Google had recently put up the factory images of the Android 5.1 Lollipop for the developers and enthusiasts to flash it on their devices. Now the Nexus 5 users are getting the Android 5.1 Lollipop OTA update. Measuring about 220.7 MB, the OTA update is being rolled out to Nexus 5 devices in phases across the world. The most signification feature that arrives with Android 5.1 is the Device Protection which keeps your device locked even after factory reset. Only after the owner signs in, the device can be unlocked. HD Voice has been activated to improve the voice call quality. There's no evidence of whether the fix for the memory leak, as reported earlier, has been patched with this...
Source: http://www.gsmarena.com/nexus_5_is_getting_android_51_lollipop_ota_update-news-11602.php