Michael D. Cadman

A Continent-Wide Migratory Divide in North American Breeding Barn Swallows (Hirundo rustica).

Populations of most North American aerial insectivores have undergone steep population declines over the past 40 years but the relative importance of factors operating on breeding, wintering, or stopover sites remains unknown. We used archival light-level geolocators to track the phenology, movements and winter locations of barn swallows (Hirdundo rustica; n = 27) from populations across North America to determine their migratory connectivity. We identified an east-west continental migratory divide for barn swallows with birds from western regions (Washington State, USA (n = 8) and Saskatchewan, Canada (n = 5)) traveling shorter distances to wintering areas ranging from Oregon to northern Colombia than eastern populations (Ontario (n = 3) and New Brunswick (n = 10), Canada) which wintered in South America south of the Amazon basin. A single swallow from a stable population in Alabama shared a similar migration route to eastern barn swallows but wintered farther north in northeast Brazil indicating a potential leap frog pattern migratory among eastern birds. Six of 9 (67%) birds from the two eastern populations and Alabama underwent a loop migration west of fall migration routes including around the Gulf of Mexico travelling a mean of 2,224 km and 722 km longer on spring migration, respectively. Longer migration distances, including the requirement to cross the Caribbean Sea and Gulf of Mexico and subsequent shorter sedentary wintering periods, may exacerbate declines for populations breeding in northeastern North America.

EXTENT OF DOUBLE-BROODING AND SEASONAL MOVEMENT OF NESTING FEMALES IN A NORTHERN POPULATION OF WOOD THRUSHES

Abstract Accurate estimates of the amount of double-brooding within a population are an important parameter for assessing the population sustainability of forest birds. In 1998 and 1999, we color-banded adult female Wood Thrushes (Hylocichla mustelina) at 70 nests early in the breeding season to assess the frequency of double-brooding in a northern population. We found that double-brooding was a common breeding strategy among Wood Thrushes in southern Ontario. Forty-seven females fledged first broods and 74% (n = 35) of them initiated egg-laying in a subsequent nest. It is possible that at least 87% (n = 41) of the females were double-brooded, based on the evidence of 6 later nests built within 50 m of successful first nests that were depredated or fledged young before their owners could be identified. Other second-brood nests were probably missed because they were overlooked and because some females moved considerable distances (100–400 m) between nestings. Most birds that failed in their early nesting attempt were not found again on the site, precluding verification of their renesting efforts.

Influence of climate on annual survival of Barn Swallows (Hirundo rustica) breeding in North America

ABSTRACT Population dynamics of migratory birds are influenced by both local weather and larger-scale patterns in climate that can operate at various stages of their annual cycle. We investigated correlations between (1) annual climatic indices and weather during the breeding season and (2) the annual survival of Barn Swallows (Hirundo rustica) breeding at 2 sites in North America. Mark–recapture data collected during a 10-yr period for each of the 2 colonies in eastern and western North America were analyzed to model annual survival probabilities. Annual survival rates of Barn Swallows breeding in Seattle, Washington, USA, were higher in years preceded by El Niño Southern Oscillation (ENSO) winters and higher in years with more positive North Atlantic Oscillation (NAO) values. ENSO was expected to primarily influence wintering conditions through rainfall amount, and NAO was expected to influence climate on the breeding grounds; thus, climatic conditions on both breeding and wintering grounds likely affected the survival of these Seattle-breeding birds. By contrast, annual survival of swallows breeding in southern Ontario, Canada, remained constant over time and were not affected by any of the climatic parameters studied, which suggests that NAO did not have a strong effect on climatic conditions there and/or that these birds winter in regions where ENSO is not strongly correlated with local weather conditions. Alternatively, there may be less geographic variation in wintering-ground locations for Barn Swallows breeding in Seattle, resulting in stronger ENSO effects on survival for the Seattle population. Our results demonstrate how correlations between climate patterns on wintering grounds and annual survival can provide information on migratory connectivity at continental scales and underline the importance of local weather conditions throughout the annual cycle on survivorship and population dynamics of aerial insectivorous birds.

Triple Brooding by Southern Ontario Wood Thrushes

Abstract Wood Thrushes (Hylocichla mustelina) typically are double brooded, and triple brooding is rare, even in the southern portions of their breeding range. We discovered one instance of triple brooding by a pair of Wood Thrushes in southern Ontario near the northern edge of their continental breeding range. Each of the three nesting attempts successfully fledged young. Females may spend only a short time (less than a week) with just-fledged broods before initiating another nest; in this circumstance, the survival of young birds from the earlier brood depends on the attentiveness of males. The rearing of three broods is an uncommon event among Wood Thrushes in our study; from 1998–2000, only one of 73 color-banded females (1.3%) with early season nesting success has been triple brooded.