Stefan Woltmann

A MULTI‐SCALE EXAMINATION OF STOPOVER HABITAT USE BY BIRDS

Most of our understanding of habitat use by migrating land birds comes from studies conducted at single, small spatial scales, which may overemphasize the importance of intrinsic habitat factors, such as food availability, in shaping migrant distributions. We believe that a multi-scale approach is essential to assess the influence of factors that control en route habitat use. We determined the relative importance of eight variables, each operating at a habitat-patch, landscape, or regional spatial scale, in explaining the differential use of hardwood forests by Nearctic-Neotropical land birds during migration. We estimated bird densities through transect surveys at sites near the Mississippi coast during spring and autumn migration within landscapes with variable amounts of hardwood forest cover. At a regional scale, migrant density increased with proximity to the coast, which was of moderate importance in explaining bird densities, probably due to constraints imposed on migrants when negotiating the Gulf of Mexico. The amount of hardwood forest cover at a landscape scale was positively correlated with arthropod abundance and had the greatest importance in explaining densities of all migrants, as a group, during spring, and of insectivorous migrants during autumn. Among landscape scales ranging from 500 m to 10 km radius, the densities of migrants were, on average, most strongly and positively related to the amount of hardwood forest cover within a 5 km radius. We suggest that hardwood forest cover at this scale may be an indicator of habitat quality that migrants use as a cue when landing at the end of a migratory flight. At the patch scale, direct measures of arthropod abundance and plant community composition were also important in explaining migrant densities, whereas habitat structure was of little importance. The relative amount of fleshy-fruited trees was positively related and was the most important variable explaining frugivorous migrant density during autumn. Although constraints extrinsic to habitat had a moderate role in explaining migrant distributions, our results are consistent with the view that food availability is the ultimate factor shaping the distributions of birds during stopover.

HIGH APPARENT ANNUAL SURVIVAL AND STABLE TERRITORY DYNAMICS OF CHESTNUT-BACKED ANTBIRD (MYRMECIZA EXSUL) IN A LARGE COSTA RICAN RAIN FOREST PRESERVE

Abstract Antbirds (Thamnophilidae) are a diverse component of neotropical forest avifaunas, and are particularly vulnerable to population declines and extirpations in fragmented landscapes. We lack estimates of apparent survival and dispersal for the majority of species, despite their value in effectively managing populations of understory birds. We studied a population of Chestnut-backed Antbird (Myrmeciza exsul) from 2004 to 2009 in a large rain forest preserve in northern Costa Rica to generate estimates of apparent annual survival (ϕ), and breeding dispersal (i.e., movement from one breeding territory to another) in continuous forest. Estimates of ϕ (± SE) of adults based on weighted model averages were high (males: 0.794 ± 0.037; females: 0.798 ± 0.050) compared to independent juveniles (males: 0.629 ± 0.159; females: 0.629 ± 0.168). Detection (recapture/reobservation) probabilities (p) were higher for males (adults: 0.916 ± 0.034; juveniles: 0.915 ± 0.049) than for females (adults: 0.544 ± 0.104; juveniles: 0.540 ± 0.115). Overall annual turnover (disappearing from the study area + territory switching) was comparable to other antbirds (∼32%). Territory switching was rare, and generally limited to short movements to adjacent or nearby territories (mean distance moved  =  372 m, range  =  145–840 m, n  =  9). Our results suggest Chestnut-backed Antbirds: (1) have relatively high adult annual survival, and (2) have limited breeding dispersal, even in a large, forested study area.

Effects of Weather on Autumn Hawk Movements at Fort Morgan, Alabama

Abstract Migrating hawks were observed at Fort Morgan, Alabama during autumn 1995–1998. The three most abundant migrants were Sharp-shinned Hawk (Accipiter striatus), Broad-winged Hawk (Buteo platypterus) and American Kestrel (Falco sparverius). The three species accounted for 84% of all observations. Most individuals (> 80%) were seen heading west. Compared with days with east and south winds, days with north winds were associated with significantly higher passage rates for all species, and passage rates of Broad-winged Hawks were significantly higher on days when both a cold front and north winds occurred than on days with north winds only. Hawks flew higher on east winds than on north and south winds, suggesting that hawks fly lower in winds that may blow them out over the Gulf of Mexico.

Population Genetics of Seaside Sparrow (Ammodramus maritimus) Subspecies along the Gulf of Mexico

Seaside Sparrows (Ammodramus maritimus) along the Gulf of Mexico are currently recognized as four subspecies, including taxa in Florida (A. m. juncicola and A. m. peninsulae) and southern Texas (Ammodramus m. sennetti), plus a widespread taxon between them (A. m. fisheri). We examined population genetic structure of this “Gulf Coast” clade using microsatellite and mtDNA data. Results of Bayesian analyses (Structure, GeneLand) of microsatellite data from nine locations do not entirely align with current subspecific taxonomy. Ammodramus m. sennetti from southern Texas is significantly differentiated from all other populations, but we found evidence of an admixture zone with A. m. fisheri near Corpus Christi. The two subspecies along the northern Gulf Coast of Florida are significantly differentiated from both A. m. sennetti and A. m. fisheri, but are not distinct from each other. We found a weak signal of isolation by distance within A. m. fisheri, indicating this population is not entirely panmictic throughout its range. Although continued conservation concern is warranted for all populations along the Gulf Coast, A. m. fisheri appears to be more secure than the far smaller populations in south Texas and the northern Florida Gulf Coast. In particular, the most genetically distinct populations, those in Texas south of Corpus Christi, occupy unique habitats within a very small geographic range.

Hybridization of White, Yellow, and Striped Bass in the Toledo Bend Reservoir

Abstract Long-term stocking of non-native Morone saxatilis (Striped Bass) in Toledo Bend Reservoir may have had adverse effects on the integrity of the native Morone chrysops (White Bass) and M. mississipiensis (Yellow Bass) genome through introgression. We examined microsatellite genotypes for evidence of hybridization in a sample of Striped. White, and Yellow Bass but found only four potential hybrids. Despite the introduction of millions of Striped Bass over four decades, there is no evidence for either a hybrid swarm or substantial introgression. Low numbers of hybrids may be the result of poor hybrid survival, little reproduction between species, or a combination of both.