This dissertation studies how sports journalists are adapting to the emerging institutional requirement to use Twitter to live-tweet during sporting events. This phenomenon is the result of the rise of the "second screen," wherein stakeholders of a live televised event convene to discuss the event online via their second screen, the computing device they use while watching the event. Institutional theory and the news ecology model framework offer a basis for considering reasons why professional journalists have shifted to live-tweeting during sporting events, which is a distinct departure from pre-social media event sharing. As journalist live-tweeting is a concept still being understood in both industry and scholarly research, this phenomenon as it relates to journalist professional duties bears study. A three-year content analysis of journalist live-tweeting from the National Association for Stock Car Auto Racing's (NASCAR) premiere racing event, the Daytona 500, via a mixed method approach, was used to determine journalist tweeting behaviors during the race and denote trends or shifts over the three years. Results indicated that there were significantly different tweet tendencies between bloggers and institutional journalists. Among the findings, bloggers were far more likely to write opinion tweets and engage with fans, while institution-affiliated journalists were far more likely to tweet information and cite sources. Tweets were more likely to be sent during caution laps than green flag laps, hashtags were not shown to be essential elements of tweets, and photo sharing was used to demonstrate credibility and access. Further, institutional print and online journalists became more homogenous in their tweeting tendencies after three years both within their peer groups and in the aggregate, offering support for institutional theory. Television broadcasters reporting in broadcast booths tweeted more frequently with each passing year, while the trend did not hold true for other types of journalists. Institutional theory, specifically the news ecology model, was shown to be a predictor in part of the research findings. Homogeneity and mimicry were seen in tweets, and Twitter presence was stable over all three years of data. However, elements of branding theory were also noted, including institutional journalist opinion sharing, and adding humor and promotional information in tweets.