Cooper's Hawks Predating Purple Martin Fledglings

[Steve Kroenke]

Cooper?s Hawks Predating Purple Martin Fledglings

It is ?easy meat? time for the resident Cooper?s hawks in our area of northwest Louisiana. Like clockwork, the Cooper?s hawks suddenly appear when the purple martins in my neighbor?s (Bob) colony and mine start fledging their young. Just like in Africa when the wildebeest cows drop their calves during a specific time frame and the lions, cheetahs, leopards, wild dogs and hyenas come for a feast! The surrounding tree canopies, utility lines and air space are filled with recently fledged martins which are being hunted daily by a large mature female Cooper?s hawk and her smaller mate. Both hawks ignore the swift adult martins and focus on the more vulnerable fledglings. The parent martins scream in terror and launch wave after wave of mobbing attacks on the hawks, but they ignore them. Martins don?t strike a raptor and only emit a harsh vocalization when they strafe the predator, though they may come within inches of its body. However, this mobbing behavior is functional: it warns other martins that a predator is on the prowl, keeps the hawk in view, and may distract the hawk to some extent. But I have never seen this mobbing behavior frighten these hawks away and sometimes nearly a hundred martins are attacking with repeated attacks and the raptors just lazily fly along looking for vulnerable fledglings. In fact, the mobbing may indicate to the hawks that food is nearby.

Between both our colonies we have in excess of 250 pairs of martins which are producing over 1000 fledglings. The first martins fledged in late May and many more are doing so during the first three weeks in June. Most should be out of their nests by the end of June with a few late broods fledging in July. So the area is being saturated with ?meat on the wing? for the Cooper?s hawks and they are having an easy time finding plenty to eat.

I am seeing several different attack strategies employed by these hawks and I have observed this hunting behavior at my previous martin colonies in Tallahassee, Florida. The most terrifying one is the high altitude approach. The big female mainly does this. She will come over at several hundred feet and make no effort to hide her approach. She wants to create terror for the martins. As she approaches our colonies, a huge flock of martins is mobbing her and screaming in terror. She ignores them and is looking for the prize. Her profile is sleek and wings curved back and somewhat pointed. And when she sees a fledgling, she then goes into action. She knows the difference between the flight behavior/vocalizations of an adult martin and a fledgling. It makes no difference if the fledgling is sitting on a utility line, in a tree or flying low down or high up, the hawk goes directly for the youngster and nearly always makes a kill. A fledgling martin is no match in flight speed with a Cooper?s hawk.

The most exciting attacks are on high flying fledglings that are being protected by numerous parent martins. The hawk behaves almost like a giant flycatcher and twists and turns in the sky as the fledgling tries to out maneuver this incredibly agile raptor. A huge spot after a tiny speck! The hawk?s long tail opens like a fan and functions as a rudder to give maneuverability while her short rounded wings allow for quick turns. Both hunted and hunter will sometimes spiral toward earth in a circuitous pursuit while the parent martins are screaming and attacking the hawk in almost suicidal mobbing. It is a frenzy of screaming martins, a frantic fledgling and a hungry determined hawk. The mobbing does no good and invariably the fledgling is caught if it is has not yet developed sufficient stamina and speed to evade this powerful, fast hawk.

I have also watched this large female hawk drop like a comet from the cosmos with her wings against her side and try to take a fledgling martin sitting on utility line. The young martins are so helpless and often just sit there until the hawk is not more than a few feet away. And the parent martins are hysterical and twist and turn with the hawk as they try to divert it from their baby. I managed to stop one attack in my front yard on a single fledgling that was sitting on a nearby utility line. My entire martin colony erupted in bloodcurdling screams of terror as I rushed out into the front yard to see the female Cooper?s hawk in a full stoop toward the fledgling. The hawk flared to grab the baby and I charged forward, clapping my hands and screaming at her. She must have almost had a coronary at the presence of her most fearful enemy and flew at top speed to a woodlot several hundred yards away. About a dozen martins overtook and mobbed her until she disappeared into the forest.

The other attack method is to find fledglings either alone or together at purple martin family assembly sites in tree canopies or on utility lines. The male hawk employs this strategy all the time and simply flies about looking for groups of fledglings at their family assembly sites or solitary young martins perched in a conspicuous location. He does not fly high but low down, through the trees or just above them. When he finds a victim, he nearly always scores a kill, just like his mate does. The parent martins scream and mob him constantly and track his movements through the tree canopies or across open areas. He ignores their mobbing.

Nearly all the observed hawk attacks are occurring within a quarter mile radius of our martin colonies and the hawks usually avoid coming too close except on just a few occasions and I have succeeded in thwarting their efforts every time. Both hawks are terrified of me and flee when I confront them. The big female will actually turn away when she is even diving at a high rate of speed toward our colonies if I rush into the yard and face her. However, there is no guarantee my passive efforts of deterring her attacks will continue to be effective and she may start making bold assaults directly inside our colonies in spite of our presence.

However, there are probably many more fledglings being caught by the hawks away from our colonies. For every one I see or believe has occurred, there are most likely many more successful attacks.

When purple martins fledge their young, it is NOT a quiet affair. It is noisy, highly animated and nearly always accompanied by extra martins, particularly SY and some ASY males which will make direct attacks on the fledglings. After the young have landed in a tree canopy or utility line, the parent martins are constantly flying about, vocalizing and defending them against the SY males. This is NOTICED by Cooper?s hawks and functions as a signal that food is available. I have watched these hawks go directly to a grouping of martins that were circling trees where fledglings are located.

I am also noticing that the parent martins are now trying to bring their young back to their nests at all times during the day, possibly as a protective measure for their fledglings. Sometimes after a hawk attack, several parents will try to get their young back in the gourds. This just creates chaos at the colony as the young often can?t get in the ?right? nest and just end up getting mixed up with other nest bound broods or sitting on the crossbars. Other martins will attack and attempt to drive the fledglings away as ONLY the parents feed their fledged young. A martin colony is not an aggregate of genetically related members like lionesses in a lion pride. Lionesses will nurse each others cubs because all the females are biologically related.

I am trying to notate each visit/attack by these Cooper?s hawk to determine a predation mortality index. So far, the hawks usually catch a fledgling every time they try unless I am able to intervene at the colony site which I have done several times and chased them away. Here are some samples of what has happened so far and it will probably continue at this pace until all the martins have fledged and left the area:

June 8?At around 6:05 am, the male hawk flew low to the south of my colony and he left upon seeing me.

At around high noon, the female hawk made a spectacular dive toward our colonies from hundreds of yards high up and I intercepted her and she fled.

At around 6:00 pm, I saw a dramatic aerial attack by the female hawk on a fledgling high over an open pasture with major mobbing by numerous martins. The martins were coming within inches of the hawk it seemed as they screamed in terror. The attack spiraled down to around 10 feet from the ground at a tree line where the hawk grabbed the youngster and its death rattle triggered a massive aerial, but failed mobbing assault by many martins.

June 9?At around 7:40 am the female hawk came from the northeast at several hundred feet above our colonies and I greeted her between our yards. She turned and went directly north where a grouping of fledglings were sitting in a dead tree. She folded her wings against her sides and plunged downward in a twisting dive while numerous adult martins pursued her to almost ground level. I am almost certain she caught one of the fledglings.

At around 8:50 am, she returned from the west (from Texas) and crossed a wide pasture. She was heading directly for an oak tree located several hundred yards from my colony which had some martin fledglings in the top canopy. There must have been a hundred martins attacking the hawk as she climbed above the tree and circled it, looking for the fledglings. The screams of terror coming from the martins were so loud that they seemed electronically amplified. Martin after martin attacked the hawk which then dropped down and appeared to grab one of the fledglings. This action caused the entire group of parent martins to mob the hawk which carried her victim across the pasture to another oak tree. Using my binoculars, I could see the hawk sitting in the top of oak. A scissor-tailed flycatcher was making repeated diving attacks at the hawk. Then the hawk carried the prey deeper in the woods.

At around 10:00 am, my colony erupted into a screaming mass of martins that climbed rapidly; Bob?s martins joined in the flight upward. They were like fighter pilots climbing to meet the enemy and engage her in battle! Coming from the southwest at several hundred yards high up with her wings curved back was the deadly female Cooper?s hawk. This is a hunting profile and usually signals a spectacular stoop. But I was standing directly under my colony and clapping my hands loudly. These hawks have fantastic eye sight and she obviously saw me and kept on going with a mob of martins on her.

At around 11:15 am, she appeared high over a woodlot several hundred yards to the northeast of our colonies. She was heading directly to area where she most likely caught a martin fledgling earlier in a dead tree. It seemed as though every martin in our colonies was after her and she ignored them. She circled over and over again the area as if she was looking for a victim. The poor martins were screaming at top volume and making repeated mobbing attacks. Then she dove down in a spiraling maneuver with numerous parent martins screaming and following her. I believe she most likely got another fledgling though I am not certain.

At around 3:40 pm, the martins in Bob?s colony exploded from their nests and streaked directly north. Coming at a slow pace about 50 feet off the ground from the north and flying down a fence line that has some sweet gum trees growing along it was the female Cooper?s hawk. She ignored the martins? mobbing and continued straight for about a 25 foot tall sweet gum tree that had some fledglings in its top canopy. I ran all the way from my yard and tried to reach Bob?s colony site in time, but the hawk streaked downward, caught a fledgling, and flew briefly to the ground. His entire colony was now a boiling witches? cauldron of screaming martins! The hawk, laden with her kill, flew across another pasture about 200 feet from Bob?s colony with a pack of martins attacking her.

June 10?At around 8:00 am the male hawk flew nearby and my presence kept him going.

At around 10:30 am, the female hawk made an awesome but unsuccessful attack on a single fledgling that was perched on a utility line near my martin colony. My martins went completely berserk and were screaming in unison. The hawk had dropped from a great distance like a meteorite and tried to snatch the fledgling from the utility line. But I fortunately managed to charge the hawk as she was flaring up and this greatly frightened her. She fled at top speed with her wings beating furiously to a woodlot as many martins pursued and mobbed her.

At around 6:00 pm, she made a brief appearance from the northeast while Bob and I were guarding our colonies and she quickly fled the area upon seeing us. Martins from our colonies chased her into the woods and continued to fly over the tree canopies and followed her as she melted farther inside. The martins will track the movements of Cooper?s hawks as long as they can see them flying from one tree to the other.

June 11?At around 7:00 am the male hawk made an unusual (for him) high flight appearance to the east of our colonies. I greeted him in my front yard and upon seeing me he quickly disappeared to the southeast. I saw no more hawk visits near our colonies.

June 12?At around 7:00 am the male hawk flew directly from the west (from Texas) and headed for same big oak tree where his mate had caught a martin fledgling several days earlier. I saw him fly up into the tree. Some martin fledglings were again assembled in the top canopy. The martin parents and other martins were screaming at top volume and mobbing the hawk which ignored their dive bombing. I couldn?t see the final outcome, but I heard the death rattle scream of a caught martin fledgling and the hawk had apparently taken its prey to ground temporarily. Then in a few minutes the hawk apparently flew up and flew back west and I could hear the screaming martins following him.

At around 8:00 am, the male hawk again appeared from the west and flew over the area where he scored his last kill. NO martin fledglings were assembled there this time! He kept on going to the east of our colonies as a flock of martins mobbed him.

June 13?At around 6:00 pm the male hawk flew nearby and I chased him away. No other nearby visits that I saw by the hawks.

June 14 and 15?No Cooper?s hawks seen near our colonies though I saw swarms of martins circling various woodlots on several occasions. One of the hawks was probably checking the area for martin fledglings.

June 16?At around 4:50 pm the male hawk flew to the north of Bob?s colony and was pursued by numerous martins. I didn?t see any evidence of predation but there were some fledglings at family assembly sites in the tops of dead trees in that area.

At around 7:50 pm the male hawk returned and caught a martin fledgling along the tree line to the north of Bob?s colony. The fledgling was caught near the top of a sweet gum tree and the mobbing attacks of probably 100 martins did not stop the hawk.

June 17?At around 6:15 am the male hawk attacked some fledglings that were just dispersing from Bob?s colony, caught one and carried it to a woodlot several hundred yards away. Within 20 minutes the hawk re-appeared with the butchered martin in its talons and carried it across an open pasture toward a forested area to the west of our colonies. Martins mobbed the prey-laden hawk. I suspect these Cooper?s hawks have nestlings or fledglings nearby and they are being fed young martins.

At around 12:50 pm, the big female Cooper?s hawk flew slowly to the west of Bob?s colony and was about to dive on a ground of fledglings sitting on a utility line. A mob of screaming parent martins strafed the hawk. I intercepted the hawk and chased her away before she could make her attack. She climbed high over our colonies and was mobbed by a huge flock of martins. She disappeared to the west.

At around 7:25, one of the hawks returned and was being tracked by a huge flock of screaming martins flying high over a woodlot located several hundred yards to the southwest of my colony. These hawks have caught martin fledglings in a large pine tree that is somewhat isolated from the main forested area. I have seen the hawks chasing fledglings around this location, usually at low altitudes. I was never able to determine if the hawk scored a kill this time.

I plan to continue keeping a record of these Cooper?s hawk attacks and seeing if the pace of predation will remain the same, continue to grow or perhaps diminish or terminate. Since the predation is occurring AWAY from our colony sites so far, the martins are still very calm and continue to behave ?normally?. However, the martins will immediately react violently to the presence of the Cooper?s hawks the moment these predators are seen and this is often at great distances from our colonies. Also, the local kingbirds and scissor-tailed flycatchers do a great job in emitting alarm vocalizations sometimes before the martins are aware of the danger. Both these tyrant flycatchers are extraordinarily vigilant around their nests/territory and will mob various raptors.

I believe that these hawks are possibly catching a minimum of three martin fledglings a day. Since we will have about 1000 martin fledglings at various stages of flying prowess over the next four or five weeks, it is possible that the Cooper?s hawks can predate around 100 of these youngsters. This would represent about a 10 % mortality rate from Cooper?s hawk predation. Mortality for martin fledglings is very high and possibly 75 % of these young die the first year. Predation is just one factor and many of the fledglings will die from other causes, particularly during migration to South America to spend the winter and then on the trip back to the United States and Canada.

There is one possible ?benefit? to having a large fledgling prey biomass available to the Cooper?s hawks. This greatly minimizes the predation impact on the parent martins. The hawks concentrate on the easy catches and this reduces the chances that a parent martin will be taken. The loss of a father or mother martin with young to feed is more detrimental than a fledgling.

Predation is mainly an emotional issue for the human landlords and I certainly am impacted by it. I will continue to try to intervene and thwart the Cooper?s hawks when they attack at my martin colony site. However, this predation at my new colony is Louisiana is MUCH DIFFERENT than that which occurred at my previous site in Tallahassee, Florida. Here, my location is completely open and the parent martins ALWAYS see the Cooper?s hawks in plenty of time to escape or attack. Though these hawks can develop effective strategies for taking adult martins at open sites by diving at great heights on the colony to surprise escaping martins or landing on the housing to flush the birds out, the martins still have a better chance of escape. In Tallahassee, my colony was surrounded by woodlots and the hawks could dive down and box the martins in and force them into a dogfight at low levels; this usually favors the Accipiters.

Cooper?s hawk predation on martin fledglings is probably occurring in many colonies throughout North America and sharp-shinned hawks and merlins also participate in their breeding ranges. Many martin landlords probably never know it because they are not there at the time of the attacks or the predation occurs outside the colony boundaries. Martin fledglings that haven?t developed full flying skills are easy prey for bird eating raptors when they are at their family assembly sites, flying around the area, or coming to roost in the evenings.

Steve Kroenke

[Laverne]

and these are the reasons such a small percentage of HY PMs return next season as SYs...

This is the natural cycle of life. There are hawks making attacks on fledglings at my colony, too. I had a projected maximum number of fledglings at 183 (not 1000s). The hawk is an efficient, very successful predator. The hawk does not kill PMs for fun or thrills or because it loves to watch them panic - it is what sustains their lives. The hawk is not being cruel.

And remember the predator plays a vital role in "the survival of the fittest". The PM fledglings that reach South America are the cream of the crop. They will no doubt feed a few more predators while in the Winter Range and then they will come back next season in tip top form and quadruple their numbers again...