Yesterday out at Al Ahsa National Park I encountered nearly a dozen Black Scrub Robins (Cercotrichas podobe), which breeds here and can be encountered throughout the year. A full survey of the park, which is actually quite expansive, would surely turn up many more. In fact, I've found this popular picnicking spot just north of the village of Al Umran to be the most reliable location for seeing them in the Eastern Province. Here's the thing though. Consult any of the literature on the distribution of Black Scrub Robins in Arabia, such as the Birds of the Middle East or Cornell's Birds of the World, and you'll see no mention of this population in Al Ahsa.
Black Scrub Robins range across the Sahel zone of North Africa from Mauritania through Sudan and into southwest and central Arabia. In the existing literature, Riyadh Province has marked the easternmost edge of its breeding range with single birds occurring further east in countries like Kuwait, Qatar, the UAE, and Oman considered winter vagrants. However, it's now understood that the species has been expanding its range to the north and east in Arabia and gradually colonizing new areas, such as the population now regularly breeding in southern Israel. The reason for this expansion is most likely the increase in irrigation and cultivation in these otherwise inhospitable desert regions. As the number of cultivated areas has increased in Saudi, it's created opportunities not only for vagrant birds to show up in places like Udhailiyah, the Aramco camp south of Al Ahsa where I saw one or two Black Scrub Robins almost daily from November 2017 until early spring 2018, but also to become firmly established, as at Al Ahsa National Park, where I've been seeing good numbers of them throughout the year since April 2018.
In eBird, besides Udhailiyah and Al Ahsa National Park, the only other records for the Eastern Province are my reports of single birds in Urayarah, Haradh, Al Munaizlah wastewater treatment plant, and my current Aramco work location in Al Ahsa as well as a couple from Rad Sularte, including an interesting one from July in a village just west of Salwa, near the border with Qatar. I contacted Jem Babbington, though, and he informed me that he has records from Al Sarrar, further north of Urayarah, as well as Qatif, Jubail, and Al Khobar. Clearly the species' eastward march continues.
Another interesting candidate for range expansion in Saudi is White-spectacled Bulbul. Yesterday I encountered 3, possibly 4, of this native bulbul species in the same quiet corner of the park where I saw one back in October of last year, suggesting that they may be breeding here. While it's possible that these birds were released or escaped in the vicinity, there is also the chance that they arrived of their own steam from the nearest populations in Riyadh Province, the furthest east they range in the Kingdom. This species, like the Black Scrub Robins, has probably taken advantage of cultivated or landscaped areas out in the desert to venture further into the Eastern Province.