DOI: 10.5061/dryad.63xsj3v0x
title: High-intensity flight feather molt and comparative molt ecology of warblers of eastern North America

description: Rapid high-intensity molt of flight feathers occurs in many bird species,
 and can have several detrimental consequences, including reductions in
 flight capabilities, foraging performance, parental care, and plumage
 quality. Many migratory New World warblers (family Parulidae) are known to
 have intense remigial molt, and recent work has suggested that
 simultaneous replacement of the rectrices may be widespread in the family
 as well. However, the phylogenetic distribution of simultaneous rectrix
 molt, and high-intensity flight feather molt more generally, has not been
 systematically investigated in warblers. We addressed this issue by
 examining flight feather molt in 13 species, representing 7 different
 warbler genera, at Powdermill Avian Research Center in southwestern
 Pennsylvania, USA. All 13 species replaced their 12 rectrices
 simultaneously, with the onset of rectrix molt occurring in the
 early-middle stages of high-intensity primary molt. As expected,
 single-brooded early migrants molted earlier than double-brooded species
 whose nesting activities extend into late summer. However, our finding
 that late-molting species replaced their primaries more slowly and less
 intensively than early-molting species was unexpected, as late-molting
 species are widely hypothesized to be under stronger migration-related
 time constraints. This surprising result appears to be at least partially
 explained by a positive association between pace of molt and daylength;
 shorter late-summer days may mandate reduced daily food intake, lower molt
 intensity, and a slower pace of molt. In comparison to other passerines,
 flight feather molt in warblers of eastern North America is
 extraordinarily intense; at its peak, individuals are simultaneously
 replacing 50-67% of their 48 flight feathers (all 12 rectrices and 6-10
 remiges on each wing) for 2-3 weeks or more. Because molt of this
 intensity is likely to present numerous challenges for flight, avoiding
 predators, foraging, and parental care, the period of flight feather molt
 for warblers constitutes a highly demanding phase of their annual cycle.
description: <b>Field methods</b> Molt data
 were collected during summer and fall banding operations, 1986-2000, at
 Powdermill Avian Research Center (40.1637ºN, 79.2674ºW) in the Laurel
 Highlands of Westmoreland County, southwestern Pennsylvania, USA. Molt
 scores were obtained from 1289 individual captures of 13 different species
 representing 7 different parulid genera (Table 1). Because all 13 focal
 species nest regularly at Powdermill and surrounding areas of eastern
 Westmoreland County (Brauning 1992, Wilson et al. 2012), individuals
 captured during molt were likely local breeders on or near their nesting
 grounds (Pyle et al. 2018). In all 13 focal species, birds were captured
 at all stages of prebasic molt, and we found no evidence that individuals
 arrested or suspended molt prior to migration. All 18 remiges on the right
 wing (9 primaries, 6 secondaries, 3 tertials) and the 6 right rectrices
 were scored using the 0-5 molt scoring system of the British Trust for
 Ornithology: 0 = old feather, 1 = old feather missing or new pin feather,
 2 = new feather emerging from sheath up to one third grown, 3 = new
 feather between one and two thirds grown, 4 = new feather more than two
 thirds grown but still sheathed at base, and 5 = fully grown new feather
 with no trace of sheath at its base (Ginn and Melville 1983).
 <b>Data analysis and statistical
 methods</b> For each individual we calculated a
 total primary molt score (0-45) and a total rectrix molt score (0-30) by
 summing the individual scores for each feather. To examine interspecific
 variation in the degree to which individuals molted all their rectrices
 simultaneously, we examined rectrix molt data from 582 individuals of all
 13 species captured during active tail molt (total rectrix molt score
 &gt; 0 and &lt; 30). For each individual we calculated two indices
 of synchrony in rectrix molt: (1) the maximum difference in score between
 any two rectrices of any molt score (MaxDiff; 0 = completely synchronous
 rectrix molt, 5 = completely staggered rectrix molt), and (2) the standard
 deviation of molt score for the 6 scored rectrices (SD; 0 = completely
 synchronous, 1.87 = completely staggered). Because neither MaxDiff nor SD
 was normally distributed, we made between-species comparisons using
 Kruskal-Wallis and Dunn’s tests, nonparametric equivalents of ANOVA and
 post-hoc multiple comparisons. JMP Pro 12.2 (SAS Institute, Cary, North
 Carolina, USA) was used for all statistical calculations.
 To determine the status of primary molt at the onset of rectrix
 molt, we examined primary feather molt scores for 126 individuals of 10
 species that were captured during the initial early stages of rectrix molt
 (total rectrix molt score &gt; 0 and ≤ 6). We made between-species
 comparisons of total primary molt score for these 10 species using ANOVA
 and Tukey-Kramer multiple comparison tests, as examination of normal
 quantile plots of residuals indicated that total primary molt scores were
 approximately normally distributed. We explored
 interspecific variation in the timing and duration of molt by focusing on
 primary molt. Primary molt in parulid warblers follows the standard
 passerine pattern, with loss of the first (innermost) primary invariably
 indicating the start of flight feather molt, with sequential loss of the
 remaining primaries occurring in a regular proximal-to-distal pattern
 (Foster 1967, Nolan 1978, Rimmer 1988). Because of the regularity and
 predictability of primary molt, and because the period of primary
 replacement nearly always encompasses molt of the rectrices, secondaries,
 and tertials as well (e.g., Foster 1967), our focus on the timing and
 duration of primary molt is appropriate. We estimated
 the day of the year when the midpoint of primary molt was achieved by
 fitting 3-parameter logistic models to the total primary molt score data
 for each species, using the Nonlinear Platform of JMP Pro 12.2. We focused
 on estimating the midpoint (or halfway date; see Jackson 2017, Erni 2018)
 rather than the onset of primary molt because the estimated midpoint was
 much less sensitive to curve-fitting assumptions; a variety of linear and
 non-linear curve-fitting approaches — including Pimm regression (Pimm
 1976), 3-parameter logistic models, cubic splines, and kernel smoothing —
 all produced similar estimates of the midpoint of primary molt. With the
 fitted logistic models, we then used JMP’s Inverse Prediction tool to
 estimate the mean ± SE day of the year that the midpoint of primary molt
 (primary molt score = 22.5) was achieved for each species.
 We estimated the pace of primary molt for each species from total
 primary molt scores of individual birds captured two or more times during
 the same season while in primary molt (primary molt score &gt; 0 and
 &lt; 45). We favored this approach over alternatives such as Pimm
 regression (Pimm 1976) and Underhill-Zucchini maximum likelihood models
 (Underhill and Zucchini 1988, Erni et al. 2013) because it allowed us to
 examine the pace of molt for individual birds and also avoid problems of
 meeting assumptions of regression and maximum likelihood models (Newton
 2009, Rohwer 2013). For each species, we fit a general linear model to
 total primary molt score with two fixed effects: day of the year as a
 continuous variable, and band identification number as a categorical
 variable. The coefficient (mean ± SE) for day of the year in these models
 represented an overall estimate of the slope of the relationship between
 primary molt score and day (change in molt score per day).  All models
 were created using the Fit Model platform of JMP Pro 12.2. With estimates
 of both the midpoint and pace of primary molt, we were able to estimate
 the mean date of onset of primary molt and mean duration of primary molt
 for each species. A total of 309 molt records from 137 individual
 bird-seasons and 12 species were included in these analyses; recapture
 data for Blue-winged Warblers (BWWA) were too limited to use this
 approach, and this species was not included in the analysis of timing and
 pace of primary molt. Recapture data for Canada Warbler (CAWA) were also
 limited, but because recaptures spanned almost the entire range of total
 primary molt scores (Supplemental Material Figure S1), this species was
 included in the analysis. Our estimates of midpoint and pace of primary
 molt, however, should be viewed cautiously for this species.
 For all 13 species we calculated two species-level estimates of
 primary molt intensity: (1) <em>Peak Intensity</em>, which we
 defined as the mean number of primary feathers on the right wing being
 replaced by individuals with total primary molt scores &gt; 15 and ≤
 35; our estimate of peak intensity therefore excludes individuals in the
 initial or terminal stages of primary molt when few primaries are being
 molted simultaneously. (2) <em>Average Intensity</em> (as
 described by Rohwer and Rohwer 2013), the mean number of primaries on the
 right wing growing simultaneously for each feather between the
 2<sup>nd</sup> and 8<sup>th</sup> primary,
 inclusive; we used this somewhat more conservative estimate, which
 includes some individuals in the early stages of molt replacing only 2-3
 primaries, to allow us to compare primary molt intensities of the 13
 species of warblers with the values calculated for other passerines by
 Rohwer and Rohwer (2013). To examine the effects of
 daylength on the pace of primary molt, we used the online SolarTopo
 calculator (van der Staay 2020) to determine daylength on particular dates
 for the latitude (40.1637ºN) of Powdermill Avian Research Center. For some
 species-level comparisons we controlled for phylogenetic relationships via
 phylogenetic independent contrasts, using a recent phylogeny of the
 Parulidae (Lovette et al. 2010) and the package ‘caper’ (version 1.01;
 Orme 2018) of R version 3.6.2 (R Core Team 2019).
description: Most variable names in the data files should be self explanatory.
 Molt scores for primaries 1-9 are noted in the fields P1-P9, for
 secondaries 1-6 in S1-S6, for tertials 1-3 in S7-S9, and for rectrices 1-6
 in R1-R6. The fields "Prefix" and "Suffix" refer to
 the bird band numbering system of U. S. Fish and Wildlife Service bands.
 The files included in the archive are the following: 
 1. <strong>AllRawData.csv</strong>. This file
 includes the raw molt data for all 1289 molt scores that comprise the
 entire data set.  2.
 <strong>Table2_PeakPrimaryMoltIntensity.csv</strong>. This
 file is a subset of 300 molt scores for calculating <em>Peak
 Intensity</em> of primary molt (Table 2), which we defined as the
 mean number of primary feathers on the right wing being replaced by
 individuals with total primary molt scores &gt; 15 and ≤ 35; our
 estimate of peak intensity therefore excludes individuals in the initial
 or terminal stages of primary molt when few primaries are being molted
 simultaneously. "NPRIMmolt" (number of primaries in molt) is the
 key dependent variable in this file.  3.
 <strong>AllRecapsSameYear.csv</strong>. This file contains 309
 molt records of 137 individuals recaptured and scored multiple times
 during the same molt year. This file was used to generate Figure S1 and
 also to generate the following file. The "Suffix" variable
 contains the indentifier for each individual. 4.
 <strong>Figure5_DaylengthIndividualPaceMolt.csv.</strong> This
 file includes estimates of the pace of primary molt for the 137 individual
 birds represented in AllRecapsSameYear.csv, and was used to generate
 Figure 5.
 5. <strong>Figure7_RohwerRohwer2013RawData.csv</strong>. This file is the raw data for calculating <em>Average Intensity</em> of primary molt, following the method used by Rohwer and Rohwer (2013). It is a 645-record subset of the AllRawData.csv. This filed was used to generate the compiled data in the following file. 6. <strong>Figure7_RohwerRohwer2013Compiled.csv</strong>. This file is a summary and compilation of the raw data in the previous file. It includes four variables: Species, Outermost Primary in Molt, Mean N Primaries in Molt, and Sample Size. From this file, <em>Average Intensity</em> of primary molt could be calculated for primaries 2-8 for each of the 13 species in the dataset.  7. <strong>Figure7_RohwerRohwer2013Comparison.csv</strong>. This file shows the values plotted in Figure 7, and includes data for passerine presented by Rohwer and Rohwer 2013.