Much of the work on L1 attrition has been done in the form of PhD and MA dissertations, which are often difficult to obtain. Here is a list of the ones that I am aware of, with—as far as possible—a source where they can be downloaded or ordered. Should your thesis, or one that you know about, not be included here, please let me know
PhD and MA dissertations on language attrition:
Aktürk-Drake, Memet. 2015. Phonological adoption through bilingual borrowing: Comparing elite bilinguals. PhD dissertation, University of Stockholm.
Ammerlaan, Ton. 1996. ‘You get a bit wobbly…’. Exploring bilingual lexical retrieval processes in the context of First Language Attrition. PhD dissertation, University of Nijmegen.
Badstübner, Tina. 2011. L1 attrition: German immigrants in the U.S. PhD dissertation, University of Arizona at Tuscon.
Baladzhaeva, Liubov. 2013. L1 attrition without L2 acquisition: A case of Russian-speaking immigrants in Israel. MA dissertation, University of Haifa.
Baldursdóttir, Bergljót. 1984. Language Shift in an Icelandic Child: A Longitudinal Case Study of Language Attrition. MA dissertation, Lancaster University.
Beganović, Jasminka. 2006. First language attrition and syntactic subjects : a study of Serbian, Croatian, and Bosnian intermediate and advanced speakers in Dutch. MA Thesis, University of Edinburgh.
Ben-Rafael, Miriam. 2001. Contact de Langues: le français parlé des francophones israéliens. PhD dissertation, University of Tel Aviv.
Bergmann, Christopher. 2017. Facets of native-likeness. First-language attrition among German emigrants to Anglophone North America. PhD dissertation, University of Groningen. This thesis has been published as a series of papers:
Bergmann, C., Sprenger, S. A., & Schmid, M. S. (2015). The impact of language co-activation on L1 and L2 speech fluency. Acta psychologica, 161, 25-35.
Bergmann, C., Meulman, N., Stowe, L. A., Sprenger, S. A., & Schmid, M. S. (2015). Prolonged L2 immersion engenders little change in morphosyntactic processing of bilingual natives. NeuroReport, 26(17), 1065-1070.
Bergmann, C., Nota, A., Sprenger, S. A., & Schmid, M. S. (2016). L2 immersion causes non-native-like L1 pronunciation in German attriters. Journal of Phonetics, 58, 71-86.
Bolonyai, Agnes. 1999. The hidden dimension of language contact: The case of Hungarian-English bilingual children. PhD dissertation, University of South Carolina. Most US dissertations can be ordered from ProQuest UMI dissertation services.
Bortolato, Claudia. 2013. Language maintenance-attrition among generations of the Venetian-Italian community in Anglophone Canada. PhD dissertation, University of Exeter.
Bylund, Emanuel S. 2008. Age Differences in First Language Attrition. PhD dissertation, Stockholm University. This thesis has been published as a series of papers:
Bylund, E. (2009). Maturational constraints and first language attrition. Language Learning 59(3): 687-715.
Bylund, E. (2009). Effects of age of L2 acquisition on L1 event conceptualization patterns. Bilingualism: Language and Cognition 12: 305-322.
Hyltenstam, K., Bylund, E., Abrahamsson, N. & Park, H-S. 2009. Dominant language replacement; The case of international adoptees. Bilingualism: Language and Cognition 12 (2): 121-140.
Bylund, E., Abrahamsson, N., & Hyltenstam, K. (2010). The role of language aptitude in first language attrition: The case of pre-pubescent attriters. Applied Linguistics 31 (3): 443-464
Calvo Capilla, M. Carolina. 2016. La atrición en las lenguas de contacto: el caso de los inmigrantes españoles en Brasil. PhD dissertation, Complutense University of Madrid.
Chang, Charles B. 2010. First Language Phonetic Drift During Second Language Acquisition. PhD dissertation, University of California, Berkeley.
Chang, Yufen. 2012. First language attrition: an investigation of Taiwanese tones and tone Sandhi. PhD dissertation, Indiana University. This dissertation can be ordered here.
Cherciov, Mirela. 2010. Between acquisition and attrition: the dynamics between two languages in adult migrants. PdD dissertation, University of Toronto.
Crezee, Ineke H. M. 2008. I understand it well, but I cannot say it proper back: language use among older Dutch migrants in New Zealand. PhD dissertation, University of Auckland.
Datta, Hia. 2010. Brain bases for first language lexical attrition in Bengali-English speakers. PhD dissertation, City University of New York.
Dostert, Susan. 2009. Multilingualism, L1 attrition and the concept of 'native speaker' . PhD dissertation, Heinrich-Heine Universität Düsseldorf.
Flores, Cristina. 2008. A competência sintáctica de falantes bilingues luso-alemães regressados a Portugal : um estudo sobre erosão linguística. University of Minho. Unpublished PhD thesis. The results from this study have been published in articles, among them:
Flores, C. (2010). The effect of age on language attrition: Evidence from bilingual returnees. Bilingualism: Language and Cognition 13(4): 533-546.
Flores, C. (2012). Differential effects of language attrition in the domains of verb placement and object expression. Bilingualism: Language and Cognition 15(3): 550-567.
Gross, Steven. 2000. The role of abstract lexical structure in first language attrition: Germans in America. PhD dissertation, University of South Carolina. Most US dissertations can be ordered from ProQuest UMI dissertation services
Gürel, Ayşe. 2002. Linguistic characteristics of second language acquisition and first language attrition: Turkish overt versus null pronouns. PhD dissertation, McGill University Montréal. Can be obtained from the McGill Linguistics Thesis Series.
Hulsen, Madeleine. 2000. Language loss and language processing. Three generations of Dutch migrants in New Zealand. PhD dissertation, Katholijke Universiteit Nijmegen A summary and contact details for the author can be found here.
Isurin, Ludmilla. 2000. Mechanisms of first language forgetting. PhD dissertation, Louisiana State University. Most US dissertations can be ordered from ProQuest UMI dissertation services.
Iverson, Michael. 2012. Advanced language attrition of Spanish in contact with Brazilian Portuguese. PhD Dissertation, University of Iowa.
Jamai, Abdeslam. 2008. Language Use and Maintenance among the Moroccan Minority in Britain. PhD dissertation, University of Salford.
Kasparian, Kristina. 2015. The Case of the Non-native-like First-Language: Neurophysiological Investigations of First-Language Attrition and Second-Language Processing. PhD Dissertation, McGill University, Montreal.The results from this study have been published in the following articles:
Kasparian, K., Vespignani, F., & Steinhauer, K. (2016). First Language Attrition Induces Changes in Online Morphosyntactic Processing and Re‐Analysis: An ERP Study of Number Agreement in Complex Italian Sentences. Cognitive science.
Kasparian, K., & Steinhauer, K. (2016). Confusing similar words: ERP correlates of lexical-semantic processing in first language attrition and late second language acquisition. Neuropsychologia, 93, 200-217.
Kasparian, K., & Steinhauer, K. (2017). When the second language takes the lead: Neurocognitive processing changes in the first language of adult attriters. Frontiers in Psychology, 8.
Kaufman, Dorit. 1992. First Language Attrition as a Creative Interplay between Two Languages. PhD Dissertation, SUNY at Stony Brook. Most US dissertations can be ordered from ProQuest UMI dissertation services.
Kang, Sang-Gu. 2011. English attrition in Korean-English bilingual children. PhD dissertation, University of Hawai'i at Manoa.
Keijzer, Merel. 2007. Last in first out? An investigation of the regression hypothesis in Dutch emigrants in Anglophone Canada. PhD dissertation, Vrije Universiteit Amsterdam. LOT dissertation series Nr. 163. This thesis has appeared in 2010 under the title First language acquisition and first language attrition: parallels and divergences with Lambert Publishing, Saarbrücken. The full text can be downloaded from the LOT website.
Köpke, Barbara. 1999. L’attrition de la première language chez le bilingue tardif: implications pour l’étude psycholinguistique du bilinguisme. PhD dissertation, Université de Toulouse-Le Mirail.
Leeuw, Esther de. 2008. When your native language sounds foreign: A phonetic investigation into first language attrition. PhD dissertation, Queen Margaret University Edinburgh.
Lippert, Susanne. 2010. Sprachumstellung in bilingualen Familien. Zur Dynamik sprachlicher Assimilation bei italienisch-deutschen Familien in Italien, Waxmann-Verlag, Reihe Mehrsprachigkeit, Münster/New York,
Leisiö, Larisa. 2001. Morphosyntactic Convergence and Integration in Finland Russian. PhD dissertation, University of Tampere, Finland.
Lubińska, Dorota. 2011. Förstaspråksattrition hos vuxna: Exemplet polsktalande i Sverige (Adult First Language Attrition: The Case of Polish Speakers in Sweden). PhD dissertation, University of Stockholm.
McCormack, Bede. 2001. The Maintenance and Loss of Reflexive Anaphors in L2 English. PhD dissertation, University of Durham, UK. Can be obtained here.
Mehotcheva, Teodora. 2010. After the fiesta is over. Foreign language attrition of Spanish in Dutch and German Erasmus students. PhD dissertation, University of Groningen.
Murtagh, Lelia. 2003. Retention & attrition of Irish as a second language. PhD dissertation, University of Groningen.
Negrisanu, Raluca Mihaela. 2008. Aspects of First Language Attrition: A Case Study of German Immigrants in East Tennessee. PhD dissertation, University of Tennessee.
Opitz, Cornelia. 2011. L1 attrition and L2 acquisition in a second language environment. PhD dissertation, Trinity College Dublin. Thesis—Appendix—Abstract.
Pelc, Linda A. 2001. L1 lexical, morphological and morphosyntactic attrition in Greek-English bilinguals. PhD dissertation, City University of New York. Most US dissertations can be ordered from ProQuest UMI dissertation services.
Schmid, Monika S. 2000. First language attrition, use and maintenance: The case of German Jews in Anglophone countries. PhD dissertation, Heinrich-Heine Universität Düsseldorf. Published under the same title in 2002 with John Benjamins, Amsterdam/Philadelphia.
Schmitt, Elena. 2001. Beneath the surface: Signs of language attrition in immigrant children from Russia. PhD dissertation, University of South Carolina. Most US dissertations can be ordered from ProQuest UMI dissertation services.
Schoofs, Petra. 2013. The mutual influence of the first and second languages in German and English L1 speakers in second language environments . PhD dissertation, University of Newcastle Upon Tyne.
Sučková, Magda. 2012. Phonetic Changes in the Speech of Anglophone Expatriates. MA thesis, Masaryk University.
Varga, Zsolt. 2012. First language attrition and maintenance among Hungarian speakers in Denmark. Aarhus university.
Ventureyra, Valerie. 2005. A la recherche de la langue perdue: étude psycholinguistique de l’attrition de la première langue chez des coréens adoptés en France. PhD dissertation Paris: Ecole des Hautes Etudes en Sciences Sociales.
Wang, Xiangyun. 2010. Patterns and Causes of Attrition of English as a Foreign Language. Doctoral Dissertation. Jinan: Shandong University.
Xie, Mianmian. 2010. First Language Maintenance and Attrition among Young Chinese Adult Immigrants: A Multi-Case Study. University of Alberta.
Xu, Xiaoyan. 2010. English language attrition and retention in Chinese and Dutch university students. PhD dissertation, University of Groningen.
Yilmaz, Gülsen. 2013. Bilingual Language Development among the First Generation Turkish Immigrants in the Netherlands. PhD dissertation, University of Groningen. This thesis has been published as a series of papers:
Yilmaz, G. & Schmid, M. S. 2015. Second language development in a migrant context: Turkish community in the Netherlands. International Journal of the Sociology of Language, 236: 101-132.
Yilmaz, G. & M. S. Schmid (2012). L1 accessibility among Turkish-Dutch bilinguals. The Mental Lexicon 7:3, 249-274.
Yilmaz, G. (2011). Complex embeddings in free speech production among late Turkish-Dutch bilinguals. Language, Interaction and Acquisition 2:2, 251-275.
Yukawa, E. 1997. L1 Japanese Attrition and Regaining. Stockholm University Doctoral Dissertation.
Zhou, Wencui. 2010. Birth language attrition/retention and relearning: a longitudinal study on perception of Mandarin Chinese tones among school age Chinese adoptees in the Netherlands. MA dissertation, University of Groningen.