Jeffrey D. Brawn

Source-Sink Population Dynamics may Complicate the Interpretation of Long- Term Census Data

We examined the reproductive success and long—term population dynamics of Neotropical migrant birds in the fragmented landscapes of Illinois. Our primary objective was to assess whether annual variation in abundances and persistence of populations within woodlots was related to variation in reproductive success. Nesting success of migrants was so low in the woodlots where we sampled that it is unlikely that these populations are self—sustaining. Moreover, populations frequently disappear from and recolonize these woodlots. These data suggest that the source—sink metaphor for population dynamics within spatially structured populations is applicable to migratory birds in Illinois. The scale of this dynamic is unknown, but may be regional (i.e., midwestern U.S.). An important implication of this result is that census data must be interpreted carefully. For one woodlot, we detected no systematic trend in the overall abundances of migratory birds over much of the 20th century. Yet the viability of populations within that woodlot likely decreased dramatically over that time owing to increasing rates of nest predation and brood parasitism. Lack of data on dispersal is a major gap in understanding the population dynamics of Neotropical migrants and prescribing effective conservation measures.

Culex pipiens (Diptera: Culicidae): A Bridge Vector of West Nile Virus to Humans

Abstract Host-feeding patterns of Culex pipiens L. collected in southwest suburban Chicago in 2005 were studied using polymerase chain reaction (PCR) and DNA sequencing techniques. Culex spp. mosquitoes, most identified to Cx. pipiens and the remainder to Cx. restuans by PCR, had fed on 18 avian species, most commonly American robin (Turdus migratorious), house sparrow (Passer domesticus), and mourning dove (Zenaida macroura). Additional blood meals were derived from four mammal species, primarily humans and raccoons (Procyon lotor). During a West Nile virus (WNV) epidemic in 2005, West Nile virus (WNV) RNA was detected in heads and thoraces of five Cx. pipiens (n = 335, 1.5%) using quantitative PCR. The hosts of these virus-infected, blood-fed mosquitoes included two American robins, one house sparrow, and one human. This is the first report of a WNV-infected Cx. pipiens mosquito collected during an epidemic of WNV that was found to have bitten a human. These results fulfill a criterion for incrimination of Cx. pipiens as a bridge vector.

Corticosterone, testosterone and life-history strategies of birds

Steroid hormones have similar functions across vertebrates, but circulating concentrations can vary dramatically among species. We examined the hypothesis that variation in titres of corticosterone (Cort) and testosterone (T) is related to life-history traits of avian species. We predicted that Cort would reach higher levels under stress in species with higher annual adult survival rates since Cort is thought to promote physiological and behavioural responses that reduce risk to the individual. Conversely, we predicted that peak T during the breeding season would be higher in short-lived species with high mating effort as this hormone is known to promote male fecundity traits. We quantified circulating hormone concentrations and key life-history traits (annual adult survival rate, breeding season length, body mass) in males of free-living bird species during the breeding season at a temperate site (northern USA) and a tropical site (central Panama). We analysed our original data by themselves, and also combined with published data on passerine birds to enhance sample size. In both approaches, variation in baseline Cort (Cort0) among species was inversely related to breeding season length and body mass. Stress-induced corticosterone (MaxCort) also varied inversely with body mass and, as predicted, also varied positively with annual adult survival rates. Furthermore, species from drier and colder environments exhibited lower MaxCort than mesic and tropical species; T was lowest in species from tropical environments. These findings suggest that Cort0, MaxCort and T modulate key vertebrate life-history responses to the environment, with Cort0 supporting energetically demanding processes, MaxCort promoting survival and T being related to mating success.

Birds defend trees from herbivores in a Neotropical forest canopy

Most forest birds include arthropods in their diet, sometimes specializing on arthropods that consume plant foliage. Experimental tests of whether bird predation on arthropods can reduce plant damage, however, are few and restricted to relatively low-diversity systems. Here, we describe an experimental test in a diverse tropical forest of whether birds indirectly defend foliage from arthropod herbivores. We also compare how the indirect effects of bird predation vary with different levels of foliage productivity in the canopy vs. the understory. For three Neotropical tree species, we observed that birds decreased local arthropod densities on canopy branches and reduced consequent damage to leaves. In contrast, we observed no evidence of bird–arthropod limitation on conspecific saplings in the less productive understory of the same forest. Our results support theory that predicts trophic cascades where productivity is high and suggest that birds play an important role in Neotropical communities by means of their indirect defense of some canopy tree species.

Interspecific associations between circulating antioxidant levels and life-history variation in birds

Antioxidants play an important role in protecting tissues against aging‐associated oxidative damage and are thus prime candidates for relating physiological mechanisms to variation in life histories. We measured total antioxidant capacity, antioxidant response to stress, and levels of uric acid, vitamin E, and four carotenoids in 95 avian species, mostly passerines from Michigan or Panama. We compared antioxidant measures to seven variables related to life histories (clutch size, survival rate, incubation period, nestling period, basal metabolic rate, body mass, and whether the species lived in a tropical or temperate climate). Life‐history‐related traits varied over at least three statistically independent axes. Higher antioxidant levels were generally characteristic of more rapid development, lower survival rate, smaller body size, larger clutch size, and higher mass‐adjusted metabolic rate, but the relationships of particular antioxidants with individual life‐history traits showed considerable complexity. Antioxidant–life history associations differed between tropical and temperate species and varied with respect to taxonomic sampling. Vitamin E showed few relationships with life‐history traits. Overall, our results partly support the hypothesis that antioxidant levels evolve to mirror free radical production. Clearly, however, the complex patterns of physiological diversification observed here result from the interplay of many factors, likely including not just investment in somatic maintenance but also phylogenetic constraint, diet, and other aspects of ecology.

Rapid amplification of West Nile virus: the role of hatch-year birds.

Epizootic transmission of West Nile virus (WNV) often intensifies rapidly leading to increasing risk of human infection, but the processes underlying amplification remain poorly understood. We quantified epizootic WNV transmission in communities of mosquitoes and birds in the Chicago, Illinois (USA) region during 2005 and 2006. Using quantitative polymerase chain reaction (PCR) methods, we detected WNV in 227 of 1195 mosquito pools (19%) in 2005 and 205 of 1685 (12%) in 2006; nearly all were Culex pipiens. In both years, mosquito infection rates increased rapidly in the second half of July to a peak of 59/1000 mosquitoes in 2005 and 33/1000 in 2006, and then declined slowly. Viral RNA was detected in 11 of 998 bird sera (1.1%) in 2005 and 3 of 1285 bird sera (<1%) in 2006; 11 of the 14 virus-positive birds were hatch-year birds. Of 540 hatch-year birds, 100 (18.5%) were seropositive in 2005, but only 2.8% (14/493) tested seropositive in 2006 for WNV antibodies using inhibition enzyme-linked immunosorbent assay (ELISA). We observed significant time series cross-correlations between mosquito infection rate and proportion of virus-positive birds, proportion of hatch-year birds captured in mist nets (significant in 2006 only), seroprevalence of hatch-year birds, and number of human cases in both seasons. These associations, coupled with the predominance of WNV infection and seropositivity in hatch-year birds, indicate a key role for hatch-year birds in the amplification of epizootic transmission of WNV, and in increasing human infection risk by facilitating local viral amplification.

Fine-Scale Variation in Vector Host Use and Force of Infection Drive Localized Patterns of West Nile Virus Transmission

The influence of host diversity on multi-host pathogen transmission and persistence can be confounded by the large number of species and biological interactions that can characterize many transmission systems. For vector-borne pathogens, the composition of host communities has been hypothesized to affect transmission; however, the specific characteristics of host communities that affect transmission remain largely unknown. We tested the hypothesis that vector host use and force of infection (i.e., the summed number of infectious mosquitoes resulting from feeding upon each vertebrate host within a community of hosts), and not simply host diversity or richness, determine local infection rates of West Nile virus (WNV) in mosquito vectors. In suburban Chicago, Illinois, USA, we estimated community force of infection for West Nile virus using data on Culex pipiens mosquito host selection and WNV vertebrate reservoir competence for each host species in multiple residential and semi-natural study sites. We found host community force of infection interacted with avian diversity to influence WNV infection in Culex mosquitoes across the study area. Two avian species, the American robin (Turdus migratorius) and the house sparrow (Passer domesticus), produced 95.8% of the infectious Cx. pipiens mosquitoes and showed a significant positive association with WNV infection in Culex spp. mosquitoes. Therefore, indices of community structure, such as species diversity or richness, may not be reliable indicators of transmission risk at fine spatial scales in vector-borne disease systems. Rather, robust assessment of local transmission risk should incorporate heterogeneity in vector host feeding and variation in vertebrate reservoir competence at the spatial scale of vector-host interaction.

Demography of Birds in a Neotropical Forest: Effects of Allometry, Taxonomy, and Ecology

Comparative demographic studies of terrestrial vertebrates have included few samples of species from tropical forests. We analyzed 9 yr of mark-recapture data and estimated demographic parameters for 25 species of birds inhabiting lowland forests in central Panama. These species were all songbirds (Order Passeriformes) ranging in mass from 7 to 57 g. Using Jolly-Seber stochastic models for open populations, we estimated annual survival rate, population size, and recruitment between sampling periods for each species. We then explored relationships between these parameters and attributes such as body size, phylogenetic affiliation, foraging guild, and social behavior. Larger birds had comparatively long life-spans and low recruitment, but body size was not associated with population size. After adjusting for effects of body size, we found no association between phylogenetic affiliation and any demographic trait. Ecological attributes, especially foraging guild, were more clearly associated with interspecific variation in all demographic traits. Ant-followers had comparatively long life-spans, but species that participate in flocks did not live longer than solitary species. The allometric associations we observed were con- sistent with those demonstrated in other studies of vertebrates; thus, these relationships appear to be robust. Our finding that ecological factors were more influential than phylo- genetic affiliation contrasts with comparative studies of temperate-zone birds and suggests that the relative importance of environmental vs. historical factors varies geographically.

Avian host community structure and prevalence of West Nile virus in Chicago, Illinois

Vertebrate host diversity has been postulated to mediate prevalence of zoonotic, vector-borne diseases, such that as diversity increases, transmission dampens. This “dilution effect” is thought to be caused by distribution of infective bites to incompetent reservoir hosts. We quantified avian species richness, avian seroprevalence for antibodies to West Nile virus (WNV), and infection of WNV in Culex mosquitoes, in the Chicago metropolitan area, Illinois, USA, a region of historically high WNV activity. Results indicated high overall avian seroprevalence and variation in seroprevalence across host species; however, there was no negative correlation between avian richness and Culex infection rate or between richness and infection status in individual birds. Bird species with high seroprevalence, especially northern cardinals and mourning doves, may be important sentinels for WNV in Chicago, since they were common and widespread among all study sites. Overall, our results suggest no net effect of increasing species richness to West Nile virus transmission in Chicago. Other intrinsic and extrinsic factors, such as variation in mosquito host preference, reservoir host competence, temperature, and precipitation, may be more important than host diversity for driving interannual variation in WNV transmission. These results from a fine-scale study call into question the generality of a dilution effect for WNV at coarser spatial scales.

The direct and indirect effects of insectivory by birds in two contrasting Neotropical forests

A goal among community ecologists is to predict when and where trophic cascades occur. For example, several studies have shown that forest birds can limit arthropod abundances on trees, but indirect effects of bird predation (i.e. decreased arthropod damage to trees) are not always observed and their context is not well understood. Because productivity is one factor that is expected to influence trophic cascades, we compared the extent to which birds indirectly limit herbivore damage to trees in two lowland Neotropical forests that differed in seasonality of leaf production and rainfall. We compared the effects of bird predation on local arthropod densities and on damage to foliage through a controlled experiment using bird exclosures in the canopy and understory of two forests. We found that birds decreased local arthropod densities and leaf damage in the canopy of the drier site during periods of high leaf production, but not in the wetter forest where leaf production was low and sporadic throughout the year. Birds had no effect on arthropod abundances and leaf damage in the understory where leaf production and turnover rates were low. In support of these experimental interpretations, although we observed that arthropod densities were similar at the two sites, bird densities and the rate at which birds captured arthropods were greater at the drier, seasonally productive site. The influence of top-down predation by birds in limiting herbivorous insects appears to be conditional and most important when the production and turnover of leaves are comparatively high.A goal among community ecologists is to predict when and where trophic cascades occur. For example, several studies have shown that forest birds can limit arthropod abundances on trees, but indirect effects of bird predation (i.e. decreased arthropod damage to trees) are not always observed and their context is not well understood. Because productivity is one factor that is expected to influence trophic cascades, we compared the extent to which birds indirectly limit herbivore damage to trees in two lowland Neotropical forests that differed in seasonality of leaf production and rainfall. We compared the effects of bird predation on local arthropod densities and on damage to foliage through a controlled experiment using bird exclosures in the canopy and understory of two forests. We found that birds decreased local arthropod densities and leaf damage in the canopy of the drier site during periods of high leaf production, but not in the wetter forest where leaf production was low and sporadic throughout the year. Birds had no effect on arthropod abundances and leaf damage in the understory where leaf production and turnover rates were low. In support of these experimental interpretations, although we observed that arthropod densities were similar at the two sites, bird densities and the rate at which birds captured arthropods were greater at the drier, seasonally productive site. The influence of top-down predation by birds in limiting herbivorous insects appears to be conditional and most important when the production and turnover of leaves are comparatively high.