1) Sultanpur National Park
Though, it's located in Farrukhnagar District of Haryana, it is one of the most popular national parks in Delhi/NCR. A permanent of 100+ and the temporary home of more than 300 species, this places is no less than heaven in winters. Every year many migratory bird species arrive here in search of feeding grounds. In winter Sultanpur Bird Sanctuary provides a picturesque panorama of migratory birds such as greater flamingo, ruff, black-winged stilt, common teal, common greenshank, northern pintail, yellow and white, northern shoveller, rosy pelican, etc. While in summers, this place is a home of birds like common hoopoe, purple sunbird, Eurasian Thick-knee, gray and black francolins, Indian and European rollers, along with many other birds like larks and pipits. Since this park remains close in monsoon, the best time to visit here is from October till March.
2) Sultanpur Flats and Basai Wetlands
A place which will surprise you in every visit. Basai used to be Haryana's pride in birding but the excess of modernization is taking its breaths away. Though still, you can visit here during any day of the year and surely you'll get a decent number of birds like White-throated Kingfisher, Saras cranes, Swallows and others. During winters, this place becomes home of many migratory birds like Bar-headed goose, Northern Shovelers, Painted storks, wooly necked storks, and common kingfisher, etc. The difference between Basai and Sultanpur flats is just 6-8kms. This is the field area spread on the outskirts of Sultanpur Bird Sanctuary in many kilometers. Though you need a guide to explore this area, this place is full of life throughout the year. From Rain Quails and francolins during monsoon to Pelicans and Flamingos during winters, you can get anything here. Even in summers, this place is full of birds like larks, pipits, bee-eaters, sandgrouses and courser, etc.
3) Okhla Bird Sanctuary & Chilla Khadar
This 4kms in size bird sanctuary is situated in Gautambudh Nagar, an entrance to Uttar Pradesh from Delhi. This place marked as a bird sanctuary in 1990. With over 320 species of birds, including several critically endangered species, this place is a haven for feathered beauties and bird watchers. After paying an entry fee of INR 30 (for Indians), you can roam anywhere in this park. This place has some big water bodies which do attract many birds throughout the year. Even during summer, when the water level goes down, summer migrants like cuckoos, larks, pipits, owlets, parakeets, and bee-eaters, etc take over here. Though due to the excess heat, you may not be able to spend more than a first quarter during summers, in winters, you can easily plan for a full day here.
An explored range of Aravali forest spread in some hundred km which is hardly 35-40km away from Delhi's center. This place has everything you cannot imagine close to a populated city. From various and at times rare birds (sighting of the Black Eagle, Indian Pitta, Indian Paradise-flycatcher, and Great tit, etc which are comparatively rare for this region) to mammals like Leopards, Jackals, and Blue Bulls, etc you can find anything here. Even the people here are very welcoming and friendly. This place has a military camp as well so one can safely visit here. As I've already mentioned above, this area is highly unexplored so you can get anything, anywhere. Just pick your camera and start searching (preferably not alone).
5) Yamuna Floodplains
There are two ways of reaching here. One is from Jagatpura side and other is after crossing the Signature Bridge. If you plan, try to come here anytime between September - March. During summers, you may not find many birds here and in monsoon, the river bank area will be under Yamuna River. You make get raptors like Long-Legged Buzzard, Steppe Eagle, Crested Serpent Eagle, and Oriental Honey Buzzard, etc here. Apart from raptors too, there are many birds (some 300+) like Striated Babbler, Little ringed plover, Small and Oriental Pratincole, Ruddy Shelduck, Baya, and Striated Weavers, etc. There is a record of some rare birds like Pallid Harrier, Common Shelduck, and Racket-tailed drongo as well.
6) Aravali Biodiversity Park
Forest IN MY CITY. Yes!! saying this is not exaggerating for this place. One of my most favorite places ABDP has given me some lifetime moments. This place becomes lush green during and after the monsoon, till March. After entering, you may find many people and families who visit for walk and gathering places but as soon as you'll leave the track, you will find yourself lost in nature. It's a huge biodiversity park spread in 380 acres. Once used to be barren land is now a green forest. This park has more than 400 species of plants, 38+ butterflies and 180+ birds (like common kingfisher, Indian Paradise Flycatcher, Peacocks, Shrikes, White-eye buzzard, and Egyptian vulture, etc. You can easily reach here from Guru Dronacharya Metro station as the walking distance from the station is hardly 5 minutes.
Fun fact - Gabbar stays here. Gabbar is the name given to an Indian Eagle Owl by the bird watchers and photographers. If you are planning to visit this park, do meet with the GABBAR.
7) Asola Bhatti Wildlife Sanctuary
It covers an area of 32.71 sq km on the Southern Delhi Ridge of Aravalli hill range on Delhi-Haryana. It is a part of the Northern Aravalli leopard wildlife corridor, an important wildlife corridor which starts from the Sariska National Park in Rajasthan, passes through Nuh, Faridabad and Gurugram districts of Haryana and ends at Delhi Ridge. This place is rich in every term. There are about 200 species of birds (like red-headed and Egyptian vultures reported from Asola along with a large number of medicinal plants, more than 80 species of butterflies, hundreds of other insects, mammals such as leopards, blue bull, blackbuck, black-naped hare, Indian crested porcupine, small Indian civet, golden jackal, and jungle cat, etc. (source - Wikipedia).
8) This you may not find anywhere on the internet but this is my personal favorite, i.e., Janakpuri District Park:
Close to my home, this place never disappoints me. Just in an area of 1-2km, I have recorded more than 90 species which includes some uncommon birds, like Oriental Honey Buzzard, Indian Paradise-flycatcher, Ultramarine Flycatcher, Verditer Flycatcher, Canary Flycatcher, Greater Spotted Eagle, and Long-tailed Shrike, etc, as well. As living in the middle of the city and capturing these many winged beauties is actually rare. Apart from getting passage migrants, this place never disappoints me as I always get good shots of common birds.. and for me, it's the FRAME which matters the most, not the species.