Isla Mujeres is about 7km long, and is made of limestone. The west coast is the sheltered side of the island, and had lots of sandy beaches, shallow water and mangroves. Most of the beach resorts and all of the marinas are on this side, and there are many piers, jetties and pilings that provide perches for the thousands of Laughing Gulls, Brown Pelicans, cormorants and terns that feed here. Also on the west side is a large lagoon, called Laguna Makax. This doesn't have much birdlife on the water itself, but is fringed by mangroves which were very productive.
The east coast faces out into the Caribbean, and therefore bears the brunt of most of the weather. In consequence it consists of low limestone cliffs, and not many birds. It is the side that will produce passing seabirds though, and it is worth checking regularly.
Salina Chica (left) and Salina Grande. Salina Chica is mostly closed off, though a small gap in a chainlink fence at the southwest corner will allow an adventurous birder to gain access to a marvelous spot. Presumably because it is closed off it holds large numbers of waterbirds.
Salina Grande is much larger, and at first glance seems devoid of birds. Both ends of the lake are much nicer though. In the north a small reedbed held a Least Bittern for a few days. In the south is a fenced off area which can be accessed through a public park. This was a very good area for waders and herons. Best birds here were Sora Rail, Solitary Sandpiper and Ringed Kingfisher.
Several species seem to be present everywhere, drawing the attention of even non-birders. On Isla Mujeres these include Great-tailed Grackle, European Collared Dove, White-winged Dove and Tropical Mockingbird. With care though, many other species can be found.
Yucatan endemics: Yucatan has a number of endemic species and subspecies, however few of these occur on Isla Mujeres. Those that I found were Yucatan Vireo, Black Catbird and the Yucatan race of Hooded Oriole. All three species were seen in the mangroves.
Seabirds: Like most tropical islands of my experience seabird diversity was low. The most obvious birds of any kind are the large numbers of Magnificent Frigatebirds soaring over the island all day. True masters of the air, they live up to their name. Less numerous, but still obvious to even non-birders are the Brown Pelicans. Gulls are represented almost exclusively by Laughing Gull, of which there are hundreds, though I did have one flyby American Herring Gull. Terns were more interesting. The southwest corner of the island had a fair sized flock of Sandwich Terns perched on the pilings outside one of the beach resorts (I counted 42 together on one day). There were a few scattered Royal Terns among them, and on Saturday 28th there was a flock of 8 Roseate Terns, apparently quite a rarity.
Herons & Ibis: The inland lakes of Salina Grande and Salina Chica held a remarkable variety of herons. The most interesting was a male Least Bittern seen at the north end of Salina Grande on 23rd & 26th. Also present at these two sites were Great White Egret, Snowy Egret, Great Blue Heron, Little Blue Heron (both blue and white phase birds), Tricolored Heron and Green Heron. The mangroves had both Black-crowned Night Heron and Yellow-crowned Night Heron, and three Cattle Egrets were at various sites in the south on 28th. White Ibis were in small numbers at the wetlands on each visit, with a single bird feeding on the lawn of a beach lodge in the south on 28th.
Flycatchers: An interesting collection of tropical flycatchers was scattered around the island, in all habitats. Most obvious in the early part of the week were the Tropical Kingbirds, they seemed to be all gone by about the 27th though, to be replaced almost immediately by Eastern Kingbirds on 28th. Two pairs of Great Kiskadee were also seen, one of which appeared to be nest-building on a transformer near the north end of Salina Chica. A Brown-crested Flycatcher on 27th turned into 2 on the 28th. A tricky identification challenge made somewhat easier when it responded to a recording of its call within seconds.
Vireos & Wood Warblers: The commonest small passerine on the island was Palm Warbler, which seemed to be present in every patch of scrub. Also common were Yellow Warbler. Warbler numbers built up during the week, reaching a peak on 27th. A total of 14 migrant wood warblers were seen, including; Worm-eating Warbler, Northern Waterthrush, Black-and-white Warbler, Prothonotary Warbler, Common Yellowthroat, Hooded Warbler, American Redstart, Cape May Warbler, Northern Parula, Magnolia Warbler, Yellow Warbler, Palm Warbler, Yellow-throated Warbler and Prairie Warbler. In addition several of the distinctive Mangrove Yellow Warblers were also seen. Mangrove Vireos were very common in the right habitat.