Incredible Bird Life!

Category: Ships Log

A closer look at the bird life on Isla Isabel

There was so much to see at Isla Isabel, we had to split it into two blogs! Here is the more biology-oriented blog about the bird life we encountered. 


It sounds like we are in the middle of a coffee break at a large conference where everyone is trying to talk at once. Except everyone has a relatively high pitched voice and all their words are coming out as squawks. We have just learned from our guides, Cesar and Patricia, that sooty terns recently invaded a portion of the blue footed boobies’ nesting grounds.  Neither party sounds happy.  Some boobies are holding their ground, but several have already abandoned their nests and eggs.


Invasion of the sooty terns.


We are on Isla Isabel, a nature park about 85 nm southwest of Mazatlan and 42 nm northwest of San Blas. It is sometimes called the “Mexican Galapagos” because there are no predators on the island and the birds that reside here are not afraid of humans.  The island is a nesting ground for blue footed boobies, brown boobies, and magnificent frigate birds. You can also see Heermann’s gulls, sooty terns, tropic birds and brown pelicans. Upon landing with our dinghy on the island, we stumbled upon a camp and met Cesar and Patricia, volunteer researchers for a blue footed booby monitoring program through the National University of Mexico (Mexico City).  Cesar is a veterinarian and Patricia is a biologist; both have volunteered two months of their time to mark nests, count eggs and chicks, and tag birds. At the camp are also two undergraduate students from the University, Andrea and Santiago; they are spending two weeks working on another blue footed booby project.  We were lucky enough to arrive on everyone’s day off, and got the five star biology tour of the entire island.


Back on the beach, blue footed boobies are everywhere; the early arrivals are already sitting on eggs or taking care of their recently hatched chicks, while the more recent arrivals are courting potential mates or walking the beach with their mate in search of a good nest site.  Finding a good nest site is important. Cesar and Patricia explain to us the boobies are very territorial about their nests, and if they think a nest is too close, they may try and kick the egg out of the neighboring nest if it is left unattended to gain more space.  This is why the invasion of sooty terns this year is causing such a ruckus; the terns are seriously crowding the boobies at one beach and stressing them out.  Cesar and Patricia said the terns typically don’t nest on this island, but for some reason they decided to try it out this year. Luckily, the boobies still have a large portion of their nesting grounds undisturbed.  The blue footed boobies typically lay one or two eggs, though as many as four have been found in a nest.  Another interesting fact we learned is that boobies are very sensitive to what “belongs” in their nest, and if they catch a whiff of anything foreign, even if it is their own egg or chick that has left the nest and is returned, they will attack it. Thus, when an egg is kicked out of a nest, the nest is usually abandoned.  Another danger for the eggs are Heermann’s gulls, which like to try and steal the eggs.


A pair of blue footed boobies courting. They have an interesting mating dance where they lift up their feet. Interesting fact we learned: the easiest way to tell males and females apart is from their pupil size; the males have a tiny pupil while the females have a larger pupil (in this pic, the male is on the right). Females also tend to be larger, but sometimes that is hard to tell.


A male blue footed booby sitting on his nest. If you look closely, you can see the egg towards his left. If you approach too close, the male will start whistling and snapping his beak at you. We thought it was interesting that the male makes a high pitched whistle and the female makes a lower pitched, honking sound.


A blue footed booby chick testing its wings. Note this is nest #138. When we were there, they were getting close to nest #1000.


We leave the beach and enter the scrubby, dense forest on a trail that leads to a fishing village on the opposite side of the island. We see a couple more booby nests in the fringes of the forest, but soon all we see are frigate bird nests in the tree tops.  The frigate birds appear massive after spending time with the boobies; their wingspan can reach 7.5 ft! As we walk through the forest we see several dead frigate birds, either on the ground or in the tree branches. Cesar and Patricia explain that the frigates are always searching for sticks for their nests; sometimes the birds fly too far into the dense forest to grab a stick and become entangled in the branches. Their wingspan is so large, they can’t free themselves and thus die in the forest.  As we are climbing a hill, Patricia spots a male frigate bird caught in the tree branches that is still alive.  She and Cesar carefully approach the bird and manage to untangle it. She explains she can’t release the bird here, because it would just get caught in the tree branches again, so they must take the bird to a clearing where it will be able to fly away safely.  She holds the massive bird with one hand and grips the beak shut with her other and carries the bird up the trail. Luckily there is a clearing close by and about 5 minutes after untangling the bird, Patricia releases him.  He flies high over the treetops and soon disappears from view.  Patricia confesses they aren’t really supposed to interfere with the birds in this way, but it is too hard to let a bird die that they can easily help. I would agree. It felt really good to see the male frigate fly away after narrowly escaping his death sentence.  Hopefully he learned his lesson and won’t attempt to enter the forest again!


Patricia and Cesar carefully untangling the male frigate caught in the tree branches.


Patricia releasing the frigate bird in a small clearing.


Approaching the fishing village, the forest thins out and gives way to grassland with just a few trees here and there.  Every tree top is covered with multiple frigate bird nests and some are even built on top of grass mounds. Males with their bright red throat pouch inflated like balloons are conspicuous everywhere. There are also many chicks with matted white feathers, making them look like they just woke up (which they may have). Patricia and Cesar tell us the adults feed and care for their chicks for an entire year. That seems like a long-term commitment for a bird (in contrast, a blue footed booby chick fledges around 3-4 months)! Unfortunately, the life of a frigate chick is also fraught with danger. For the chicks that are in nests above the forest floor, if they happen to fall out, their parents will not be able to reach them in the forest and the chicks will starve.  Likewise, if an egg rolls out of a nest to the forest floor, it is a goner. Apparently, enough frigates survive though, so evolution hasn’t deterred frigate birds from building their nests in the forest canopy.


Frigate birds nesting in the tree tops. The males have the bright red throat pouches.


A male frigate bird trying to attract a mate with his inflated throat pouch.


A frigate chick eyeing the odd creatures with black boxes pointed at him.


As we pass through the fishing village, we learn the fishermen are instrumental in helping the monitoring project succeed. It is the fishermen that bring the volunteer researchers drinking water (there is no freshwater on the island) and any other supplies they may need from San Blas, which is the nearest town. The Mexican Navy also helps by transporting the researchers to and from the island and by delivering a large resupply every few weeks, but they come infrequently, so the fishermen fill in when needed.

The fishing village on the southern side of the island. Behind is the hill with the lighthouse erected on top.


We continue past the village and up a hill to one of the best viewpoints on the island, where a lighthouse is erected. This also happens to be the only spot on the island where you can catch a cell signal, so Santiago and Andrea have brought their phones.  From the top, we have a spectacular view of the island; it is one of those places you could just sit for hours. There are also a few scattered brown and blue-footed booby nests on the rocky ground, many with fuzzy white chicks.

The lighthouse on top of the hill (it is not so much a house as a metal tower with a light on top).


Requisite pic on top of the hill with gorgeous background.


A female brown booby we met at the top of the hill.


A female brown booby with her chick. The chick seems to have outgrown his mother!


Back at the researcher’s camp, we return to the turf war between the boobies and the terns.  It will be interesting to see in coming years if the terns continue to return or if this was just a one-time fluke.  As we are all hot and sticky from the hike, we end the day snorkeling around the surrounding rocks.  There are a lot of beautiful fish to see, but the birds are still foremost in our thoughts.  We feel so lucky to have spent the day hanging out with Cesar, Patricia, Andrea and Santiago and getting to learn about the amazing bird life on Isla Isabel from them! This was a stop we won’t soon forget!



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