Natal Philopatry and Juvenile Survival in Swainson's Warblers - BioOne

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THE WILSON JOURNAL OF ORNITHOLOGY Vol. 129, No. 4, December 2017

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The Wilson Journal of Ornithology 129(4):850–859, 2017

Natal Philopatry and Juvenile Survival in Swainson’s Warblers (Limnothlypis swainsonii) Nicholas M. Anich,1,7 Thomas J. Benson,2 John A. Gerwin,3 Neil A. Chartier,3,4 Bryan M. Reiley,2 Jeremy L. Everitts,5 James C. Bednarz,6 and Sharna F. Tolfree3

1

Wisconsin Department of Natural Resources, 2501 Golf Course Road, Ashland, WI 54806, USA. 2 Illinois Natural History Survey, Prairie Research Institute, University of Illinois, 1816 South Oak Street, Champaign, IL 61820, USA. 3 North Carolina Museum of Natural Sciences, 11 W. Jones St., Raleigh, NC 27601, USA. 4 North Carolina State University, Fisheries, Wildlife, and Conservation Biology Program, Turner House, Campus Box 7646, Raleigh, NC 27695, USA. 5 National Wild Turkey Federation, 625 Bonnie Lane, Conway, AR 72034, USA.

ABSTRACT.—Understanding natal dispersal and rates of philopatry can help provide minimum estimates of juvenile survival, a vital demographic parameter for which little information is available. However, despite the large number of hatch-year migratory birds banded every year at their natal sites, few are resighted or recaptured. Here, we report on the return of 22 Swainson’s Warblers 6 Department of Biological Sciences, University of North Texas, Denton, TX 76203, USA. 7 Corresponding author; e-mail: [email protected]

SHORT COMMUNICATIONS (Limnothlypis swainsonii) banded as nestlings or fledglings between 1997–2008 at breeding sites in Arkansas, North Carolina, and South Carolina, USA. Of 284 nestling Swainson’s Warblers banded, we found 13 (4.6%) exhibiting natal philopatry. Of 95 fledglings banded, we detected 9 (9.5%) in later years. The median distance moved from the original capture to an encounter location the subsequent breeding season for all second-year birds we detected was 1.5 km (range ¼ 54 m to 15.4 km). The mark-recapture estimate for first-year apparent survival was 0.119 (SE ¼ 0.032). Estimated recapture probability was between 0.55 and 0.65. Only one returning juvenile we detected was female. Assuming a 50:50 sex ratio, the annual apparent (i.e., minimum) survival estimate for firstyear males was 0.238. Body mass, wing size, and banding date were not good predictors of whether a juvenile would return to the site the following year, and there was no evidence of differences among study sites. Because land use and hydrology limits suitable habitat, Swainson’s Warbler populations at our study sites were mostly isolated from other suitable habitats and populations; thus, individuals of this species may benefit from natal philopatry more than most passerines. Received 26 August 2016. Accepted 8 February 2017. Key words: dispersal, juvenile survival, Limnothlypis swainsonii, natal dispersal, natal philopatry, Swainson’s Warbler.

Despite the large number of young migratory passerines banded every year in ornithological studies, very few are ever resighted or recaptured. Consequently, we know relatively little about their natal dispersal. Generally, most ornithologists (e.g., Wood 1947) assumed that the majority of migratory passerines bred far from where they were hatched. More recently, natal philopatry (when an individual returns to attempt breeding near its place of origin; Weatherhead and Forbes 1994) has been demonstrated for some species, typically at low rates. In a review of 51 reported return rates for 32 species of migratory passerines, Weatherhead and Forbes (1994) found that 0– 13.5% of nestlings or fledglings returned to their natal site to breed, and the rate was greater in isolated populations (median ¼ 10.5%) than in non-isolated populations (1.75%). Natal dispersal is an important driver of population dynamics; however, there are still many species for which we lack data. Understanding natal dispersal and rates of philopatry can also help provide minimum estimates of juvenile survival, a vital yet poorly understood demographic parameter (Faaborg et al. 2010, McKim-Louder

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et al. 2013, Cox et al. 2014). We emphasize that determining robust estimates of first-year survival is difficult, as an unknown number of individuals may survive but are never detected because of long-distance dispersal away from the natal site (Cooper et al. 2008). Here, we present information on apparent first-year survival for Swainson’s Warblers but acknowledge these are likely to be underestimates of true survival, as there are no data on long-distance dispersal with which to refine our estimates. On an individual level, particularly in regions where suitable habitat is limited or populations are scattered and isolated, returning to a site known to be occupied by conspecifics can increase the likelihood of finding suitable habitat or mates (e.g., Mayfield 1983). Returning to a known site may also confer benefits associated with site familiarity (Greenwood 1980). Natal philopatry may affect the long-term occupancy of a site both directly, through returning individuals, and indirectly because returning juveniles provide cues for settlement through social information (Nocera et al. 2006) and conspecific attraction (e.g., Ward and Schlossberg 2004). However, even with these benefits, return rates of banded nestlings may remain low because of low juvenile survival. Moreover, first-time breeders often have a competitive disadvantage with older birds, particularly if they arrive on breeding territories later than older birds (Smith and Moore 2005). Dispersal also may be a mechanism to avoid inbreeding, although Wheelwright and Mauck (1998) found complete avoidance of inbreeding in a population of Savannah Sparrows (Passerculus sandwichensis) with an 11.2% nestling return rate. Natal philopatry may be more common than is currently recognized, but this can be difficult to detect in migratory non-cavity-nesters because it typically involves finding and banding nestlings from a large number of nests, or encountering a large sample of young in mist nets, followed by searches of the study site for several subsequent breeding seasons. Here, we report and discuss the return of 13 Swainson’s Warblers (Limnothlypis swainsonii) banded as nestlings and nine banded as fledglings to four breeding sites: two in Arkansas, one in North Carolina, and one in South Carolina. We also estimate apparent

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survival (uncorrected for long-distance dispersal) based on these returns. METHODS We studied Swainson’s Warblers in eastern Arkansas at the Alligator Lake area of Dale Bumpers White River National Wildlife Refuge (WRNWR; 348 02 0 N, 918 05 0 W), and across the entire St. Francis National Forest (SFNF; 348 39 0 N, 908 40 0 W). Swainson’s Warblers inhabited upland and bottomland hardwood forests at SFNF, and bottomland hardwood forest at WRNWR (Anich et al. 2012). In North Carolina, we worked in bottomland hardwood forest at Roanoke River National Wildlife Refuge (RRNWR; 358 53 0 N, 778 01 0 W; Chartier 2014). In South Carolina, we worked in a bottomland hardwood forest at Woodbury Wildlife Management Area (WWMA; 338 52 0 N, 798 22 0 W; Bishop et al. 2012). Our sites in Arkansas were 70 km apart, and our sites in Carolina were 310 km apart. We banded 140 nestling Swainson’s Warblers between 20 May and 8 August, 2005–2007 in Arkansas, banded 90 nestlings between 21 May and 3 August, 2006–2008 in North Carolina, and banded 54 nestlings from mid-May to 31 July, 1998–2001 in South Carolina, as part of larger demographic studies (Benson et al. 2010a, Bishop et al. 2012, Chartier 2014). We banded nestlings approximately 3–5 days before the estimated fledging date. We moved nestlings away from the nest to measure and band them, and returned them to the nest within 15 mins. We banded each individual with an aluminum U.S. Geological Survey (USGS) band. We also banded each juvenile with a single color band in South Carolina, and in Arkansas in 2005 and early 2006. In North Carolina, we banded nestlings with a single color band and fledglings with 3 color bands. In Arkansas and North Carolina, we recorded unflattened wing chord to the nearest 0.5 mm and measured mass to the nearest 0.1 g with spring scales and electronic balances. In South Carolina, we measured mass only. We were unable to identify the sex of birds banded as nestlings or fledglings. Video camera data (Benson et al. 2010b, Pappas et al. 2010, Chartier 2014) indicated brooding females generally returned to

the nest 15–30 mins after we replaced the nestlings. Nests were monitored at 1- to 4-day intervals to determine the status and fate, and we recorded GPS coordinates for all nests. In Arkansas, we searched for returning Swainson’s Warblers from early April to the end of August 2006–2009 at both sites. We walked and drove roads in suitable habitat listening for songs of Swainson’s Warblers, visited known territories, and explored potentially suitable habitat. As the Swainson’s Warbler is a habitat specialist, and the dense understory was limited at our study sites, we were able to search the majority of suitable habitat at our sites every year. We used targeted mist-nets with song playback to catch and color-band male Swainson’s Warblers, including both unbanded birds and birds that were banded as nestlings (Benson and Bednarz 2010). In 2008, severe spring flooding hampered our ability to access WRNWR and caused a reduction in habitat quality which may have reduced returns at this location (Anich and Reiley 2010, Reiley et al. 2013). In 2008 and 2009, we increased our search effort in relatively high-elevation areas (including nearby private land, and areas outside the levee) after the flooding to find displaced individuals. We captured and banded several fledglings during passive mist-netting between mid-June and mid-August 2006, and also caught one fledgling during a targeted capture of an adult male. In North Carolina, we resighted or captured returning Swainson’s Warblers as we conducted targeted mist-netting with song playbacks, nest searches, and radio telemetry from 9 May to 25 July, 2006–2009 (Chartier 2014). We captured and banded fledglings in passive mist-nets run from mid-June to mid-August, 2006–2007. We thoroughly searched the known available habitat each spring for returning males. We also searched for birds on lands adjacent to our study site using song playbacks, traveling 5 linear km upriver and 11 km downriver but found no Swainson’s Warblers or suitable habitat outside our primary study site. In South Carolina, we surveyed our study site each year to assess occupancy of territories and identify banded warblers. We conducted radio telemetry work from mid-April to early August, 2004–2008. When a bird with a USGS band plus a single color band was detected, we recaptured and identified that bird. The majority of our returns

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SHORT COMMUNICATIONS

were detected this way. We captured and banded fledglings during extensive constant-effort mistnetting between late May and early August 1996– 2008 (except for 2002–2003, when our effort was much reduced). For the period of most recaptures (2004–2008), we mist-netted 6 hours per day, 4days per week, during 7 weeks between mid-April and early August. We deployed 10–12 nets per session. In contrast to our other locations, there was additional potentially suitable Swainson’s Warbler habitat upriver and downriver from the South Carolina study site that was not thoroughly searched each year. While singing males were conspicuous and easily captured by target netting, females were extremely secretive and effort to resight females differed among sites. In Arkansas, we only banded 10 adult females at these sites 2005–2007, compared to 121 males. In North Carolina, we specifically targeted females as part of an extrapair paternity study and banded 50 adult females at targeted mist-netting around nests (Chartier 2014). In South Carolina, we captured 106 adult females using passive netting. At all sites, we recorded GPS coordinates for bird capture locations, generally with 8 m accuracy. We calculated distance and direction from the nest where a bird was hatched or the net where a bird was originally caught to the location at which we relocated the bird in a subsequent year. Although our estimated distance and direction traveled imply a straight line, Swainson’s Warblers winter in Central America or Caribbean islands (Anich et al. 2010), so our observations suggest returning migrants homed to natal areas and subsequently settled at some distance from their natal territory. We tested for a pattern in the direction of natal dispersal using Rao’s Spacing Test of Uniformity (Rao 1976, Agostinelli and Lund 2013) in R (R Core Team 2015). We estimated first-year apparent survival for nestlings that were banded in nests and fledglings that were caught in mist nests with Cormack-JollySeber (CJS) methods in program MARK (Lebreton et al. 1992, White and Burnham 1999). We constructed capture histories for only those nestlings known to have fledged (n ¼ 150) along with fledglings that were captured in mist nets (n ¼ 97). Because of our limited sample size, we only fit models with constant, rather than time-varying, survival and recapture probability. We fixed adult

survival at 0.58 based on prior estimates from our populations in Arkansas (Benson 2008, Benson and Bednarz 2010). We treated individuals banded as nestlings and those banded as fledglings as different groups, and evaluated candidate models that included separate apparent survival and recapture probabilities for these groups. We also evaluated models that treated study site as a covariate on apparent survival and recapture probability. For nestlings, for which we had consistent measurements between Arkansas and North Carolina, we examined the potential influence of three covariates on first-year apparent survival: day of year banded, mass, and wing length. RESULTS In Arkansas, 105 of our 140 banded nestling Swainson’s Warblers survived to fledge. In subsequent years, we detected nine banded nestlings (8.6% of those that fledged), all males, that returned to their natal site. Given that our focus on adult birds in Arkansas was almost exclusively on males, and assuming a 50:50 sex ratio (i.e., that 50% of banded nestlings were females that we made no effort to resight), firstyear male survival was at least 17.2% based on this unadjusted return rate. The median distance between banding and recapture locations was 3.7 km (n ¼ 9, SE ¼ 1.4 km; range ¼ 1.2 to 15.4 km, Table 1). In North Carolina, 45 of our 90 banded nestling Swainson’s Warblers survived to fledge. In subsequent years, we detected three males banded as nestlings (6.7% of those that fledged) that returned to their natal site. The median distance between banding and recapture locations was 1.5 km (n ¼ 3, SE ¼ 1.0 km; range ¼ 81 m to 3.4 km, Table 1). In South Carolina, we did not record which of our 54 banded nestlings fledged versus those that did not; thus, these nestlings were excluded from mark-recapture models. In subsequent years, we detected one male banded as a nestling that returned to our study site (Table 1); the distance between the banding and recapture locations was 1.2 km. Overall, of 284 nestling Swainson’s Warblers banded, 13 (4.6%, all males) were recaptured in a

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TABLE 1. Information on Swainson’s Warblers that were banded as nestlings and returned to the study site in a subsequent year to breed. Observations occurred at four study sites: 1. Dale Bumpers White River National Wildlife Refuge, Arkansas (WRNWR), 2. St. Francis National Forest, Arkansas (SFNF), 3. Roanoke River National Wildlife Refuge, North Carolina (RRNWR), and 4. Woodbury Wildlife Management Area, South Carolina (WWMA). Site

Bird ID

Sex

Initial banding date (in nest)

Recapture date

Distance moved (m)

Direction moved (degrees)

WWMA WRNWR WRNWR WRNWR WRNWR WRNWR WRNWR WRNWR SFNF SFNF RRNWR RRNWR RRNWR

74645 82979 82991 82995 10771 26403 26436 26455 10533 10566 46059 46071 90338

M M M M M M M M M M M M M

06/17/2000 07/04/2005 07/11/2005 07/22/2005 05/24/2006 05/23/2007 06/19/2007 07/17/2007 06/26/2006 07/10/2006 07/12/2007 08/03/2007 06/12/2008

05/13/2001 04/28/2006 04/28/2006 04/27/2006 05/10/2007 05/29/2008 05/29/2008 05/13/2009 04/29/2008 05/22/2009 04/20/2008 06/22/2008 05/01/2009

1,214 2,066 3,408 1,208 3,727 2,387 4,895 4,774 15,421 4,264 81 3,437 1,544

42 83 80 70 40 248 11 188 329 298 259 159 197

subsequent breeding season at their natal site. The overall median distance moved from the nest or original capture to capture in a subsequent breeding season was 3.4 km (n ¼ 13). There was no consistent pattern in direction of dispersal (U ¼ 103.5, P . 0.1). While mist-netting passively, we caught and banded four fledglings in Arkansas, seven fledglings in North Carolina, and 84 fledglings in South Carolina. None of the four fledglings banded in Arkansas, and one of the seven fledglings banded in North Carolina (14.3%) were detected in subsequent years. Of the 84 South Carolina fledglings banded, we detected 8 (9.5%) in later years. Across all sites, 9.5% of birds banded as fledglings were later detected back at their natal site. The median distance between banding and

recapture locations was 267 m (n ¼ 9, SE ¼ 234 m; range ¼ 54 m to 2.0 km, Table 2). There was no consistent pattern in direction of dispersal (U ¼ 143.0, P . 0.1). There was no evidence of variation in apparent survival (DAICc ¼ 3.99) or recapture probability (DAICc ¼ 2.37) among study locations. A model incorporating whether birds were banded as nestlings or fledglings was statistically indistinguishable from a model without this factor (DAICc ¼ 1.96; nestlings: 0.110 6 0.036 [SE], fledglings: 0.125 6 0.044 [SE]). There was some evidence of a difference in recapture probability between birds banded as nestlings versus fledglings (DAICc ¼ 0.69), so we calculated model-averaged estimates based on this and the top-ranked model (constant survival and recapture probability). Our model-

TABLE 2. Information on Swainson’s Warblers that were banded as fledglings and returned to Woodbury Wildlife Management Area, South Carolina (WWMA) or Roanoke River National Wildlife Refuge, North Carolina (RRNWR) in a subsequent year to breed. Site

Bird ID

Sex

Initial banding date (in mist net)

Recapture date

Distance moved (m)

Direction moved (degrees)

WWMA WWMA WWMA WWMA WWMA WWMA WWMA WWMA RRNWR

80671 95566 89668 89968 89745 89684 89687 89900 46035

M M M M M F M M M

7/19/1997 8/1/2003 7/21/2004 7/28/2005 7/14/2006 7/21/2006 7/30/2006 7/25/2007 6/19/2007

5/17/1999 6/15/2005 5/10/2005 5/4/2006 5/16/2007 6/12/2007 5/5/2007 5/22/2008 4/27/2008

1,576 248 457 135 2,014 502 138 54 267

198 28 55 150 183 79 216 93 222


averaged estimate for first-year apparent survival was 0.119 (SE ¼ 0.032), and recapture probability was estimated as 0.546 (SE ¼ 0.171) and 0.654 (SE ¼ 0.166) for birds banded as nestlings and fledglings, respectively. For the analysis restricted to banded nestlings with appropriate covariate data (n ¼ 150), there was no evidence for an effect of day of year, or morphological measurements on apparent survival based on DAICc (0.64–1.94) and 95% confidence intervals for parameter estimates that included zero. DISCUSSION Although our sample size of returning birds was relatively small, there is limited information on natal philopatry, particularly for migrant passerines for which values are under-reported in the literature (Weatherhead and Forbes 1994, Faaborg et al. 2010, Cox et al. 2014). Our return rate of banded nestlings (4.6% of 284 nestlings) falls within the range of values reported for other migrant passerines ([0–11.5%]; Weatherhead and Forbes 1994). Some investigators have reported no natal philopatry (Holmes et al. 1992). However, other studies found nestlings returning to their natal site to breed, including 1.7% of 296 Kirtland’s Warblers (Setophaga kirtlandii; Berger and Radabaugh 1968), 4.9% of 3,354 Tree Swallows (Tachycineta bicolor; Shutler and Clark 2003), 7.3% of 9,289 Prothonotary Warblers (Protonotaria citrea; McKim-Louder et al. 2013), 7.8% of 1,190 Willow Flycatchers (Empidonax traillii; Sedgwick 2004), 8.1% of 173 Blackpoll Warblers (Setophaga striata; Eliason 1986), and 11.2% of 1,615 Savannah Sparrows (Wheelwright and Mauck 1998). Relatively few passerine studies band large numbers of young as fledglings, however our return rate of banded fledglings was similar to values reported by other investigators to include 8.1% of 432 Bank Swallows (Riparia riparia; Freer 1979), and 12.2% of 1,594 White-crowned Sparrows (Zonotrichia leucophrys; Morton 1992). Much of first-year mortality likely occurs relatively early during the post-fledging period (Cox et al. 2014). Thus, it was surprising that we found no evidence of a difference in first-year apparent survival between birds marked as nestlings and individuals that had survived part of the

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critical post-fledging period to be first marked as a fledgling. In our case, relatively small sample sizes for returns probably hampered our ability to detect a difference. The factors we examined, including body mass, wing length, and fledging date were not good predictors of whether a juvenile would return to the site the following year. Relationships of survival with fledging date and wing development have been observed in other species (e.g., McKimLouder et al. 2013, Jones et al. 2017). Again, it is possible that our inability to detect such relationships is constrained by our limited sample size. However, finding no relationship is not unusual, according to a review by Maness and Anderson (2013) that looked at how similar variables related to postfledging survival. Results were quite variable among the 70 passerine studies they examined, and the number of instances in which these variables showed a relationship with survival was nearly equal to the number of instances where no relationship was detected. We observed only one returning female, despite presumably marking equal numbers of males and females (i.e., assuming a 50:50 sex ratio [Koenig 2016]; there are no data on the actual sex ratio of Swainson’s Warbler [Anich et al. 2010]). Our single female detection may be related to sex differences in philopatry, as in birds, females are generally thought to disperse farther (Greenwood 1980, Mumme 2015). However, part of this difference may have been caused by our increased focus on males, especially in Arkansas, and the generally reduced detectability of females, which do not sing and were rarely observed away from the nests we located. Our results are in contrast to those of McKim-Louder et al. (2013), who found similar rates of philopatry between sexes in Prothonotary Warbler, a cavity nester and the closest relative of Swainson’s Warbler (Lovette et al. 2010). Considering only males, doubling our apparent survival estimate (i.e., effectively removing females) yields 0.238 (SE ¼ 0.064) for male firstyear apparent survival. If females are rarely philopatric and few returned to our study site, then including them may be lowering our survival estimate and 0.238 may be the best minimum estimate of first-year survival in this species; although clearly more study is needed on dispersal

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and survival of juvenile male and female Swainson’s Warblers. The likelihood that some unknown number of individuals exhibit long-distance dispersal means actual rates of first-year survival are higher than our estimate presented here. The frequency distributions of natal dispersal distances for most species indicate that most are detected relatively close to their natal site, with a long tail of small numbers of birds seen at greater distances from the site (Paradis et al. 1998). We have no way of knowing, given current technology, whether this represents an accurate picture of the distribution of first-year birds or whether it is confounded by the difficulty in searching far afield to locate distant dispersers (Koenig et al. 1996). With Swainson’s Warblers in particular, we are hesitant to even speculate about the frequency and distance of long-distance dispersal, as we are not aware of a recapture of a bird at a breeding site other than its original banding location. The relatively low percentage of returning male nestlings that we detected could be explained by three possible scenarios: 1) juvenile survival is higher than our estimate, but relatively few juvenile males exhibit natal philopatry, 2) juvenile male survival is low, but many surviving males exhibit natal philopatry, 3) juvenile males have dual dispersal strategies with some individuals exhibiting philopatry and others predisposed to colonize sites farther from their natal territory. The distance distribution of 679 returning Prothonotary Warblers, including examining the potential bias from failing to detect long-distance dispersers, prompted McKim-Louder et al. (2013) to conclude the second scenario was more likely in that species. However, other investigators have found philopatry to be rare at their sites (first scenario), for example, Holmes et al. (1992) report never encountering any of the .300 nestling Blackthroated Blue Warblers (Setophaga caerulescens) they banded. In other systems, the third scenario may be operating, as Xenophontos and Cresswell (2016) found the models of Cyprus Wheatears (Oenanthe cypriaca) indicated greater survival when corrected for projected undetected longdistance dispersers. We suggest that a variety of factors may affect the extent to which natal philopatry acts to sustain local populations, including an individual species’ natural history, population size, reproductive strategies, and land-

scape context of available habitat (Weatherhead and Forbes 1994, Paradis et al. 1998). There were several birds that we did not detect until 2 (n ¼ 4) or 3 (n ¼ 1) years after banding as young. Although we were able to identify and search the majority of suitable habitat within our study region (Brown et al. 2009, Anich and Reiley 2010, Chartier 2014), the temporal gap in detections is interesting (and indeed, useful for CJS models). There are three possible explanations for a gap in detections. 1) A bird may have been holding territories at the outer reaches of our study site and was not detected because we failed to adequately search its territory. This is likely the case in one instance, but in the other instances, we know that we searched their eventual territory in the preceding year. 2) A bird could have been a non-territorial floater, lurking around the study site but not defending a territory and difficult to detect. We had several instances of ‘‘surprise’’ captures (of a second, presumably non-territorial male we had not previously detected) in mist nets while target netting during our long-term studies, which suggests that some percentage of males could be floaters. 3) A bird could have held a territory elsewhere in the range the following year, and then, perhaps not having paired or nested successfully, returned to their natal site. We never witnessed birds of any age dispersing among our study sites (Benson and Bednarz 2010) and are not aware of reports of it in Swainson’s Warbler, but reports of long-distance movements among study sites are not uncommon in related species, such as Kirtland’s Warbler (Setophaga kirtlandii; Probst et al. 2003). We believe all three of these scenarios are likely potential explanations for these temporal gaps in detection and have implications when considering true rates of survival and philopatry. Our median observed dispersal distance (1.5 km) was similar to the 1.4 km reported for Prothonotary Warblers (McKim-Louder et al. 2013), but farther than the 755 m reported for male White-crowned Sparrows (Morton 1992), and 228.5 m for Savannah Sparrows (Wheelwright and Mauck 1998). These estimates are all imperfect and affected by the scale of the study site because of the difficulty with detecting birds that disperse beyond the study site. Nevertheless, our somewhat larger distances may be explained, in part, by the relatively large territories and home ranges held by Swainson’s Warblers (Anich et al.

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2009, Chartier 2014). Bowman (2003) found a positive relationship between dispersal distance and territory size of bird species. These distances are also likely related to the extent of suitable habitat at a site. At WRWNR because of hydrological constraints (Benson et al. 2011), many territories were situated just above the normal flood line at both sites, often along a ridge of high ground or narrow strip of dense understory vegetation, resulting in somewhat linear territories. Weatherhead and Forbes (1994) found that natal philopatry was more likely in isolated subpopulations. Because land use and hydrology limit suitable habitat at our sites (Thompson 2005, Benson et al. 2011, Chartier 2014), Swainson’s Warblers there are isolated from other suitable habitats and populations, and therefore may benefit by returning to their natal sites, where we were more likely to detect them. Although return rates of nestlings are relatively low, these birds likely play an important role in maintaining populations at our isolated sites. Continuing efforts to band nestlings and monitor banded populations will lead to a better understanding of natal philopatry, natal dispersal, and juvenile survival in other species. ACKNOWLEDGMENTS Funding for the Arkansas research was provided by the Arkansas Game and Fish Commission (AGFC), the U.S. Forest Service, and U.S. Fish and Wildlife Service (USFWS) through a State Wildlife Grant. Additional funds were received from a cost-share program with USFWS and Arkansas State University. We thank the Department of Biological Sciences at Arkansas State University for administrative support. In Arkansas, we thank M. Albrechtsen, K. Ballantyne, A. Beeler, N. Bieber, J. Brown, W. Edwards, E. Huskinson, K. Jones, W. Kohler, C. McCarroll, J. O’Connell, A. Overfield, B. Paterson, C. Roa, S. Rune, J. Sardell, and A. Zachary for assistance in the field. Funding for the North Carolina research was provided by U.S. Fish and Wildlife Service, North Carolina State University, North Carolina Museum of Natural Sciences, North Carolina Wildlife Resources Commission, Georgia Ornithological Society, U.S. Geological Survey, and the Carolina Bird Club. In North Carolina, we thank M. Morales, R. Klimstra, S. Collins, C. Etheridge, A. Yoke, S. Pottier, and R. DeJardins for assistance in the field and R. Lancia and J. Richter for their technical and logistical assistance. Funding for the South Carolina research was provided by North Carolina State University, North Carolina Museum of Natural Sciences, U.S. Fish and Wildlife Service, South Carolina Department of Natural Resources, and National Audubon Society. For related field assistance we thank J. Thompson Bishop, R. DeJardins, E.

Corliss, A. Greene, K. Jensen, J. Klimstra, J. Norwalk, and A. Savage. W. Cresswell, M. McKim-Louder, and an anonymous reviewer offered comments that improved this manuscript.

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The Wilson Journal of Ornithology 129(4):859–862, 2017

Use of Two Song Types by Townsend’s Warblers (Setophaga townsendi) in Migration Stewart W. Janes1 ABSTRACT.—The use of song by wood-warblers (Parulidae) with multiple song types has received little attention during migration. On the breeding area, the two song types serve to attract mates and defend territory, but the function(s) during migration, if any, are unclear. I studied the singing of Townsend’s Warblers (Setophaga townsendi) during spring migration through southern Oregon and northern California. Males preferentially used Type II songs (79%) whether alone or in flocks of up to ~20 individuals. Type I songs of wood-warblers tend to be acquired early in the hatch year while Type II songs appear to be subject to modification over an extended period of time. The preference for Type II songs in migration may reflect an important period in the development of Type II song. More than one dialect of Type I song was recorded within flocks indicating that flocks were composed of individuals from different breeding populations. Type II songs are recognizably similar in breeding populations from central Oregon to Alaska. The similarity of Type II songs across the range of the species may result from song refinement through interaction with other singing males while in transit. Received 18 March 2016. Accepted 14 February 2017. Key words: bird song, migration, Setophaga townsendi, song categories, Townsend’s warbler, Type I song, Type II song.

Many wood-warblers (Parulidae) have multiple songs that can be assigned to two general categories based upon the contexts in which they are delivered (Spector 1992). First category songs tend to be delivered early in the breeding season prior to the arrival of the females and appear to serve a function in mate attraction (Kroodsma 1981, Kroodsma et al. 1989). Second category 1 Department of Biology, Southern Oregon University, Ashland, OR 97520, USA; e-mail: [email protected]

songs tend to be delivered before sunrise after pairing and in territorial encounters and appear to serve a function in territorial defense. Both songs may be delivered after sunrise following pairing. The terminology for song categories of various wood-warblers is not standardized. In Townsend’s Warblers, the terms ‘‘Type I’’ and ‘‘Type II’’ are used, but in other species different terms are applied. Spector (1992) suggests using the terms first and second category songs in discussions of multiple species. Type I and II songs of Townsend’s Warblers correspond to first and second category songs, respectively. While use of song categories on the breeding area has been studied in some detail, a general pattern of song use in migration is not clear. Blackthroated Green Warblers (Setophaga virens), for example, employ both song categories in transit but increase their use of first category songs as they near the breeding area (Morse 1991). Hermit Warblers (S. occidentalis) use first category song in passage, but the use of second category song was not reported (Pearson 1997). Townsend’s Warblers (S. townsendi) breed in coniferous forests from central Oregon and Idaho north to Alaska and winter in western Mexico and Central America (Wright et al. 1998). Some winter in coastal Oregon and California. In migration, loose flocks and solitary birds pass through the lower elevation coniferous forests of interior southwestern Oregon and northern California, and singing is common. Several distinct dialects of Type I songs create a mosaic across the breeding range of Townsend’s Warblers in Oregon and Washington (Janes and