Within-Site Variation in Feather Stable Hydrogen Isotope (δ2Hf) Values of Boreal Songbirds: Implications for Assignment to Molt Origin

Abstract

Understanding bird migration and dispersal is important to inform full life-cycle conservation planning. Stable hydrogen isotope ratios from feathers (δ2Hf) can be linked to amount-weighted long-term, growing season precipitation δ2H (δ2Hp) surfaces to create δ2Hf isoscapes for assignment to molt origin. However, transfer functions linking δ2Hp with δ2Hf are influenced by physiological and environmental processes. A better understanding of the causes and consequences of variation in δ2Hf values among individuals and species will improve the predictive ability of geographic assignment tests. We tested for effects of species, land cover, forage substrate, nest substrate, diet composition, body mass, sex, and phylogenetic relatedness on δ2Hf from individuals at least two years old of 21 songbird species captured during the same breeding season at a site in northeastern Alberta, Canada. For four species, we also tested for a year × species interaction effect on δ2Hf. A model including species as single predictor received the most support (AIC weight = 0.74) in explaining variation in δ2Hf. A species-specific variance parameter was part of all best-ranked models, suggesting variation in δ2Hf was not consistent among species. The second best-ranked model included a forage substrate × diet interaction term (AIC weight = 0.16). There was a significant year × species interaction effect on δ2Hf suggesting that interspecific differences in δ2Hf can differ among years. Our results suggest that within- and among-year interspecific variation in δ2Hf is the most important source of variance typically not being explicitly quantified in geographic assignment tests using non-specific transfer functions to convert δ2Hp into δ2Hf. However, this source of variation is consistent with the range of variation from the transfer functions most commonly being propagated in assignment tests of geographic origins for passerines breeding in North America.

Prealternate molt-migration in Rusty Blackbirds and its implications for stopover biology

“…However, the role of plumage in sexual selection or intrasexual communication in this species is not known, and the inconsistent sex effect in our study suggests that it may be more complicated than female-choice sexual selection. It should also be noted that the geographic location of molt could be stochastic for individuals of the same population from year to year (Pyle et al 2009, Nordell et al 2016, so molt may not necessarily always occur at this stopover site.…”

“…at or near breeding and overwintering sites). However, there is increasing evidence that molt can also occur during migratory stopover, or after movement to specific ''molting grounds, '' in a variety of taxa (''moltmigration''; e.g., Salomonsen 1968, Leu and Thompson 2002, Reed et al 2003, Pyle et al 2009, Quinlan and Green 2011, Lourenço and Piersma 2015, Nordell et al 2016, Pillar et al 2016. For instance, several passerine species in western North America, such as the Bullock's Oriole (Icterus bullockii; Pillar et al 2016) and Black-headed Grosbeak (Pheucticus melanocephalus; Siegel et al 2016), pause their southward migration to wintering areas in the monsoon region of northwestern Mexico to complete prebasic molt.…”

Variation in Stable Hydrogen Isotope Values in a Wetland-Associated Songbird

“…An interaction between year and other sources of variation has been found in some studies, with species-specific (Nordell et al 2016) and age (Gow et al 2012;Haché et al 2012) effects being different among years. Most studies have not investigated differences between sexes, but those that did have been inconclusive (Langin et al 2007) or found no effect (Nordell et al 2016). A further assumption is that δ 2 H f values should not vary between feather types in the same individual, but small differences have been found between primary and rectrix feathers in a songbird (Haché et al 2012) and large differences were seen between various primary feathers in a raptor (Meehan et al 2003).…”

“…Specifically, δ 2 H f can vary among individuals of different ages (Meehan et al 2003;Langin et al 2007;Gow et al 2012;Haché et al 2012), and between years at the same site (Haché et al 2012;Van Dijk et al 2014;Tonra et al 2015). An interaction between year and other sources of variation has been found in some studies, with species-specific (Nordell et al 2016) and age (Gow et al 2012;Haché et al 2012) effects being different among years. Most studies have not investigated differences between sexes, but those that did have been inconclusive (Langin et al 2007) or found no effect (Nordell et al 2016).…”

“…Stable isotope ratios are often used for provenance studies because they exhibit predictable geographic variation, can be measured from a single capture, and are relatively inexpensive to measure (Hobson 1999). Feathers assimilate the isotope ratio of the local environment and are often retained for an entire migration cycle (Hobson and Wassenaar 2008); additionally, the location of feather growth is known for many species of migratory birds (Pyle 1997; but see Leu and Thompson 2002;Nordell et al 2016). Stable isotopes, especially stable hydrogen, have been widely used to assign migratory birds captured on the nonbreeding grounds to their location of feather growth on the breeding grounds (Chamberlain et al 1997;Hobson and Wassenaar 1997;Rushing et al 2014;Gonzalez-Prieto et al 2017) and to quantify natal dispersal movements (Studds et al 2012;Haché et al 2014).…”