Do North American landbirds molt on their breeding territories before migrating or do they have discrete locations where they go to molt? The answer is surprising a lot of people

by GrrlScientist for Forbes | @GrrlScientist

This adult male northern cardinal (Cardinalis cardinalis) has a less-than-spectacular appearance because he is molting. This study suggests that northern cardinals, which are resident, still move to molt โ€” although only by a short distance.
(Credit: Keith / Creative Commons Attribution 2.0 Generic license.)

Summer is a busy time for birds. The adults must help their new brood(s) of youngsters learn valuable life skills so they survive the coming winter; many species change their diet from insects to other foods, such as seeds (which means they must change the architecture and function of their gut accordingly), and birds must molt some or all of their feathers before they can successfully migrate to their wintering grounds.

Molt is an energetically demanding process. Additionally, because molt compromises flight abilities, this also is a dangerous time for birds, so they tend to โ€œlay lowโ€, making it difficult for human observers to keep track of them during this life stage.

โ€œBirds become very retiring at this time, as if on vacation, to recover from the breeding season,โ€ said the lead author of a recent study, ornithologist Peter Pyle, a staff biologist at The Institute for Bird Populations (IBP), in a press release.

โ€œMolt has been under-studied by ornithologists,โ€ Mr. Pyle said. โ€œWe still have no clue where most North American landbirds undertake their post-breeding (prebasic) molt.โ€

Molt-migration is well known for some species like waterfowl, and has also been documented in a few landbird species. For example, we know that some western songbirds migrate to southeastern Arizona in late summer to take advantage of lush green growth after monsoons whilst they molt. But according to a recent study โ€” and contrary to what many people believe โ€” many North American landbirds donโ€™t molt on their breeding territories. Instead, the birds either disperse or migrate to a discrete molting location before continuing their migratory journeys to their wintering grounds. Since we didnโ€™t even know of the existence of discrete molting areas for most landbirds, weโ€™ve long assumed that they donโ€™t exist.

To learn more about bird molt and to identify potential molt-migrations, Mr. Pyle and his collaborators examined 17 yรฉars of data collected under the auspices of the Monitoring Avian Productivity and Survivorship (MAPS) program. MAPS oversees a continent-wide bird banding effort by thousands of volunteer citizen scientists and professional biologists.

Mr. Pyle and his collaborators analyzed data from more than 760,000 capture records of 140 species of landbirds from 936 MAPS bird-banding stations across North America (Figure 1).

FIGURE 1. Bird-capture stations used in breeding and molting probability analyses. Dots indicate distribution of 936 Monitoring Avian Productivity and Survivorship (MAPS) stations and dashed lines indicate delineation of western and eastern North American regions for analyses.

This extensive data set was used to estimate the probability of capturing the same individual at a MAPS station during breeding and during molt. This data set was also used to investigate regional timing of molt (Figure 2), as well as geographic and elevational movements from breeding to molting grounds.

FIGURE 2. Captures of breeding and molting birds in relation to day of year. Birds in breeding condition peaked in Juneโ€“July, while birds in molting condition were largely captured in July and August; no substantial differences in these patterns were noted between West and East regions.

Ornithologists and birders have known that some landbird species to undertake โ€œmolt-migrationsโ€ โ€” movement to a specific molting location that is neither where they nested nor where they will winter โ€” to take advantage of wetter conditions or more abundant food. These โ€œmolt areasโ€ include Californiaโ€™s Sierra Nevada mountains or regions of the desert Southwest, which experience late summer monsoon rains. But these landbirds were always perceived to be the exceptions.

But โ€” surprise, surprise! โ€” not so. When Mr. Pyle and his collaborators analyzed the geographic movements of North American birds, they found that most of the 140 species that they examined did move to a different location during molt (Figure 4), and a few moved to higher elevations whilst others moved lower.

โ€œIn some cases this may be hundreds of kilometers away, in others it may be down the block,โ€ Mr. Pyle explained.

Mr. Pyle and his collaborators used their data set to predict molt-migration: species whose names are in the lower left of each panel (Figure 4) are more likely to migrate to molt, whereas those whose names appear at the upper right are are less likely to migrate to molt.

FIGURE 4. (A) Predicted probabilities that landbirds captured at their breeding sites would also be captured molting. Dashed lines show mean species predicted molt probabilities; shaded regions span ยฑ95% credible intervals. The mean probability that a bird captured at its breeding site would be molting was similar in the West (0.47; 95% cred. int.: 0.38โ€“0.57) and East (0.42; 95% cred. int.: 0.32โ€“0.52). Six species showing evidence for directional or elevational movements to molt are illustrated. (B) Boxplots summarize distributions of predicted molt-probability means for species categorized according to the literature as nonmigratory (Resident; shown in black), migratory and reported to molt on the breeding grounds (Breed; shown in blue), or migratory and reported to molt away from the breeding grounds (Nonbreed; shown in orange).

The data revealed some basic trends based upon species and geography. Probably most important is that many landbird species in western North America typically move longer distances to molt than do most landbirds in eastern North America (although these molt-migrations still remained within each speciesโ€™s breeding range; Figure 8). This is consistent with previous reports for some western North American landbirds but evidence for molt-migration is an unexpected finding for most eastern landbirds, particularly for non-migratory resident species like northern cardinals.

Individual species also showed some interesting patterns. For example, Nashville warblers, Leiothlypis ruficapilla, that breed west of the Rocky mountains were more likely to nest at lower elevations than where they molted, whereas Nashville warblers breeding east of the Rockies typically showed the opposite trend. Individuals also showed inconsistencies in molt-migration. This may be because more experienced birds snap up high-quality territories and can remain in place to molt, whereas less-experienced birds, failed breeders, or breeders in poor-quality habitats must move elsewhere to find suitable conditions for molt.

FIGURE 8. Examples of spatial (latitude and longitude) and elevational shifts from breeding to molting locations for selected species. Sample sizes are given for each species. Maps were developed by implementing 1ยฐ resolution, spatialโ€“conditional autoregressive models with responses representing differences between probabilities of captured individuals being in breeding condition during Mayโ€“August and being in molting condition if captured on August 1. Bluer areas represent relatively high breeding probabilities relative to molting probabilities; redder areas represent relatively high molt probabilities relative to breeding probabilities. Breeding locations aggregated within 1ยฐ cells are indicated with black โ€œxโ€ symbols; molting locations are indicated with black open squares; symbol sizes are scaled according to numbers of captures in breeding or molting condition, respectively. Locations in which all captures lacked both breeding or molting condition are shown with gray open circles scaled to total numbers of captures. Latitude, longitude, and elevation plots represent differences in each response variable (mean ยฑ95% confidence intervals) between stations where birds were captured in molting relative to breeding condition.

These findings are interesting for a number of reasons. For example, individual birds make different molt-movement choices, even on an annual basis, in response to reproductive success, weather events or environmental and food conditions each year โ€” choices and factors that we currently donโ€™t clearly understand.

โ€œRather than characterizing molt as occurring on discrete breeding or wintering grounds, it can perhaps best be thought of as a process occurring along a continuum, with most species, populations, or individuals showing some level of movement from breeding territories to molt,โ€ said a co-author of the study, Jim Saracco, a staff ecologist at IBP, in a press release.

Additionally, molt is an important event in the lives of birds, and thus, molting areas would appear to be critically important, which then raises some questions: What makes a particular area attractive to molting birds? Do different species have similar habitat requirements whilst molting? Are these molt areas receiving the conservation protection and attention they need, particularly as they influence molting habitat requirements?

โ€œOur findings highlight the need for better understanding of molt-movement patterns and habitat needs during molt to better inform full life-cycle conservation strategies,โ€ Dr. Saracco pointed out.

โ€œ[W]e need more information on the particular habitats or foods that are most important [to molting birds], and the conservation status of these areas,โ€ Mr. Pyle agreed.

Although most migrant landbirds showed southward and/or eastward movements between breeding and molting grounds, Stellerโ€™s jay (Cyanocitta stelleri) showed an unusual northward movement to its molting grounds.
(Credit: Alan D. Wilson / CC-BY-SA 3.0.)


Peter Pyle, James F. Saracco, and David F. DeSante (2018). Evidence of widespread movements from breeding to molting grounds by North American landbirds, The Auk: Ornithological Advances, 135(3):506โ€“520 | doi:10.1642/AUK-17โ€“201.1

Originally published at Forbes on 27 July 2018.



๐†๐ซ๐ซ๐ฅ๐’๐œ๐ข๐ž๐ง๐ญ๐ข๐ฌ๐ญ, scientist & journalist

PhD evolutionary ecology/ornithology. Psittacophile. SciComm senior contributor at Forbes, former SciComm at Guardian. Also on Substack at 'Words About Birds'.