by Western Grebes Aechmophorus occidentalis
Western Grebes winter in the coastal marine waters of western North America, where they often form large flocks. Birds disperse from roosting flocks at dusk and begin foraging after sunset. Western Grebes are known to feed on pelagic-schooling fish, primarily herring (Clupea harengus). Observations with a night vision scope, confirm that grebes are solitary nocturnal foragers. This is particularly interesting because it is difficult to imagine how a predator, specializing on schooling fish, can locate and capture such mobile, cryptic, and evasive prey, in complete darkness.
Daytime-feeding birds which feed on pelagic fish, conduct dives in a series of bouts. Most dives are search dives where the bird does not encounter fish. It is likely that as many as 80% of dives are search dives and can be considered an energetic cost of foraging. I suggest that night-feeding grebes save a great portion of the energetic costs associated with search dives, by not initiating a dive unless they know that prey is available in surface waters. By using bioluminescence to track the presence of prey fish in surface waters, grebes do not initiate dives unless prey is nearby.
Grebes forage at night because they feed on fish that vertically migrate. During the day the fish remain at depths approaching 100 meters, but at night they follow their prey, zooplankton, which vertically migrate to surface waters. Since grebes are diving birds they benefit by having their prey closer to the surface. This does not explain, however, how they are able to locate and capture their prey in darkness.
In March and April of 1996, while conducting night observations, I noticed that there was much bioluminescence in the waters. When I encountered fish in surface waters, I could see bioluminescent trails left by individual fish fleeing the approach of the boat. Night foraging grebes appear to be looking into the water. Unlike daytime postures, their neck is held straight and angled forward. I suspect that grebes do not dive until they encounter fish in surface waters. Fish accelerating to flee the approaching grebe leave a trail of bioluminescence that betrays their presence. I suggest that Western Grebes save energy by not conducting search dives at night. By using bioluminescence as a cue to locate prey, grebes do not initiate dives until prey is available in surface waters.
This speculation is supported by research by Hobson (1966) and Hobson et al. (1981) who showed that nocturnally foraging reef fish used bioluminescence to betray the presence of prey. They also observed that bioluminescent illumination is strong enough for a human diver to detect and identify fish underwater. Other researchers have demonstrated the same advantage for fish predators feeding on invertebrate prey (Mensinger and Case 1992, Abrahams and Townsend 1993).
The Western Grebe has several unique morphological characteristics which set it apart from other diving birds sharing a similar foraging niche. The Western Grebe's bill appears to be adapted for spearing fish and the length and structure of its neck allow the sudden forward thrust of the head. These three morphological characteristics appear be correlated traits that allow the bill to functions as a spearing device.
Western Grebes have not been observed spearing prey. On one occasion, however, Bluegill Perch removed from the stomach of a grebe had a puncture wound passing completely through each fish. The only other birds known to hunt in this manner are anhingas and herons, which have similar bills, long necks, and convergent ostiological adaptations in the neck. Western Grebes spend 8 months of the year in coastal marine waters where they feed on schooling fish in the pelagic zone. It is difficult to understand how these birds would use a spearing mechanism to impale pelagic fish since this would require a lateral attack, usually achieved by stalking prey. Pelagic schooling fish are usually captured in active pursuit by diving birds.
It may be that night foraging grebes spear fish from the surface, something like a swimming heron. If they spear fish underwater, it has been suggested that they somehow stalk pelagic schooling fish. It is difficult to imagine how they might do this, but it offers a challenge for future research into the foraging strategy of this mysterious bird.
Nocturnal foraging by Western Grebes was first documented by James Clowater while he was observing the grebes wintering in Saanich Inlet in 1994.You can read more about this research in his Master's Thesis.
Download the Adobe Acrobat pdf file here: