The House Crow (Corvus splendens) is native to the Indian Subcontinent but has been introduced to various countries including Singapore. 
Although it is usually described as an entirely black bird, it has a greyish collar. 
These birds are often observed in flocks around urban areas.

1st, 2nd & 3rd photo: Pulau Ubin, Singapore, 24 October 2022
Last photo: Sungei Buloh Wetland Reserve, Singapore, 25 October 2022