Revisiting Winter Wing Molt in Western Grebes (Aechmophorus Occidentalis DQG&ODUN·V*UHEHV A

Revisiting Winter Wing Molt in Western Grebes (Aechmophorus occidentalis DQG&ODUN·V*UHEHV A. clarkii )

DIANA L. HUMPLE1,*, HANNAHROSE M. NEVINS2,3 AND LAIRD A. HENKEL2

1Point Blue Conservation Science, 3820 Cypress Drive #11, Petaluma, CA, 94954, USA

2California Department of Fish and Wildlife – Marine Wildlife Veterinary Care and Research Center, 1451 Shaffer Road, Santa Cruz, CA, 95060, USA

3Wildlife Health Center, One Shields Ave., University of California, Davis, CA, 95616, USA

*Corresponding author; E-mail: [email protected] Abstract.—Species that undergo simultaneous flight-feather molt are susceptible to mortality during the flightless period. Remigial molt was examined in non-breeding Western Grebes (Aechmophorus occidentalis) and Clark’s Grebes (A. clarkii) using beachcast carcasses from California to Washington to determine where and when molt occurs and help identify regions of maximum risk. Molt occurred in 18% of individuals and in all age classes and months examined, including winter, not previously recognized as a significant period for Western and Clark’s grebe molt. Molt was encountered in all regions, with the highest proportions in Oregon/Washington during fall (35%), and central (18%) and southern (17%) California during winter. The percentage of Western and Clark’s grebes in molt collected during the first 3 days of an oil spill was greater than later in the response (27% vs. 12%), suggesting a greater vulnerability to oiling during this part of their life cycle. Oil spills and other mortality events can provide insights into avian biology not otherwise available, and knowledge of molting regions can minimize wildlife impacts of spills through prevention and response. Received 15 November 2012, accepted 6 August 2013. Key words.—Aechmophorus clarkii, Aechmophorus occidentalis, Clark’s Grebe, flight-feather molt, oil spills, simultaneous molt, Western Grebe. Waterbirds 36(4): 426-431, 2013

Western Grebes (Aechmophorus occidentalis) often limited by opportunities to sample birds and Clark’s Grebes (A. clarkii) are sister taxa at sea. that breed on lakes throughout western North Previously, Aechmophorus grebes (grebes) America and winter in predominantly coastal have been thought to undergo remigial molt waters from Alaska to Baja California, and un- following the breeding season in late sum- dergo simultaneous molt (Storer and Nuech- mer or fall. Molt has been documented in terlein 1992), the nearly synchronous loss and late summer at colonies in California (Storer replacement of all primaries and secondar- and Nuechterlein 1985) and Manitoba (N. La ies (hereafter remigial molt; Howell 2010). Porte, pers. commun.), and in fall in coastal This strategy has energetic (Guillemette et al. British Columbia and Washington (Henny et 2007; Fox et al. 2008) and behavioral (Hahn al. 1990; Stout and Cooke 2003). There is lim- et al. 1992; Döpfner et al. 2009) consequences. ited evidence of remigial molt in grebes dur- Because birds become ﬂightless, they may be ing winter: it was reported in approximately more vulnerable to changes in food resourc- 10% of grebes from the January/February es, weather, or predation. Increased nutrient 1969 Santa Barbara oil spill (Sibley 1970), in demands or predator avoidance (Panek and a single grebe from southern California in Majewski 1990; Guillemette et al. 2007) may February (Storer and Nuechterlein 1985), in drive some species to speciﬁc locations to molt a single grebe in midwinter from an unknown (“molt migration”; Jehl 1990; Peterson et al. location (Palmer 1962), and in Pyle (2008). 2006). Such strategies are deployed by many We examined remigial molt patterns in species that undergo simultaneous molt (Little non-breeding beachcast Aechmophorus grebes and Furness 1985; Storer and Jehl 1985; Pe- collected opportunistically during oil spills and tersen et al. 2006). The aggregation of vulner- other mortality events in California (Humple et able birds in molting areas has important con- al. 2011), Oregon and Washington (Phillips et servation relevance (Petersen et al. 2006), yet al. 2011). We also investigated whether grebes understanding timing and location of molt is might become more susceptible to mortality

426 WING MOLT IN GREBES 427 from oil spills because of their lack of mobility April), and spring/early summer (May-June). Grebes and compromised thermoregulation. were collected throughout the year in southern and central California but predominantly in winter; in November from northern California; and in October from Oregon/ METHODS Washington. Collection effort was a result of the timing of mortality events and beachcast grebe deposition; dates We assessed remigial molt on 615 carcasses of non- do not necessarily relate to regional occurrence. breeding beachcast Western (n = 543), Clark’s (n = 62), We summarized the prevalence of molt by species, and unknown Aechmophorus grebes (n = 10) from Cali- time period, region, and age. We assessed whether the fornia, Oregon and Washington. Carcasses were from proportion of molting grebes classified as adult differed six sources from predominantly coastal locales: 1) Ven- from the proportion of all grebes classified as adult. For tura Oiled Bird Incident [VOBI] in southern Califor- grebes with sufficient details recorded, we characterized nia in January and February 2005 (n = 358; Humple et molt stage, symmetry (i.e., occurring in both wings), and al. 2011); 2) M/V Kure oil spill in northern California synchrony (i.e., similar stages in all feathers). We charac- in November 1997 (n = 25); 3) a harmful algal bloom terized molt stage as early (most feathers missing, in pin, in central California in November 2007 (n = 79; Jessup and/or grown up to 20% of full length), middle (most et al. 2009); 4) a harmful algal bloom at the Oregon/ feathers grown 21-75%), or late (most feathers grown Washington border in October 2009 (n = 61; Phillips et 76-99%); and synchrony as completely synchronous if all al. 2011); 5) carcasses collected by Coastal Ocean Mam- growing feathers were the same percent grown, fairly syn- mal and Bird Education and Research Surveys (Beach- chronous if all were within 25% of one another, and asyn- COMBERS) during non-event periods in central Cali- chronous if they were greater than 25% of one another. fornia (2007-2010; n = 24); and 6) rehabilitation centers To assess if molting grebes were more likely to suc- in southern and central California (2002-2009; n = 67). cumb to oil spill mortality, we tested whether the propor- Molt was examined post-mortem, either during the tion molting differed between those collected alive and VOBI spill response (n = 207) or during later necropsies dead in VOBI. To assess whether molting grebes strand (n = 408), on freshly dead carcasses. During necropsy, more quickly than non-molting grebes, we evaluated we examined gonads and bursa of Fabricius to classify remigial molt during two phases of the VOBI search and individuals to sex and age class; grebes were aged by collection period, 12-26 January: early (days 1-3) and late 2 bursal involution in males and bursal involution and (days 4-15). For all comparisons we used a Pearson r gonad development in females. Age classifications were test using the software program R (R Development Core adult, immature, and – for females only – subadult, rep- Team 2009). resenting probable 1-2 year-old individuals based on bursal presence (as in immatures) but with at least one RESULTS mature gonad component (Humple et al. 2011). Gonad development was not helpful for aging males. We defined three California regions: southern (Mex- In total, 18% (n = 111) of Aechmophorus ico border through Morro Bay); central (north of Morro grebes examined were undergoing remigial Bay through Sonoma County); and northern (north of Sonoma County). Time periods were defined as fall molt: 19% of Western and 16% of Clark’s (September-October), early winter (November-Decem- grebes. Molting occurred in all regions ber), midwinter (January-February), late winter (March- and time periods examined (Table 1), but

Table 1. Proportion (sample size) of beachcast Western Grebes (Aechmophorus occidentalis) and Clark’s Grebes (A. clarkii) in molt, by region and time of year. Excluded are 10 Aechmophorus grebes not identified to species and six Western Grebes of unknown collection date. Sep-Oct Nov-Dec Jan-Feb Mar-Apr May-Jun Subtotal Western Grebe OR/WA border 0.35 (60) — — — — 0.35 (60) Northern CA — 0.04 (23) — — — 0.04 (23) Central CA 0 (7) 0.20 (76) 0 (2) 0 (8) 0 (2) 0.15 (100) Southern CA — 0 (1) 0.18 (340) 0.13 (16) 0 (2) 0.18 (360) Subtotal 0.31 (67) 0.16 (100) 0.18 (342) 0.08 (24) 0 (4) 0.19 (537) Clark’s Grebe OR/WA border 0.50 (2) — — — — 0.50 (2) Northern CA — 0 (1) — — — 0 (1) Central CA 0 (1) 0.24 (25) — 0 (4) 0.50 (2) 0.22 (32) Southern CA — — — — — 0 (4) Subtotal 0.33 (3) 0.23 (26) 0.09 (23) 0 (8) 0.50 (2) 0.16 (62) 428 WATERBIRDS frequency and stage of molt varied among In VOBI, a slightly greater proportion of regions and season. We found highest fre- Western Grebes collected dead had molt- quencies of molt for Western Grebes in fall ing remiges (20%; n = 205) than captured in Oregon/Washington (35%), in early win- alive (15%; n = 122), but this was not signifi- 2 ter in central California (20%), and in mid- cant (r 1 = 1.19, P = 0.28). Dead or debili- winter in southern California (18%); and for tated grebes were collected with significantly Clark’s Grebes in early winter in central Cali- greater proportions in molt during the first fornia (24%). Grebes were in a mix of early 3 days (27%) compared with the subsequent 2 (16%), middle (36%), and late (48%) stage 12 days (12%) of the response (Fig. 1; r 1 = molt in southern California in midwinter (n 11.12, P < 0.001, n = 314). = 56) and predominantly early molt (88%) in central California in early winter (n = 16). Molting grebes were predominantly adults DISCUSSION (66% of all aged grebes in molt); this was higher than the proportion of adults (51%) Our study reveals that remigial molt is 2 in the overall sample of aged grebes (r 1 = relatively common among non-breeding 4.60, P = 0.03). Molt also occurred in some Aechmophorus grebes during fall and winter immature and subadult grebes (Table 2). in coastal California, Oregon and Washing- Symmetrical molt occurred in 94% of ton, and that timing of molt is variable. Pri- molting grebes (n = 84). A few grebes had or reports of molt peaking in late summer one wing more advanced than the other (Storer and Nuechterlein 1985) or at fall (nine right, three left). All individuals molt- staging areas (Stout and Cooke 2003) may ing on only one wing had just initiated represent regional variation or sampling bi- molt (i.e., feathers missing or in pin; n = 5). ases. We found the greatest proportion of Feather growth was completely synchronous molting grebes at the Oregon/Washington in 64%, fairly synchronous in 30%, and asyn- border in fall (35%); it is unknown if these chronous in 6% of molting grebes examined individuals were staging or would have over- (n = 77). The most asynchronous individu- wintered there. Our estimate of the frequen- als (n = 3) had some feathers missing or in cy of winter molt (17%) was slightly higher pin while others were 50-95% grown. For than Sibley (1970) reported (10%). These individuals whose molt was not completely results provide a foundation for understand- synchronous within a feather tract, there was ing seasonal molt patterns in grebes that will variation in which feathers (inner vs. outer be enhanced by future studies using samples secondaries or primaries) were leading. from additional time periods and regions.

Table 2. Proportion (sample size) of beachcast Western Grebes (Aechmophorus occidentalis) and Clark’s Grebes (A. clarkii) in molt, by age and time of year. Excluded are 10 Aechmophorus grebes not identified to species and six Western Grebes of unknown collection date. Sep-Oct Nov-Dec Jan-Feb Mar-Apr May-Jun Subtotal Western Grebe Adults 0.43 (35) 0.32 (28) 0.15 (72) 0.13 (8) 0 (1) 0.25 (144) Subadults 0.67 (3) 0.25 (8) 0.38 (13) 0 (2) n = 0 0.35 (26) Immatures 0.05 (21) 0.08 (51) 0.17 (23) 0.08 (12) 0 (3) 0.09 (110) Unknown1 0.38 (8) 0.08 (13) 0.18 (234) 0 (2) n = 0 0.18 (257) Clark’s Grebe Adults 0.50 (2) 0.31 (13) 0 (5) 0 (4) n = 0 0.21 (24) Subadults n = 0 0 (1) 0 (1) n = 0 n = 0 0 (2) Immatures n = 0 0.11 (9) 0 (4) 0 (2) 0 (1) 0.06 (16) Unknown1 0 (1) 0.33 (3) 0.15 (13) 0 (2) 1 (1) 0.20 (20) 1The number of unknown-aged grebes is high because 194 Western Grebes (and six Clark’s Grebes) were not necropsied and, therefore, not aged. WING MOLT IN GREBES 429 chicks may preclude molt from occurring then (Stout and Cooke 2003; Howell 2010), and nutritional or other constraints may influence whether molt and breeding can take place simultaneously (Stiles and Wolf 1974; Howell 2000). Timing of molt in non-breeding grebes may relate to breeding phenology (Howell 2010), which can vary substantially among regions and years. Nesting can occur during fall in northern California (Storer and Nuechterlein 1985; Robison 2012) and winter in southwestern states (Unitt 1984; Parmelee and Parmelee 1997). Winter-molting grebes could be from colonies with this atypical breeding phenology, perhaps with adults still molting post-breeding and immatures undergoing either their preforma- tive or second prebasic molt (if closer to a year old). Unlike species with a restricted breeding season that have bimodal timing of flight-feather molt by age class (Kemper et al. 2008), we would expect the variability Figure 1. Proportion of beachcast Aechmophorus grebes in grebe nesting phenology to produce a in wing molt (of sample that was assessed for molt) complex molt phenology. Grebe colonies at across 15 days of search and collection during the Ven- higher latitudes may have a contracted molt tura Oiled Bird Incident (VOBI) in 2005, including (A) those arriving at spill response center dead (DOA: Dead period (e.g., in Manitoba; Storer and Nuech- on Arrival) and (B) those brought to spill response cen- terlein 1992), but insufficient knowledge of ter alive (LOA: Live on Arrival). the breeding provenance of the molting individuals in this study (Eichhorst 1992; Contrary to expectations that immatures Humple 2009) precludes such interpreta- molt in winter and adults in summer or fall tion. Nonetheless, we are unaware of other (Storer and Nuechterlein 1985; Pyle 2008), temperate species that exhibit such variabil- we observed remigial molt in all age classes ity in adult flight-feather molt timing. during all time periods examined. In fall Our results suggest grebes with remigial and winter, remigial molt was most prevalent molt are more vulnerable to oil spills than in adults. Our lack of known-age validation non-molting individuals. Based on the dif- of bursal involution and gonad maturity tim- ference in proportion of molting grebes col- ing in grebes (Pyle 2008; Humple et al. 2011) lected during the first few days of the spill limited our ability to classify subadult males, compared with later in the response (27% and likely influenced age classifications in vs. 12%), and the similar pattern during the both sexes. Santa Barbara oil spill (Sibley 1970), we sug- Temporal and geographic molt patterns gest petroleum exposure may also result in may relate to age and breeding phenology quicker mortality in molting birds. Simul- of different populations (Hahn et al. 1992; taneous molt in foot-propelled divers is un- Pyle 2008). It has been suggested that adult likely to influence foraging, but heightened grebes undergo molt in late summer and vulnerability to oiling may derive from poorer immatures in winter (Storer and Nuechter- body condition due to the increased nutrient lein 1985; Pyle 2008). Although grebes do requirements of molt (Fox et al. 2008; Craik not need flight during breeding, competi- et al. 2011) or altered foraging behavior due tion with their young for food resources or to predator avoidance (Panek and Majewski the use of their remiges for back-brooding 1990). Molt may also alter thermoregulation 430 WATERBIRDS in seabirds and increase sensitivity to disrup- Red-Breasted Mergansers in the Gulf of St. Law- tions in waterproofing (Nevins and Young rence, Canada. Waterbirds 34: 280-288. Döpfner, M., P. Quillfeldt and H. Bauer. 2009. Chang- 2011), resulting in even greater susceptibil- es in behavioral time allocation of waterbirds in ity to oil-induced hypothermia and death. wing-molt at Lake Constance. Waterbirds 32: 559- We would also expect greater vulnerability of 571. molting birds to harmful algal blooms whose Eichhorst, B. A. 1992. An analysis of Western Grebe surfactants can cause similar plumage fouling banding and recovery data. North American Bird Bander 17: 108-115. (Jessup et al. 2009). In the absence of such im- Fox, A. D., P. Hartmann and I. K. Petersen. 2008. pacts, birds are not necessarily at greater risk Changes in body mass and organ size during remi- during simultaneous molt (Iverson and Esler gial moult in common scoter Melanitta nigra. Jour- 2007), and may adapt behaviorally or physi- nal of Avian Biology 39: 35-40. ologically to mediate heightened nutritional Guillemette, M., D. Pelletier, J. M. Grandbois and P. J. Butler. 2007. Flightlessness and the energetic demands (Guillemette et al. 2007; Döpfner et cost of wing molt in a large sea duck. Ecology 88: al. 2009). If molting birds are at greater risk 2936-2945. of debilitation and mortality during oil spills, Hahn, T. P., J. Swingle, J. C. Wingfield and M. Rame- the proportion of molting grebes presented nofsky. 1992. Adjustments of the prebasic molt here might be inflated. Yet these results schedule in birds. Ornis Scandinavica 23: 314-321. Henny, C. J., L. J. Blus and R. A. Grove. 1990. 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