The Integration of Gastrointestinal and Renal Function in Nectar-Feeding Birds
Regular Whizbangers know that I love hummingbirds. I spent hours watching these tiny feathered warriors at our feeder last summer. When I came across an abstract about their kidney function, it had to be blogged.
Most birds we encounter have very little fluid in their diets. They primarily ingest seeds and bugs, and they maximally retain water from their food. Nectar-feeding birds have an all-liquid diet. Hummingbirds, sunbirds, and honeyeaters must deal with high water loads during their daylight hours. For example, a hummingbird requires the calories in an amount of nectar 1.6 times the bird’s body weight during ideal environmental conditions to meet minimum metabolic needs. With cool temperatures or other stresses, intake may go as high as 3.3 times its body mass. Imagine the mythical 70 kg male drinking more than 200 liters of fluid each day! My kidney stone patients freak out about 2.5 to 3 liters daily!
Birds have much different anatomy than mammals as well (see diagram below). Food enters the crop where digestion begins, then moves into the proventiculus or stomach. After a pass through the gizzard, it hits the intestine where absorption occurs. The remaining material passes into the cloaca from where it leaves the body. Water absorbed from the intestine can be filtered by the kidney. Urine passes into the cloaca. From there it can be directed into the lower intestine for more processing or pass directly out of the body.
These birds have different renal structures from mammals as well. Two types of nephrons occur in birds, looped or mammalian nephrons which reach into medullary pyramids and produce a countercurrent multiplier system for concentrating urine, and reptilian nephrons without loops. Hummingbird kidneys consist primary of unlooped nephrons, so their kidneys are built for maximal urine dilution.
This paper shows that the two classes of nectar feeding birds have different strategies for dealing with massive water intake. Both groups of birds drop their filtration rate to zero overnight when they do not feed. It then increases again during the day as they drink. Hummingbirds primarily absorb water and filter it out via their kidneys, while Passeriformes (those sunbirds and honeyeaters) reduce gastrointestinal water absorption. Imagine instrumenting tiny 4.5 g birds to see how much fluid remains in the intestine as they eat more. Yup, this group did it! The hummingbirds had stable intestinal water excretion across all levels of intake, while water absorption decreased dramatically with higher intake in the Passeriformes, allowing them to send it straight on through the intestine.
These birds have evolved different strategies to handle the same problem, namely high water intake. There are interesting physiological lessons here. And besides, birds are fun! And bird kidneys? Well, what more could you want in a blog post...?