The X6100 is a very advanced SDR transceiver that, unlike most other portable transceivers, features a Cortex A7 CPU on board. Basically it makes it a Unix controlled transceiver able to do things not experienced before in a tiny, self contained and portable unit. Among other features it has a switching power system which is synchronised to a synthesizer to avoid interference in the receiver.
Most of the birdies you hear is noise leaking out of the case and picked up by the antenna connector. When you connect an external antenna this noise will be reduced because now the pickup point is moved from the open connector to the antenna feed point. There are also a few birdies produced within the receiver itself which is normal for wide-band receivers able to cover the full HF spectrum.
If you experience birdies that interfere with reception of weak signals on the bands, make sure you have the lates firmware installed as part of these problems already have been adressed in later FW versions. Xiegu is continuing the work of optimising the algorithms used to reduce both kind of birdies mentioned in this article.
Latest Firmware is available on the Xiegu.eu Downloads Page.