October 3, 2011 | 29
Hummingbirds are among the weirdest birds of them all. You already have a rough idea of how weird they are – there’s that hovering and humming, oh, and nectar-eating (or nectarivory). But there are a number of facets of hummingbird biology and diversity that aren’t so well known, and in the interests of enlightening you, here are just three of them. I was originally going to write a lot more but I decided to be lazy and spread things out over as many articles as possible. And there’s certainly no shortage of things to talk about as goes the weirdness of hummingbirds.
As usual I’ll start by apologising to the knowledgeable types for whom these supposed revelations are old hat – I have a great dislike of being presented with “did you know?” facts that I arrogantly regard as common knowledge. Anyway, here we go.
– Hummingbirds have to supplement their diet of nectar, since they’d otherwise suffer from deficiencies. There’s no protein in nectar, so hummingbirds have to catch and eat arthropods (more on that below). Nectar is also low in such elements as calcium, and birds famously need lots of calcium, especially females during times of egg production. The females of most birds store calcium in a special tissue known as medullary bone, but hummingbirds are known to have particularly small amounts of this tissue (Adam & des Lauriers 1998), perhaps because their low calcium budget makes it difficult to accrue and maintain. So, what to do? It seems that hummingbirds (egg-producing females in particular) obtain their calcium requirements by eating all kinds of mineral-rich materials including soil, sand and fine grit particles, mineral dust and wood ash (e.g., Verbeek 1971, des Lauriers 1994, Adam & des Lauriers 1998, Graves 2007). They typically perch or hover near the ground and lick up these substrate particles by flicking the long tongue in and out. Studies of the materials favoured by hummingbirds show that they are high in calcium, soluble salts, potassium and magnesium (Adam & des Lauriers 1998).
– A specialised diet of nectar also means that hummingbirds have to produce a lot of urine (note that they also drink a lot of water). A lot. In fact, the urine they produce every day might amount to anywhere between 56 and 149% of body mass (Calder 1979, Calder & Hiebert 1983). For comparison, the urine normally produced by a human in a day amounts to somewhere between 1.5 and 2.5% of body mass. This copious peeing means that hummingbirds are forced to expel a significant amount of the electrolytes their bodies contain: studies of Broad-tailed hummingbirds S. platycercus indicate that they need to replace about 14% of their sodium and potassium requirements every single day (Calder & Hiebert 1983). So this might also link to that ingestion of sediment and ash particles discussed above. There’s also a report of a hummingbird drinking sea water from a sheltered bay (Bacon 1973) – perhaps another effort to compensate for electrolyte loss. If you’re wondering, yes, there are various images and videos online of peeing hummingbirds, but to be honest they don’t show anything particularly remarkable.
– Mandibular kinesis (the bending or bowing of the bones of the lower jaw) has evolved a few times in birds (most notably in pelicans and nightjars). I don’t think you’d expect it to be present in hummingbirds: after all, they mostly obtain food by poking their long, slender bills into flowers and licking up nectar with a long, protrusible tongue. But, as we’ve just seen, hummingbirds need to supplement their nectar diet with arthropod prey. Their generally elongate, slender jaws aren’t great for grabbing arthropods, so it seems that they’ve evolved a unique form of mandibular kinesis that allows them to catch insects while in flight. No, they don’t try to grab insects with a tweezer-like motion – rather, they open their jaws wide and scoop the prey into the back the mouth (in swift-like or caprimulgiform-like fashion – hmmm…). As they open the jaws wide, the lower jaw tip bends downwards while its two halves bow outwards (Yanega & Rubega 2004). Jaw flexion thus occurs in two dimensions simultaneously: a unique, and very neat, feature. Incidentally, there are no special joints that allow this extension of the gape – it’s all to do with deforming already flexible bones. Remember that we only know about this amazing flexibility because we can use high-speed video to study live hummingbirds: we wouldn’t know it otherwise (Yanega & Rubega 2004).
For previous Tet Zoo mentions of hummingbirds (and ‘mentions’ are pretty much all that have appeared so far), see…
Refs – -
Adam, M. D., & des Lauriers, J. R. 1998. Observations of hummingbirds ingesting mineralrich compounds. Journal of Field Ornithology 69, 257-261.
Bacon, P. R. 1973. Hummingbird drinking sea water. Auk 90, 917.
Calder, W. A. 1979. On the temperature-dependency of optimal nectar concentration of birds. Journal of Theoretical Biology 78, 185-186.
- . & Hiebert, S. M. 1983. Nectar feeding, diuresis, and electrolyte replacement of hummingbirds. Physiological Zoology 56, 325-334.
des Lauriers, J. R. 1994. Hummingbirds eating ashes. Auk 111, 755-756.
Graves, G. R. 2007. Jamaican hummingbirds ingest calcareous grit. Journal of Caribbean Ornithology 20, 56-57.
Verbeek, N. A. M. 1971. Hummingbirds feeding on sand. Condor 73, 112-113.
Yanega, G., & Rubega, M. (2004). Feeding mechanisms: Hummingbird jaw bends to aid insect capture Nature, 428 (6983), 615-615 DOI: 10.1038/428615a