Late breeding season definitive prebasic molt in males, and late breeding season brood care by females, in central California Wilson’s warblers

Abstract I made observations of a central California population of Wilson's Warbler, Cardellina pusilla, after July 1 over 10 breeding seasons. I sighted males in definitive prebasic molt from July 4 (in 2007) to September 1 (in 1999). Most territorial males molted on their breeding territories, and individual molt lasted up to 46 days. Following prebasic molt, territorial males engaged in subdued “post‐molt singing,” which lasted about 7 days in some males, and which I first heard on August 13 (in 2004) and last heard on September 6 (in 1999). I sighted no female in definitive prebasic molt, or in fresh basic plumage, during the study. Of 13 females sighted ≥ July 21, 11 were in late breeding season uniparental brood care, and I could not rule out late brood care for the other two. Most, and possibly all, females not engaged in late season uniparental brood care apparently vacated their breeding territories before July 21. This departure was much earlier than for resident males, the last of which I sighted on September 10 (in 1999). Early‐departing females presumably underwent prebasic molt after July 21 at locations not known. Remaining late‐nesting females must have molted much later than resident males and likely later than early‐departing females, and at locations unknown. I last sighted two uniparental brood‐tending females, still in worn plumage, on August 26 and 29, respectively. Two unique findings of this study are a male/female difference in location of prebasic molt, and a likely dichotomy of prebasic molt timing between females leaving their breeding territories early and those remaining in uniparental brood care. Another finding, post‐molt singing in most and possible all territorial males, is a largely unrecognized behavior, but one previously reported in several passerine species. Post‐molt singing may reliably indicate completion of prebasic molt.


| INTRODUC TI ON
There exists wide variation in the definitive prebasic molting regimens of migratory passerines. Many migratory passerine species undergo complete definitive prebasic molt, or at least the molt of flight feathers, on their breeding grounds before flying to wintering sites (Rohwer et al., 2005;Ryder & Rimmer, 2003;Vega Rivera et al., 1998). Other species undertake "molt-migration," which can involve a wide range of timing and locations (Flockhart, 2010;Pyle et al., 2018;Rohwer et al., 2008;Tonra & Reudink, 2018;, but often involves recognized "molting grounds" (Steele & McCormick, 1995;Tonra & Reudink, 2018). Finally, some passerine species migrate to wintering sites before beginning prebasic molt (Jenni & Winkler, 2020;Rohwer et al., 2005). Although these are considered to be the three basic strategies of definitive prebasic molt, at least in western North American passerines (Carlisle et al., 2005;Pyle, 1997), many variations occur, and altitudinal molt migration, in addition to latitudinal molt migration, is receiving increasing research attention (Pageau et al., 2020). Also, there can be wide interannual stochastic variations in molting strategies, including differences in timing and the proportion of individuals engaging in different strategies, and strategies can vary in response to environmental factors including breeding success and food supplies (Pageau et al., 2021).
Males and females of many passerine species initiate definitive prebasic molt at about the same time of season, and require about the same amount of time to complete molt, with differences varying by only a few days (Butler et al., 2008;Flockhart, 2010;Heise & Rimmer, 2000;Vega Rivera et al., 1998). In studies that have found significant sex difference in the mean timing of prebasic molt, most have found that males initiate and complete molt earlier than females (Borowske et al., 2017;Rimmer, 1988;Ryder & Rimmer, 2003;Svensson & Nilsson, 1997). However, few studies have reported mean migratory passage times, which potentially can reflect timing of definitive prebasic molt, to be earlier for females than for males in some species (Carlisle et al., 2005;Swanson et al., 1999). The greatest mean difference in timing of definitive prebasic molt between the sexes that I found in the literature, based on field observations, was a mean 12 days earlier for male Seaside Sparrows (Ammospiza maritimus, Borowske et al., 2017).
The majority of studies on definitive prebasic molting, or timing of autumn migratory passage which can relate to timing of molt, have involved late or post-breeding season mist netting (Carlisle et al., 2005;Junda et al., 2020;Pyle et al., 2018;Ryder & Rimmer, 2003;Yong et al., 1998). One limitation of these studies is that they cannot directly evaluate the molting and behavior of individuals that possibly remain on breeding grounds after the majority of individuals have departed for migration. Thus, possible late nesting and delayed molting in those individuals cannot be assessed. Also, contrary to direct observation of a color-banded and sexed study population, migrating, sexually monomorphic species can be difficult to sex by rapid, non-invasive means in the autumn. Possibly based on that difficulty, and the fact that in many species timing of molt between sexes is minimal in any case, some mist-netting studies have not distinguished molting data based on sex (Pyle et al., 2018;. I here report observations and conclusions made during the late breeding season (>1 July) over 10 years of comprehensive field observation. This study was part of a broader study into the breeding ecology of Wilson's Warblers (Figure 1), carried out over a greater number of years. However, information here reported specifically relates to timing and behaviors associated with definitive prebasic molt and late breeding season nesting. Since this study was based on direct field observation, it has been able to reveal some facets of molting and late breeding behavior that studies mentioned above were not designed to explore, and some findings of this study vary from general consensus established by prior studies.  (1999, 2001, 2003, and 2004) (Pyle, 1997), and/or if a bird was a recapture from a previous year, in which case it was an ASY bird. However, in this study I did not use my aging data to evaluate behaviors. During the 10 years of my study, I obtained data from 40 color-banded AHY females and 85 AHY color-banded males (Table 1).
This research followed a flexible procedural design involving an intent to comprehensively observe, record, and evaluate sighted behaviors on a daily basis. Field observation plans could change day to day, as I structured these plans based on an iterative process which considered what field observations might return the most important information based on prior observations. about 30 sec to 18 min. Flight in these molting males was noticeably dissimilar from that seen in foraging individuals, which usually was rapid and darting. I also considered absence of rectrices to suggest, but not confirm, possible flight feather molt. Rectrices can be shed as a means of predator evasion (Awasthy, 2010;Møller et al., 2006).  Nolan (1978) called "noticeable molt." Also, I observed no other females at my study site that displayed any feather loss, and two of the three tailless females I sighted had lost their rectrices before the date of first observed molting in any male (July 4, 2007). These observations confirmed that rectrices can be lost in Wilson's Warblers for reasons other than molting, and such loss thus could not be a reliable indicator of prebasic molt.

| Observations of male and female molt and plumage
Detached contour feathers, often plucked loose by a preening male, I considered to be indicative of body feather molt. I also considered the contrast between worn, patchy body feathers and fresh basic plumage body feathers to indicate continuing prebasic body feather molt. All above-mentioned conditions (weak, fluttering flight, shedding of body feathers, etc.) are indicative of noticeable molt (Nolan, 1978), observable in the field. I considered observation of either noticeable flight feather molt or contour feather molt, or both, to indicate that prebasic molt was in progress. Although I sighted a male beginning noticeable prebasic molt as early as July 4, a few males continued to provision nestlings past mid-July while remaining in worn plumage, with no noticeable molt.
The contrast between males in noticeable molt and males that had completed prebasic molt was evident, and I considered both strong flight and fresh basic plumage to indicate that prebasic molt was complete or near completion. However, males in my study area also sang, usually in subdued volume, for short time periods after completing prebasic molt, and all such singing males that I sighted were in fresh basic plumage and flew well. I thus considered this "post-molt singing" (Metcalf, 2003(Metcalf, -2004 to be a reasonable indicator that prebasic molt was complete, even when an individual was not sighted.
In 1999 and 2001, on or after July 4, the first day I ever observed molting in males, I made concentrated efforts to observe molting in five and six color-banded males, respectively, occupying territories centrally located in my study area. If I had not yet observed one of these targeted males in prebasic molt, or had not yet heard it in post-molt singing, I spent additional time observing its territory, and made additional passes by the territory during observation days. The purpose of these more intensive searches was to learn the likely extent to which prebasic molt in males might happen on their breeding territories, as observing the extent of molting in these samples likely could be applied to males in the entire study population. In addition to information related to molt in males, I recorded the sighting dates, molt and plumage status, and any late season brood care of females sighted during the late breeding season on or after July 21. TA B L E 1 Timing or tabulation of relevant information related to Wilson's Warbler late breeding season pre-basic molt, plumage, or brood care

Relevant information Dates or tabulations
Latest dates I sighted males in worn plumage (three samples) 21 July (in 1998, 2005), 22 July (in 1998) Latest date I sighted a male in worn plumage and in biparental brood care 21 July (1998) Earliest dates I sighted males in prebasic molt (two samples) 4 July (2007); 5 July (2008) Latest dates I sighted males in prebasic molt (two samples) 26 Aug. (1999)

| Breeding success of late-nesting uniparental females
Of the 11 females confirmed to have been involved in late breeding season (≥ July 21) brood care, 9 fledged their broods, and 2 nests were depredated. This late breeding season nesting success rate (82%, 9/11, from 1998 to 2009) by primarily uniparental females was higher than nest success rates determined for early breeding season biparental broods at my study site (16 %, from 199558% in 1999;Ammon & Gilbert, 1999). Most broods tended in the late breeding season by primarily uniparental female nesters were of reduced size (mean = 2.83±0.75 SD, range = 2-4, n = 6), while most early breeding season biparental broods contained four eggs and/or young (Ammon & Gilbert, 1999).

| Post-molt singing
Post-molt singing was singing by territorial males which I heard when the males had completed prebasic molt. Since all males sighted in post-molt singing were in fresh basic plumage, I considered the singing to be a likely indicator that prebasic molt was complete. I indicate the earliest and latest dates that I heard postmolt singing (Table 1) Since prebasic molt in most passerine species has been found to occur at similar times for males and females (Butler et al., 2008;Flockhart, 2010;Heise & Rimmer, 2000;Vega Rivera et al., 1998) trade-off between current reproductive productivity and future fitness (Hemborg, 1999;Svensson & Nilsson, 1997 Based on mist-netting operations, Otahal (1995), Yong et al. (1998), andBenson et al. (2006) found no significant differences between mean autumn migratory passage times of the sexes of adult Wilson's Warblers, while (Carlisle et al., 2005) determined that adult males preceded adult females by a mean 5 days. Migratory passage times in autumn can indirectly reflect earlier events on breeding grounds (Benson & Winker, 2001). These earlier events might include molting times and strategies. Additionally, the advantages of completing molt as early as possible to facilitate earlier migration and establishment of winter territories are recognized (Rappole et al., 1979).
Selection pressures therefore should logically favor molting and migration times which were as rapid as possible, although it is recognized that factors contributing to migratory timing can be complex, and can confound the premise that migration should be as rapid as possible (Carlisle et al., 2005;Otahal, 1995 (Chilton et al., 2020) and Townsend's Solitaires (Bowen, 2020). Postmolt singing, as I found it occurring in Wilson's Warblers, appears to be brief, usually lasting no more than a week. Such brief singing may be found in numerous other species that otherwise are not known to sing during the non-breeding season.
Regarding a possible adaptive function of post-molt singing, Metcalf (2003Metcalf ( -2004 suggested it might just be a hormonal response to changes in photoperiod. Also, it might be presumed to have a territorial function, with territorial males signaling a warning to males prospecting for future territories. This function might apply to species whose males molt on their breeding grounds, as did male Wilson's Warblers at my study site. However, since post-molt singing in Orange-crowned Warblers occurs on molting grounds, and based on my observations usually not on breeding grounds (unpubl. data), other adaptive functions must apply for some species.
The distinction between post-molt singing, as I describe it for Wilson's Warblers, and extended singing during the non-breeding season, as occurs in some other avian species, is not altogether clear, and merits further study. Similarly, the adaptive functions of singing after the breeding season, whether brief or extended, also would be of interest.
I conclude with an observation that the approach of this study involved comprehensive "boots on the ground," field observation, and it seems unlikely that the study's relevant findings could have been obtained by another approach. This suggests that much remains to be learned about birds based on comprehensive field observation.

DATA AVA I L A B I L I T Y S TAT E M E N T
Cumulative count data for different sexes, molts, and plumage stages are accessible on Dryad (https://doi.org/10.5061/dryad 547d7 wm9X).