Tern Hunting of Fish
Terns are a group of bird species that hunt fish close to the water surface.Fish have adaptations to avoid being eaten,including cryptic coloration (most fish are dark along the back,even if they are silvery on the sides),habitually swimming at depths below those at which they are in danger from above,and taking evasive action if they see a bird diving toward them.Terns have their own cryptic coloration,most having white or light grey underparts so that they are difficult to see from below the surface,and they dive vertically toward the fish at the highest possible speeds.
Terns also have to learn to adjust for parallax:sunlight changes direction at the air-water interface,which makes fish appear always to be at shallower depths than they really are.This is an especially difficult problem when the water surface is wavy,which makes the image of the fish move around horizontally as well as vertically-as one can see by looking down from a bridge or dock into rippled water. Terns must learn by trial and error to compensate for parallax and dive accurately toward a target that appears to be moving unpredictably.It has been shown that terns' success in catching fish declines with increasing wind speed,presumably because increasing waviness of the surface makes the fish increasingly hard to locate.At the other extreme,terns' fishing success declines again at very low wind speeds,either because the terns have more difficulty hovering to locate the fish in still air,or because the fish can see the terns more easily when the surface is smooth.Except in the most favorable circumstances,terns' fishing success is usually quite low-typically only one fish caught for every three or four dives,with many attempts broken off even before the tern hits the water.
At most times and in most places,terns are unable to catch fish because the fish are swimming deeper below the surface than the terns can dive.Terns' ability to catch fish usually depends on factors that bring the fish toward the surface.Many fishes follow vertical movements of their invertebrate prey,or zooplankton (tiny water- based animals),but these prey habitually come to the surface at night when terns are usually unable to catch fish.Sand eels,one of the main prey species for terns,come to the surface to spawn (lay eggs), and several other fishes spawn in shallow water near the shore. These spawning events provide good opportunities for terns to catch fish,but they are usually localized and transitory.Fish larvae and juvenile fish are more likely to swim near the surface than adult fish, but these prey are often too small to be good food sources for terns.
An important factor that brings larger fish close to the surface is vertical movements of the water caused by currents running over obstacles or through narrow passages.Terns regularly concentrate over tide“races"such as that at Dungeness,England,where currents running around narrow strips of land generate turbulent eddies (circular currents)that bring fish to the surface,or at gaps in barrier beaches such as those of the north Norfolk coast (also in England), where tidal currents running in and out twice each day produce similar eddies.Other sites suitable for tern feeding are where tidal currents run around islands or over shallow rocks,reefs,or sandbars, bringing fish close to the surface at predictable times of the tidal cycle.Roseate terns in particular feed regularly over such places,and in some areas travel long distances to feed at sites where fish can be caught predictably.
The most widespread factor making fish come toward the surface, however,is predatory fish chasing them from below.In many coastal areas,flocks of terns gather over schools (groups)of predatory fish and follow them as they feed,diving frantically in the short intervals when the prey fish are forced close to the surface by the predators pursuing them from below.Many tropical terns habitually follow tuna, bonito,or other predatory fish that hunt in schools.The sooty tern is thought to be dependent for much of the year on tuna and the dolphins that accompany them,ranging widely over tropical oceans and coming together wherever the tuna come near the surface to feed.
Terns are a group of bird species that hunt fish close to the water surface.Fish have adaptations to avoid being eaten,including cryptic coloration (most fish are dark along the back,even if they are silvery on the sides),habitually swimming at depths below those at which they are in danger from above,and taking evasive action if they see a bird diving toward them.Terns have their own cryptic coloration,most having white or light grey underparts so that they are difficult to see from below the surface,and they dive vertically toward the fish at the highest possible speeds.
1.
►Terns are a group of bird species that hunt fish close to the water surface.Fish have adaptations to avoid being eaten,including cryptic coloration (most fish are dark along the back,even if they are silvery on the sides),habitually swimming at depths below those at which they are in danger from above,and taking evasive action if they see a bird diving toward them.Terns have their own cryptic coloration,most having white or light grey underparts so that they are difficult to see from below the surface,and they dive vertically toward the fish at the highest possible speeds.
2.
►Terns also have to learn to adjust for parallax:sunlight changes direction at the air-water interface,which makes fish appear always to be at shallower depths than they really are.This is an especially difficult problem when the water surface is wavy,which makes the image of the fish move around horizontally as well as vertically-as one can see by looking down from a bridge or dock into rippled water. Terns must learn by trial and error to compensate for parallax and dive accurately toward a target that appears to be moving unpredictably.It has been shown that terns' success in catching fish declines with increasing wind speed,presumablybecause increasing waviness of the surface makes the fish increasingly hard to locate.At the other extreme,terns' fishing success declines again at very low wind speeds,either because the terns have more difficulty hovering to locate the fish in still air,or because the fish can see the terns more easily when the surface is smooth.Except in the most favorable circumstances,terns' fishing success is usually quite low-typically only one fish caught for every three or four dives,with many attempts broken off even before the tern hits the water.
3.
►Terns also have to learn to adjust for parallax:sunlight changes direction at the air-water interface,which makes fish appear always to be at shallower depths than they really are.This is an especially difficult problem when the water surface is wavy,which makes the image of the fish move around horizontally as well as vertically-as one can see by looking down from a bridge or dock into rippled water. Terns must learn by trial and error to compensate for parallax and dive accurately toward a target that appears to be moving unpredictably.It has been shown that terns' success in catching fish declines with increasing wind speed,presumably because increasing waviness of the surface makes the fish increasingly hard to locate.At the other extreme,terns' fishing success declines again at very low wind speeds,either because the terns have more difficulty hovering to locate the fish in still air,or because the fish can see the terns more easily when the surface is smooth.Except in the most favorable circumstances,terns' fishing success is usually quite low-typically only one fish caught for every three or four dives,with many attempts broken off even before the tern hits the water.
4.
►Terns also have to learn to adjust for parallax:sunlight changes direction at the air-water interface,which makes fish appear always to be at shallower depths than they really are.This is an especially difficult problem when the water surface is wavy,which makes the image of the fish move around horizontally as well as vertically-as one can see by looking down from a bridge or dock into rippled water. Terns must learn by trial and error to compensate for parallax and dive accurately toward a target that appears to be moving unpredictably.It has been shown that terns' success in catching fish declines with increasing wind speed,presumably because increasing waviness of the surface makes the fish increasingly hard to locate.At the other extreme,terns' fishing success declines again at very low wind speeds,either because the terns have more difficulty hovering to locate the fish in still air,or because the fish can see the terns more easily when the surface is smooth.Except in the most favorable circumstances,terns' fishing success is usually quite low-typically only one fish caught for every three or four dives,with many attempts broken off even before the tern hits the water.
5.
►Terns also have to learn to adjust for parallax:sunlight changes direction at the air-water interface,which makes fish appear always to be at shallower depths than they really are.This is an especially difficult problem when the water surface is wavy,which makes the image of the fish move around horizontally as well as vertically-as one can see by looking down from a bridge or dock into rippled water. Terns must learn by trial and error to compensate for parallax and dive accurately toward a target that appears to be moving unpredictably.It has been shown that terns' success in catching fish declines with increasing wind speed,presumably because increasing waviness of the surface makes the fish increasingly hard to locate.At the other extreme,terns' fishing success declines again at very low wind speeds,either because the terns have more difficulty hovering to locate the fish in still air,or because the fish can see the terns more easily when the surface is smooth.Except in the most favorable circumstances,terns' fishing success is usually quite low-typically only one fish caught for every three or four dives,with many attempts broken off even before the tern hits the water.
6.
►At most times and in most places,terns are unable to catch fish because the fish are swimming deeper below the surface than the terns can dive.Terns' ability to catch fish usually depends on factors that bring the fish toward the surface.Many fishes follow vertical movements of their invertebrate prey,or zooplankton (tiny water- based animals),but these prey habitually come to the surface at night when terns are usually unable to catch fish.Sand eels,one of the main prey species for terns,come to the surface to spawn (lay eggs), and several other fishes spawn in shallow water near the shore. These spawning events provide good opportunities for terns to catch fish,but they are usually localized and transitory.Fish larvae and juvenile fish are more likely to swim near the surface than adult fish, but these prey are often too small to be good food sources for terns.
7.
►An important factor that brings larger fish close to the surface is vertical movements of the water caused by currents running over obstacles or through narrow passages.Terns regularly concentrate over tide“races"such as that at Dungeness,England,where currents running around narrow strips of land generate turbulent eddies (circular currents)that bring fish to the surface,or at gaps in barrier beaches such as those of the north Norfolk coast (also in England), where tidal currents running in and out twice each day produce similar eddies.Other sites suitable for tern feeding are where tidal currents run around islands or over shallow rocks,reefs,or sandbars, bringing fish close to the surface at predictable times of the tidal cycle.Roseate terns in particular feed regularly over such places,and in some areas travel long distances to feed at sites where fish can be caught predictably.
8.
►The most widespread factor making fish come toward the surface, however,is predatory fish chasing them from below.In many coastal areas,flocks of terns gather over schools (groups)of predatory fish and follow them as they feed,diving frantically in the short intervals when the prey fish are forced close to the surface by the predators pursuing them from below.Many tropical terns habitually follow tuna, bonito,or other predatory fish that hunt in schools.The sooty tern is thought to be dependent for much of the year on tuna and the dolphins that accompany them,ranging widely over tropical oceans and coming together wherever the tuna come near the surface to feed.
9.
⬛Terns also have to learn to adjust for parallax:sunlight changes direction at the air-water interface,which makes fish appear always to be at shallower depths than they really are.⬛This is an especially difficult problem when the water surface is wavy,which makes the image of the fish move around horizontally as well as vertically-as one can see by looking down from a bridge or dock into rippled water. ⬛Terns must learn by trial and error to compensate for parallax and dive accurately toward a target that appears to be moving unpredictably.⬛It has been shown that terns' success in catching fish declines with increasing wind speed,presumably because increasing waviness of the surface makes the fish increasingly hard to locate.At the other extreme,terns' fishing success declines again at very low wind speeds,either because the terns have more difficulty hovering to locate the fish in still air,or because the fish can see the terns more easily when the surface is smooth.Except in the most favorable circumstances,terns' fishing success is usually quite low-typically only one fish caught for every three or four dives,with many attempts broken off even before the tern hits the water.
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