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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123. No. 2. June 2011 
DISCUSSION 
Interspecific variation between Saltmarsh and 
Nelson's sparrows was low. but consistent with 
seven diagnostic nucleotide differences and a 
1 .2% sequence divergence at the COl gene. The 
DNA barcode RFLP approach developed in this 
study, given the order of magnitude smaller 
intraspecific variation (0-0.2%), is appropriate 
for differentiating the two species. Similar levels 
of interspecific differentiation were found for 
these two species in previous studies using 
mtDNA markers (Rising and Avise 1993, Klicka 
and Spellman 2007). The low interspecific 
difference between Saltmarsh and Nelson’s spar¬ 
rows likely reflects their recent divergence 
(Rising and Avisc 1993) and is comparable to 
the differentiation of other young and hybridizing 
avian sibling species pairs (Kerr et al. 2007). 
We found Nelson’s specific mtDNA in 8% of 
individuals identified morphologically as Salt- 
marsh Sparrows, indicating they were of hybrid 
origin or genetically introgressed. We found 
evidence of introgression in all but one of six 
marshes (Rye. New Hampshire) within the 
overlap zone at a frequency of 3-19%. The 
relatively high proportion of introgressed individ¬ 
uals in the southern portion of the overlap zone 
(e.g., 14% at Chapman’s Landing. New Hamp¬ 
shire and 19% at Parker River NWR, Massachu¬ 
setts) indicated genetic introgression may exceed 
morphological variation (Rhymer and Simberloff 
1996. Sattler and Braun 2000. Bronson et al. 
2003). Introgressed individuals were morpholog¬ 
ically more similar to pure Saltmarsh Sparrows 
than to Nelson's Sparrows, consistent with the 
findings of Shriver et al. (2005). The lack of a 
clear "intermediate" hybrid phenotype in the 
individuals we identified with Nelson’s mtDNA 
suggests successful backcrossing. which appears 
to be asymmetrical and biased toward Saltmarsh 
Sparrows. Shriver et al. (2005) reported similar 
patterns of microsatellite DNA introgression. Our 
finding of an introgressed individual at John H. 
Chafee NWR. Rhode Island raises concern for the 
potential expansion o! the hybrid zone, as this 
marsh is 150 km south of the southernmost point 
of the currently assumed hybrid zone (Parker 
River NWR, Massachusetts). 
The morphological similarities between hybrids 
and Saltmarsh Sparrows may have led to past 
underestimation of the extent of introgression 
between Nelson’s and Saltmarsh sparrows and 
underscore the difficulties of characterizing hy¬ 
brid zone dynamics. Hodgman et al. (2002), in an 
extensive survey of New England marshes 
monitored 40 points at Parker River NWR and 
documented a maximum of one Nelson’s Sparrow 
at 3% of their points in comparison to a maximum 
of 10 Saltmarsh Sparrows at 78% of their points. 
Their findings extended the southern range of 
Nelson's Sparrow and identified Parker River 
NWR as the most southern point of the overlap 
zone. Eighteen (19%) of the 95 putative Salinuush 
Sparrows sampled from Parker River NWR in our 
study were identified as introgressed (having 
Nelson’s mtDNA). Our findings suggest a higher 
incidence of hybridization and. therefore, likely a 
higher proportion of Nelson’s individuals at 
Parker River NWR than previously recorded 
Previous research on Nelson's-Saltmarsh hybrid¬ 
ization focused on a narrow portion of the overlap 
zone between the Weskeag River and Webhamtet. 
Maine (Shriver et al. 2005). The discovery’ by 
Rising and Avise (1993) of an individual with 
discordant morphology and mtDNA at Parker 
River NWR led the authors to conclude the need 
for further information on interbreeding between 
the two species. Shriver et al. (2005) suggested 
hybrids are present wherever Nelson's and Salt¬ 
marsh sparrows occur sympatrically: the genetic 
data were not available to support their hypoth¬ 
esis, as they did not sample marshes south of 
Webhannet, Maine. Our results confirm that 
introgression is occurring throughout the overlap 
zone of Nelson’s and Saltmarsh sparrows and 
there may be a southern expansion of Nelson s 
alleles. 
The high proportion of introgressed sparrows in 
Parker River NWR and the potential tor an 
expansion of the hybrid zone have increased 
significance when the results are compared with a 
recent population genetic study (Walsh 2009 1 . 
Microsatellite analyses indicated the sparrows al 
Parker River NWR are highly connected, by 
dispersal, to sparrows from other marshes along 
the northeastern coast. This may have implica¬ 
tions for the spread of hybrids, given the large size 
°f this marsh and the potential for large number' 
ot dispersers originating from Parker River NW R- 
Emigration from Parker River NWR may result in 
the spread of Nelson's alleles further south. and 
may impact pure Saltmarsh Sparrow populations 
originally believed to be outside of the overlap 
zone. Dispersal patterns of hybrids and parental 
populations largely influence the spatial and 
