Walsh et al. • HYBRIDIZATION IN NELSON’S AND SALTMARSH SPARROWS 
321 
temporal dynamics of hybrid zones. High dispers¬ 
al rates can lead to maintenance or expansion of a 
Vibrid zone even with selection against hybrids, 
and introgression can spread to allopatric popula¬ 
tions via hybrid dispersal (Barton and Hewitt 
1985. Rowher et al. 2001), 
The potential for hybrid expansion warrants 
iuither consideration of the conservation of 
genetically ‘pure’ populations, as hybridization 
can lead to reduced fitness of parental species 
through outbreeding depression and loss of local 
Captations, as well as the replacement of pure 
-rxies by hybrid swarms (Rhymer and Simberl- 
'il 19%. Allendorf et al. 2001). These conse- 
Mtfnces are exacerbated when the hybrid zone is 
moving (Buggs 2007). a situation where Salt- 
marsh Sparrows are vulnerable due to their small 
population sizes, potential for asymmetrical 
^crossing, human modifications of their hab- 
li,il - mrd climate change, Additional research into 
'he behavioral characteristics and fitness of 
^hrids is needed to evaluate the potential threat 
of hybridization as one of many stressors 
currently impacting this threatened species. 
Our study was focused only on detecting 
mndirectional transfer of Nelson's mtDNA into 
^ilimarsh Sparrows and diil not address the 
P’ l| enti:il lor introgression of Saitmarsh Sparrow 
Jl| ch into Nelson's Sparrow populations. Our 
‘Elusive use of mtDNA enabled us only to detect 
■ ndsw >th maternal Nelson’s ancestry, possibly 
10 an undercstimalion of the extent of 
Tidization in Saitmarsh Sparrows. Our data 
v ' c collected during targeted Saitmarsh Sparrow 
'‘ Mco| ogy (Lane and Evers 2007. Lane el al. 
and population genetics (Walsh 2009) 
^-xarch and not as a study designed specifically 
■ in ' est igate the question of hybridization. Our 
IP In - S are consistent with patterns of previous 
using both mtDNA (Rising and Avise 
, ' and nuclear markers (Shriver et al. 2005). 
" Provide important new insight into the 
1 ence of introgression and the potential 
of this hybrid zone. 
acknowledgments 
thank N. s. Pau of Parker River National Wildlife 
^ lge S - Paton of John H. Chafee National Wildlife 
, and the Marine Nature Center in Oceanside. New 
Hu,' °l '’ Uppon ' n coordinating sample collection. We 
\V „!| c _ Maine Department of Inland Fisheries and 
for allowing sample collection in protected 
also thank C. M. Schmitz and the many 
' irnsai R achel Carson National Wildlife Refuge for help 
in the field. W. G. Shriver provided a number of helpful 
comments on a previous druft of the manuscript. We are 
grateful to D. L. Berlinsky for generous use of laboratory 
facilities. Funding for this project was provided by the U.S. 
Fish and Wildlife Service and New Hampshire Sea Grant. 
The findings and conclusions in this article are those of the 
authors and do not necessarily represent the views of the 
U.S. Fish and Wildlife Service. 
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