Dr Philipp Boersch‑Supan

Blog post - A feather at a time

This blog post was originally published in the BTO’s Lifecycle magazine

Most adult birds moult all their feathers once a year, which provides an energetic challenge on a par with reproduction and migration. Despite this, moult has been a neglected subject in ornithology and much remains to be learned about how it fits into the annual cycle of birds. Here I highlight the value of recording scores of individual primary feathers.

Documentation of basic moult data such as the timing, location, sequence, and completeness of feather replacement is crucial for any attempt at unravelling the physiological and environmental causes of moult and how these might differ between species and regions. As changing climates alter birds’ breeding and migration seasons, how is the timing of moult changing? Ringing records provide a critical source of information and we have been looking at how we can best make use of them. Specifically, we are interested in quantifying the variation in passerine moult timing and duration, and investigating potential environmental drivers thereof. Estimating the date when moult starts and how long it lasts in wild bird populations is challenging; usually the full progression cannot be observed in individuals. Rather, we have to infer moult timing across a population from snapshots of different individuals throughout the season that are usually only caught once.

RECORDING MOULT
One of our first insights from this project is that the way moult is recorded matters. The guidelines in the Ringers’ Manual allow two types of moult information. Moult codes consider moult progression across the entire bird in broad categories relevant to all ages, whereas primary scores track the progression of flight feather moult in more detail.

SIMULATION
To determine the relative value of these two approaches, we simulated moult in a large virtual population of adult birds. We then sampled this virtual flock as if we were catching them using different levels of effort (i.e. numbers of birds caught over the course of a season) and noted either the moult code (‘O – old plumage’, ‘M – active main moult’, ‘N – new plumage following main moult’) or the scores of the individual feathers for each (0–5). Finally, we used a statistical model to estimate moult timing for each sample. Because we know how moult progresses in our simulated population, we can assess how accurate our statistical estimates of moult commencement and duration are using the two recording methods. Estimating moult dates and durations is sensitive to when birds are observed in their moult cycle. Ideally, records from a season cover birds in all stages of active moult as well as those individuals that haven’t started (code ‘O’, score 0) and those that have finished (code ‘N’, score 50). We found that this was particularly important when using moult codes alone.

Analyses of moult timing (phenology) based on primary scores require a much smaller sample size to yield useful results (left), and deliver date estimates with higher precision, particularly for small sample sizes (right).

RESULTS
On average, a much larger sample of birds with just a moult code is required for the phenology model to yield any results – 50 birds sampled randomly across the season offers only a 50:50 chance of successfully estimating moult start date and duration (see graph). Primary scores hold much more information, because wing moult progresses in a near-linear fashion in many passerines. A random sample of about 30 scored birds virtually guarantees reliable estimates of moult phenology. Further, when estimation succeeds, the estimated dates and duration are much more precise for a given sample size when using primary scores. Samples larger than c. 50 individuals with primary scores yield standard errors smaller than about five days. For moult codes, a threefold larger sample is required to obtain the same level of precision. Although these numbers might seem small in a national context, by the time you start breaking the sample down by sex, habitat, location… every record becomes valuable. So, next time you are processing captured birds please consider recording a full set of primary scores, and remember, scores of pre- and post-moult birds are as important as those of actively moulting birds to understand the phenology of feather replacement.

REFERENCES
Underhill, L. & Zucchini, W. (1988) A model for avian primary moult. Ibis 130, 358–372. DOI
Boersch-Supan, P. et al. (2021) Bayesian inference for models of avian moult timing. Poster presented at EURING 2021. DOI