This paper analyzes the impact of GitHub Copilot, a generative AI pair programmer, on open source software (OSS) development using data from GitHub. We find that using Copilot increases project-level code contributions by 5.9%, which is due to increased individual code contributions (2.1%) and increased developer engagement (3.4%). However, it also has the side effect of increasing code integration coordination time by 8%, which is due to increased code discussions by the AI pair programmer. We also observed differences in the effects between core and peripheral developers, with peripheral developers experiencing a relatively smaller increase in project-level code contributions and a larger increase in coordination time. In conclusion, AI pair programmers have a dual impact on project-level code contributions and coordination time in OSS development, and have a positive effect on overall productivity, but the effects vary depending on the developer role.