I’m in Loreto, Baja California Sur, Mexico. I’ve just returned from a dive trip and I am still trying to warm up. Our dive boat pulls alongside the dock and as it does so there is a flash of brown and then a splash.
I look up just in time to see a large missile enter the water. It’s a brown pelican (Pelecanus conspicillatus), the largest of the plunge diving birds. It emerges a second later, fishless, and clumsily becomes airborne again, looking embarrassedly over its shoulder at us as if to say, “that was only a practice run”.
Plunge diving birds include the gannets (boobies too), the brown pelican, terns, and the kingfishers. The kingfisher is the odd one out. Kingfishers usually sit on a perch and only plunge when prey swims into range.
The others all actively patrol a large area looking for victims swimming within their catchable depth range. We have brown boobies, redfooted boobies, masked boobies, and terns here in Fiji and one species of kingfisher.
But they all face the same problems. First, you need to be able to see your potential prey. They have this covered. They are birds after all and nearly all birds have superb vision. Humans have around 200,000 photo receptors per mm2 in the retina.
The house sparrow has 400,000 while the common buzzard reaches an astonishing million in the same area. Many birds can detect Ultraviolet (UV) light. This provides them with visual clues that we can’t see.
They may be able to judge the health of potential mates through the strength and distribution of UV reflecting pigment in their feathers. Certain fruits have waxy coatings that reflect UV light and allow fruit-eating birds to detect them more easily.
It is also thought they may be able to communicate to each other using UV reflecting markers. Some marine fish may also signal to each other using UV clues but not surprisingly, such signals tend to occur on the flanks, rather than the dorsal surfaces.
One study compared the plunge-diving birds with sea gulls to see if they had this detection capacity but interestingly only the seagulls had this ability (evolved not to detect the occasional UV flash from a fish, but to help them with their terrestrial invertebrate foraging).
Bird and reptile color vision is very different from that in mammals. These classes retain an oil droplet in their cones just above the light receiving visual pigment in the receptor cell.
The droplet acts as a filter and changes the wavelengths received by the pigment. Many seabirds have red oil droplets which improves contrast and presumably makes it easier for them to locate fish. So, now we’ve located our fish, what next? Simple right? Dive!
But of course, it isn’t as easy as that. The bird has a complex 3-D problem to resolve – it needs to compute the speed of its dive, the depth and direction of movement of the fish and possibly consider surface refraction.
All of this takes place in much less time than it took you to read this. Target located, the bird folds its wings and dives at where the fish is going to be. The higher the dive height, the deeper the bird will penetrate the surface and the higher the entry velocity.
This entry velocity can be high. Northern gannets reach 24 meters per second (53.7 mph) on entry. This raises new questions. How does a bird enter the water at that speed without breaking its neck? As is often the case, people have studied this.
Chang and co-authors wrote in 2016: “First, we use a salvaged bird to identify plunge-diving phases. Anatomical features of the skull and neck were acquired to quantify the effect of beak geometry and neck musculature on the stability during a plunge-dive.
Second, physical experiments using an elastic beam as a model for the neck attached to a skull-like cone revealed the limits for the stability of the neck during the bird’s dive as a function of impact velocity and geometric factors.
We find that the neck length, neck muscles, and diving speed of the bird predominantly reduce the likelihood of injury during the plunge-dive.” The salvaged bird, a gannet, was frozen and dropped into a tank while recording the entire action on video.
I’m not quite sure where the bird was salvaged from – they probably picked one up from their local booby recycling plant. The portion of the dive with the most risk is between the beak entering the water and entry of the chest. During this brief interval the neck is subject to substantial compressive forces which are relieved as the chest enters the water.
These forces may be partly reduced by air sacs in the chest close to the skin. Gannets and boobies can reach 10m depth from their dive, at which point they are neutrally buoyant and pursue prey by flapping their partially closed wings.
So now we have it, our bird is in the water, but the fish is still several feet away and taking evasive action. Boobies and gannets will swim after their prey (not so the brown pelicans) but as anyone who has tried to see underwater without a mask, terrestrial eyes don’t do well underwater.
Evolution to the rescue. Many plunge diving birds alter the shape of their eye to accommodate the new regime, within 80- 120 microseconds a diving gannet can change from vision optimised for air to optimised for water.
And how do we know this? Machovsky-Capuska et al (2012) videoed gannets being pushed into an aquarium on the edge of a gannet colony (yes, the Ethics Committee approved this). “Test birds were captured at the periphery of the colony using a shepherd’s hook and transferred by hand to the setup. The experimenter, holding the gannet, aligned the bird’s head so that its bill pointed ca 45° downwards.
Then, in a single smooth motion, the experimenter submerged the gannet’s head in the water for 2–5 s, to a depth of water ca 10 cm above the eye and ca 5–10 cm from the aquarium’s wall.” Sounds like water-boarding to me.
So why does this all matter? Machovsky- Capuska et al. analyzed underwater footage of 95 Australian gannet dives on bait balls in New Zealand’s Marlborough Sounds (yup – the same region that the wonderful Sauvignon Blanc comes from).
Gannets were much more successful in capturing prey (47 versus 25) when they swam after their victims than they were in capturing prey during the initial strike. The authors argue, perfectly reasonably in my opinion, that the dramatic and rapid change from air to underwater vision (a 45-diopter change) gave them the ability to locate and seize prey.
As I reached for my camera to try and record this diving activity several bluefooted boobies joined in the fun.
Even more accomplished plunge divers than the brown pelicans, the air was soon full of these avian javelins. Blue-footed and other boobies apparently have air sacs in their brain to help cushion the impact with the water.
These guys needed it – fearlessly launching some attacks from up to 100m. I wish I could convey the excitement of this hunt through photography and words, but they are inadequate. The pelicans made a loud splash when they hit, in contrast, the booby entry noise was more like a “swish”.
Maximum feeding activity only lasted around 20 minutes, but it was a very happy Paddy who wrapped up his photography and headed back to the hotel for a well-deserved margarita. Have a great week, keep smiling.
• PADDY is the author of Fiji’s Natural Heritage and a former contributor to The Sunday Times. He taught at USP for many years and now works as a personal trainer and university professor in Denver, Colorado. Dr. Ryan is a PADI divemaster and a professional photographer whose work and articles have been published around the world. He is madly in love with Fiji and returns whenever he can.
