Rare breeding behaviour in Californian bird

Most migratory bird species overwinter in one region, then spend the summer in an area rich in resources for raising young – showing fixed and tightly controlled patterns of movement ecology and breeding biology. 

Previously, only two bird species were known to buck this trend. Known as itinerant breeders, these species beat the notion that the migration/breeding cycle is inflexible by nesting in one area, migrating, and then nesting again in another region.

Now, new research published in The Auk: Ornithological Advances adds another bird, the phainopepla, to this exclusive club.

Named after the Greek words for shining robe, the phainopepla (Phainopepla nitens) is a type of silky flycatcher found in two very different habitats in southwestern US: desert areas and woodland habitats. 

The phainopeplas in the desert breed in nest territories and subsist largely on desert mistletoe (Phoradendron californicum) berries. In woodland areas, the species nest in a loose colony and have a wider diet that includes insects.

These little birds are accomplished mimics, and their calls have raised suspicions of itinerant breeding. In woodland areas, they’ve been seen uttering the calls of desert birds, while in the desert they appeared to have incorporated woodland bird calls into their repertoire. 

So, to find out whether the phainopepla were breeding in two locations, rather than being two separate populations of the same species, lead author Daniel Baldassarre from Princeton University and colleagues used a combination of GPS tracking and population genomics.

Throughout March and April 2017, Baldassarre captured and fitted 24 adult phainopeplas breeding in the Mojave Desert with tiny GPS tags, which have only become small enough to use on songbirds in recent years.

The team then had to wait until the following autumn.

They were rewarded with the return of five phainopeplas – and the GPS data confirmed the birds had in fact migrated from desert to woodland areas as suspected.

“Seeing the GPS tracks for the first time was amazing, but the biggest thrill for me was re-sighting the first tagged bird that returned to the capture site,” says Baldassarre. 

“We were a bit unsure how likely they were to come back to the same spot, so to see that a tagged bird had returned was an exhilarating moment. When he hit the net it was like, okay, we’re in business here.”

Genomic analysis of blood samples taken from 96 individuals revealed no genetic differentiation among the desert and woodland populations, indicating significant movement and gene flow across the phainopeplas’ range.

The data from the GPS trackers not only enabled the researchers to confirm that phainopepla individuals migrate between and breed in desert habitats, but also to quantify the environmental conditions experienced during the phases of their annual cycle.

These included solar radiation, precipitation, wind and forest cover, which varied markedly between the desert and woodland areas where they breed, and also between their stopover sites during the spring and fall migration period.

This is quite unlike other itinerant breeders; the tri-coloured blackbird (Agelaius tricolor) and red-billed quelea (Quelea quelea) migrate and breed between relatively homogenous environments and ecologies.

“For the phainopepla,” the researchers write, this movement between habitats “entails a shift not only in climatic conditions, but in diet, habitat, nesting substrate, predator community, and social system as well, indicative of an unusually high degree of behavioural and physiological plasticity.”

Looking at the bigger picture, Baldassarre suggests that phainopeplas may thus be able to deal with climate change better than other birds.

“They’re highly mobile,” he says, “and they can deal with different physical and social environments. It makes this interesting beyond just being a weird bird behaviour.”