A group of researchers, led by Andrew Sharp of Utah State University, has advanced the understanding of the migration behaviour of one of America's most colourful songbirds by fitting geolocators to hundreds of Painted Buntings from the two regions it inhabits.
The team sought to understand the degree to which individuals from the species' two distinct populations in the USA remain together as they migrate over the course of the annual cycle. This is known as migratory connectivity. It is considered strong when birds that are close together at one point in the year remain so during other phases in the annual cycle, but weak when they use different areas or mix with individuals from other areas. Weak migratory connectivity is common in migratory passerines.
Migratory connectivity has implications for conservation, so it is important to understand the connection between breeding populations and areas used in wintering areas and along migration routes. Population trends witnessed on the breeding grounds may be connected to factors on the wintering grounds and vice versa, and conservation measures could hinge on the degree to which individuals from different parts of the range stay together or mix with other populations during the annual cycle.
Painted Buntings exist in two distinct and widely separated populations in the USA, with many birds from the interior population undergoing a moult migration (Steve Bell).
However, little information on migratory connectivity exists for most species, due to low bird ringing recovery rates, which do not offer sufficient data, and the cost and practical challenges presented by tracking studies.
The US interior population of Painted Bunting breeds from northern Mexico to Mississippi and north to Kansas, and is separated from the much (60 times) smaller eastern population by a 500-km gap. The eastern birds occupy an area 25 times smaller than the interior population, breeding close to the Atlantic coastline from northern Florida to North Carolina.
Painted Bunting has declined across the board since 1970, but it is the eastern population that has seen the steepest fall in numbers.
There is an interesting difference in the annual cycle of birds from the two populations. At least a large proportion, possibly all, of the interior birds make a prolonged stop between the breeding areas and the wintering grounds in Central America to undergo the annual complete moult, whereas eastern Painted Buntings moult before undertaking any post-breeding movement towards their wintering areas in southern Florida and the Caribbean.
Researchers used light-level geolocators to investigate the relationship between moulting behaviour and migratory connectivity in Painted Bunting at different spatial scales and times of year. Their study is the first to have tracked birds from both the interior and eastern populations of the species, investigating connectivity across the bird's entire range, between breeding and moulting grounds in interior birds, and between breeding and wintering grounds within each population.
Between 2017 and 2019, a total of 295 Painted Buntings were fitted with geolocators between 11 sites across the species' range. These small devices do not transmit data remotely, so researchers had to recapture as many birds as possible to gather data. Recovered geolocators with viable data amounted to 28% of those deployed on eastern birds and 16% of the devices fitted to birds from the interior population.
The data revealed sharp division between the eastern and interior populations, as expected, with no evidence of birds from the two regions mixing, meaning strong migratory connectivity within each population.
Although the data confirmed the assumption that eastern and interior populations do not mix in the non-breeding period, it was revealed that birds from different breeding areas within the two populations mix rather freely within the respective wintering ranges. In other words, migratory connectivity was strong on a range-wide scale but breeding-to-winter migratory connectivity was weak within both of the populations, although there was evidence that birds breeding in North Carolina winter further south than birds from elsewhere in the eastern range.
The moult migration connectivity of the interior population was a focus of the research, and the geolocator data showed that birds from different areas stay together to moult in segregated groups after their initial post-breeding movement. All Painted Buntings breeding in Oklahoma migrated to north-western Mexico to moult, with tracked birds from Arkansas seeming to moult on their breeding grounds as eastern birds do.
It is thought that Okhaloma birds may be driven to moult away from the breeding grounds by the increasingly arid conditions in the region by mid-summer, whereas birds further east benefit from a productive landscape even towards the autumn.
The researchers said their results highlight the need to consider how migratory connectivity changes throughout the annual cycle, and how a small region, like north-west Mexico in the case of Painted Bunting, could have huge conservation potential for birds that breed over a wide area.
Sharp, A J, Contina, A, & 8 others. 2023. The strength of migratory connnectivity in Painted Buntings is spatial scale dependent and shaped by molting behavior. Avian Behavior, Ecology and Evolution. DOI: https://doi.org/10.5751/JFO-00233-940107