avian Fat Reserves?

We tested the hypothesis of optimal winter fat storage in birds. The hy- pothesis predicts that when the survivorship benefits (fasting capacity) but not the survi- vorship costs (higher cost of flight, lowered agility, increased exposure to predators) of fat are eliminated by predictable resources (e.g., tree- and shrub-borne food), species will be leaner than species exploiting unpredictable resources (e.g., ground-borne food subject to sudden covering by snow). The predicted pattern was found in an earlier study of avian communities in central North America, where snowfall is frequent. Here we tested the predictions of the hypothesis that (1) when ground- and tree-feeding guilds are compared between geographic regions of harsh and mild winter climate, only ground-feeders will have relatively high fat reserves, resulting in a significant guild x climate interaction term in a two-way analysis of variance, and (2) ground- and tree-foraging guilds will both show low fat reserves in a mild winter environment. To test these predictions, visible subcutaneous fat class was measured in both guilds in Wisconsin and Michigan (harsh winter environments) and southwestern British Columbia, northwestern Washing- ton, and Tennessee (mild winter environments). Both predictions were supported. We suggest that small birds wintering in North America approach local optima in energy storage strategy and winter survivorship. The energy storage strategy thus appears to be a major life-history trait in the nonbreeding period. The trade-off between the costs and benefits of winter fat has two potential implications for understanding population limitation. First, trade-offs that reduce reserves can lead to lower survival in severe winter weather than would occur at maximum fat levels. Second, our study suggests that individual birds are sensitive to, and can control, both predation and starvation risks. Recent theory of population limitation indicates that as food abun- dance declines, birds feed to avoid starvation, but at the expense of increased mortality from predation. Together these studies suggest that populations are limited by interacting (predation, food supply) instead of single (food supply) factors. Study of trade-offs used by individuals to maximize survival can provide unique perspectives on population limitation.

Cerulean warbler reproduction, survival, and models of population decline

Abstract We present and compare demographic data for cerulean warblers (Dendroica cerulea) from 5 study sites across the range of the species from 1992 to 2006. We conducted field studies to collect data on daily nest survival, nest success, and young fledged per successful nest, and we used data to estimate fecundity. Daily nest survival, nest success, young fledged, and fecundity varied widely across the cerulean range and among years. Study sites in agriculture-dominated landscapes (Mississippi Alluvial Valley, IN, and MI, USA) had negative growth rates in all years monitored because measured values of nest success and young produced per successful nest were incapable of offsetting apparent mortality. Ontario (Canada) and Tennessee (USA) populations had greater nest success and fecundity but still appeared to be incapable of producing stable populations (λ = 1) under field-measured and assumed conditions. We had survival data only for one site (Ontario); thus, additional survival data are greatly needed to enable more reliable estimates of population growth. Conservation strategies for cerulean warblers in agriculture-dominated landscapes (e.g., Mississippi Alluvial Valley, IN, and MI) may require major landscape-level habitat reconfiguration to change agriculture-dominated landscapes to forest-dominated landscapes to increase fecundity. Conservation strategies in predominantly forested landscapes in the core of the range (e.g., TN) require a focus on minimizing habitat loss and developing management prescriptions capable of improving fecundity. In both cases, based on sensitivity and elasticity analyses, efforts to improve survival during the nonbreeding season would have the greatest positive effect on population growth.

Geographic Variation in Winter Fat of Dark‐Eyed Juncos: Displacement to a Common Environment

Main small temperature—zone birds fatten in winter and use this reserve when food becomes unavailable. The winter body mass of migratory Dark—eyed Juncos (Junco hyemalis) usually is greater where severe weather is more frequent, i.e., at higher latitudes or altitudes. The mechanism responsible for this geographic variation is unknown, and we asked whether geographically separated populations are composed of birds with different capacities or tendencies to fatten. We sampled the lipid index (LI, grams of lipid per gram of lean dry mass) of free—living juncos at three latitudes in two winters and found geographic variation. However, the pattern of variation in LI at the three latitudes was not consistent, either within or between winters. We also displaced wintering juncos from northern, intermediate, and southern sites and exposed them to a common environment in Indiana. One group of displaced birds was released in natural junco habitat (during three winters), and another was held outdoors in captivity (during two winters). Many released birds remained nearby, and when we recaptured some of them in late winter the LIs of the three geographic subgroups were statistically indistinguishable. It was also indistinguishable from the LI of juncos sampled at that time from the free—living Indiana population. Among the displaced birds that we held captive, we found no differences among the geographic subgroups either in LI or total body mass. In one of the years, late—winter LI and body mass of the captives did not differ from values for the released group or from values of newly caught Indiana juncos, but in the second year values for the captives were lower. Our results indicate the geographic variation in winter fattening of juncos is not a population—specific trait that persists independently of current environmental information and that it probably reflects a response to local environmental conditions. To our knowledge, this is the first experiment using free—living birds to test the role of the environment in maintaining geographic variation in a life history trait in the nonbreeding period.

Testing optimal body mass theory: Evidence for cost of fat in wintering birds

Optimal body mass models for small wintering birds are central to animal ecology, and offer insights into maximizing individual fitness in a complex environment. Such models assume both costs and benefits of fat deposition, and consider how they affect winter survival probability. Hypothesized mass-dependent costs of elevated fat include increased wing load and subsequent reduced ability to avoid predators, as well as increased predator exposure while feeding to fatten. A likely benefit of winter fat is increased fasting capacity during resource shortages. Here I test optimal body mass theory by searching for both cost and benefits of winter fattening, utilizing interspecific variation in winter fat in natural populations. If increased predation risk is a mass-dependent cost of fattening, wintering birds occupying dense (closed) winter habitat offering low exposure to predators should show (1) higher fat reserves, and (2) higher wing load, than wintering birds occupying less dense (open) habitat offering ...

NESTING SUCCESS AND BREEDING BIOLOGY OF CERULEAN WARBLERS IN MICHIGAN

Abstract The Cerulean Warbler (Dendroica cerulea) is a Nearctic-Neotropical migratory bird species that has declined significantly over the long-term. Poor reproductive success may be an important factor contributing to the observed decline, but reproductive output has been measured for very few breeding populations. From 2003 to 2005, I intensively monitored 22–23 breeding territories/year in each of two large forest habitats in southwestern Michigan: oak- (Quercus spp.) hickory (Carya spp.) (2003: Barry State Game Area) and black locust- (Robinia pseudoacacia) black cherry (Prunus serotina) (2004–2005: Fort Custer U.S. Army Michigan National Guard Reservation). I also gathered descriptive data on nonsong vocalizations and age of territorial males. I describe four distinctive call notes, by sex, including the social and environmental contexts in which they were used. Using two independent methods of aging, there was a strong preponderance of after-second-year males at both study sites. Only 9 (n = 7 nests), 12 (n = 14), and 30 (n = 25) fledglings were produced during the 2003, 2004, and 2005 breeding seasons, respectively. Nest heights were the highest recorded for this species (mean = 19–20 m). During the same period, male reproductive success was 0.30, 0.32, and 0.80 male fledglings/breeding male and 0.60, 0.63, and 1.58 fledglings/breeding pair. Productivity estimates, not thought to be self-sustaining, were even lower than those of a well-studied Cerulean Warbler population in southern Ontario. Thus, reproductive output was low in two geographic regions—representing three different forest types—in the northern portions of the Cerulean Warblers breeding range. The preponderance of after-second-year males at the Michigan study sites and in southern Ontario suggests a need for regional models of Cerulean Warbler population dynamics.

Spatial variation in winter survival of Song Sparrows Melospiza melodia

The resident population of Song Sparrows breeding on Mandarte Island, British Columbia fluctuated markedly between 1975 and 1989. A major population crash occurred during a week of unusually severe winter weather in early February 1989. Over 90% of Song Sparrows alive on the island in mid-January 1989 disappeared by 9-12 February 1989. The high mortality was accompanied by directional natural selection favoring female birds with longer wings. No selection was detected among males. We addressed the alternative hypothesis that Song Sparrows died of disease rather than poor weather. The population of Fox Sparrows Passerella iliaca wintering on Mandarte also declined sharply during the same period as Song Sparrows. The contemporaneous decline in the population size of these two species supports the conclusion that severe weather caused the population crashes. However, Fox Sparrows survived the poor weather better than Song Sparrows, possibly because of greater fat reserves in the former species. The effect of cold weather on population size of Song Sparrows was local. Sexspecific adult mortality was highest on Mandarte, lowest at a nearby mainland site provided with supplemental winter food, and intermediate on similar islands near Mandarte.

Isolation and characterization of bacteria from the feathers of wild Dark-eyed Juncos (Junco hyemalis)

ABSTRACT We dislodged microbes from samples of composites of ventral feathers from different birds of overwintering Dark-eyed Juncos (Junco hyemalis) after mist-net capture in south-central Kansas, USA. Bacterial loads were measured by standard plate counts and >300 isolates were purified by repetitive streak-plating on R2A medium (+ cycloheximide). Biochemical and physiological characterization included identification by 16S rRNA gene phylogeny. Nearly half of the isolates grew on keratin and 80% exhibited lipase activity, suggesting that these isolates can degrade feathers and thus may affect survival and reproduction. Individual bacterial loads from 8 juncos varied within a 3-fold range, 105–106 colony-forming units g−1 feather. At 97% DNA sequence identity (species-level), 63 operational taxonomic units were detected among 202 sequences; the Chao1 estimate was 123. The Shannon diversity index (H; 97% identity) was 3.75, Simpsons diversity index (1/D) was 16.1, and Goods coverage was 82.4. Gram-positive bacteria dominated the culture collection, balanced between low and high G+C clades. Bacillus spp. were abundant, including B. asahii, B. cereus, B. megaterium, and B. pumilus. Lysinibacillus, Paenibacillus, and Staphylococcus also were isolated. Remarkably, substantial numbers of Actinomycetes were isolated, including representatives of Clavibacter, Curtobacterium, Microbacterium, and Rathayibacter, genera recognized as being populated by xylem-filling crop plant pathogens. Apposed to these were feather isolates implicated as beneficial to host plants, Frigoribacterium and Kitasatospora, being antagonists to plant pathogens or acting as plant growth promoters. High G+C Gram-positive bacterial isolates included Blastococcus, Cellulomonas, Humicoccus, Nocardioides, Promicromonospora, and Rhodococcus. Proteobacteria dominated the Gram-negative bacteria, with Alphaproteobacteria most abundant, including the potential plant pathogens Agrobacterium and Sphingomonas, and the oligotrophs Aurantimonas, Brevundimonas, Methylobacterium, Rhizobium, and Rhodobacter. Gammaproteobacteria included Pantoea, Pseudomonas, and Stenotrophomonas. Ours is the first report of abundant helpful and harmful phyllosphere bacteria on wild bird feathers. The clear implication is that free-living migratory birds may carry bacteria throughout their geographic ranges and may transmit pathogens and beneficial bacteria to plants.

Use of Fecundity Measured Directly Throughout the Breeding Season to Test a Source–Sink Demographic Model

Populations of landbirds (bird species that occupy terrestrial habitats for most of their life cycle) are declining throughout North America (north of Mexico) and Europe, yet little is known about how demography is driving this trend. A recent model of 5 geographically separated populations of Cerulean Warblers (Dendroica cerulea) that was based on within-season sampling of nest survival and fledgling success shows that all populations are sinks (annual reproduction is consistently less than annual adult mortality). I tested this indirect model by directly measuring fecundity (number of female fledglings/female) during the breeding season for 2 years in a Cerulean Warbler population occupying a mature forest in southwestern Michigan (U.S.A.) I determined territories of male birds on the basis of male plumage characters and phases of the nesting cycle (2007) and on uniquely color-banded males (2008). I transferred locations of identified males to topographic maps. I counted all fledglings in territories from May to July each year. The model I tested may apply only to single-brooded species; therefore, I searched the literature to estimate the percentage of single-brooded species in North America. The breeding season of Cerulean Warblers was short- nearly all nests were initiated from mid-May to late June. Nest predation and brood parasitism were primary and rare causes of nest failure, respectively. Significantly fewer Cerulean Warblers fledged from parasitized than from nonparasitized nests. Fledgling survival required to maintain the population size was well above previously published values for Neotropical migrants. Single-brooded species comprise 62% of North American breeding bird species for which the number of broods per year is known; I believe my results may apply to these species. The consistency between identification of populations as sources or sinks on the basis of either model estimates or direct measurements suggests that a demographic model relying on within-season sampling of fecundity is adequate to determine population status of single-brooded avian populations. In addition, on the basis of results of previous studies, annual adult survival rate of the Cerulean Warbler is typical of parulid warblers that are not declining. Thus, low fecundity, here determined with different quantitative methods, can drive status of landbird species with high-observed survival.