Terry Suchma
03-23-2006, 09:23 AM
Specialized Behavior May Affect The Species' Survival
A paper by Rick Cruz
The Purple Martin is probably one of the most unique birds in the New World. Their continuous vocalizations, graceful flight, and colonial nesting even attracted them to Native Americans long before Europeans arrived. Today they are still the most sought after bird for backyard bird lovers.
Yet the Purple Martin has a major Achilles heel, or two, or three.
In my opinion, it is too specialized.
In this day and age specialization can lead to extinction. I know this sounds alarmist, but, history has shown modern species that are too specialized can not adapt fast enough to a changing environment. This eventually interrupts their reproductive capabilities until their numbers dwindle beyond the hope of recovery. I feel that The Purple Martin stands at this threshold.
What makes a martin so specialized? The Purple Martin’s entire life cycle. From start to finish this bird’s life faces a list of difficulties to overcome in a modern world.
DEPENDENCY ON MAN
Before the arrival of Europeans in North America, Native Americans began supplying nesting gourds that encouraged martins to nest in close proximity with Man. Europeans built bird houses and within the next centuries all Purple Martins, East of the Rocky Mountains, now nest only in human supplied housing. Abandoning their former cavities in trees would have been necessary as forests were cleared for farming. This dependency for nest sites leaves them at the whims of Man. Sheer ignorance of their housing needs and passive management leads to increased predation. Over time, fewer people provide housing and their numbers dropped.
NESTBOX PLACEMENT SELECTIVITY
Purple Martin Housing that is placed in an open area of at least 40 feet in diameter and placed within 100 feet of a human dwelling is considered very important martin attraction requirements. This almost excludes any human home within a standard city or suburban lot or yard. People on the outskirts of towns and cities seem to have a better chance of attracting them.
SPECIALIZED DIET
Martins feed exclusively on flying insects. If this is not specialization enough, they prefer to feed at a higher altitudes than other swallows, taking larger insect prey than their cousins. This feeding technique requires martins to spend more time away from their nests, leaving their eggs and young more vulnerable to predation. This diet is also susceptible to inclement weather. If temperatures drop below 45 degrees Fahrenheit or persistent rain continues for more than three days, martins will die of starvation and hypothermia. So, if a prolonged cold snap occurs at the height of nesting season, wide-pread loss of life and nest abandonment can be expected.
REPRODUCTIVE CAPABILITY
They may not be able to overcome natural attrition. The goal of any creature is to replace itself, so over the course of a martin’s life span, if two offspring survive, a martin couple has done their job. There are obstacles to this goal. Martins only lay one clutch of 2-7 eggs per season. If this clutch is lost for some reason or another, an attempt at another might not pan out. Due to a breeding cycle of 70 days, there usually is not much time left before migration nears. This long nesting cycle also provides that the young remain in the nest longer, leaving them more vulnerable to hazards. So, one pair of martins only gets one chance per season to succeed. In comparison, American Robins and Eastern Bluebirds fledge 2-3 clutches in a season.
COLONY BREEDING SPECIES
The martins’ nesting and breeding cycle concentrates and isolates the breeding population, making it more vulnerable to predators and reproductive failures. A large single concentration of birds staying in one location for an extended period of time, attracts predators much the same way a waterhole does on the African plains. Once predators are attracted to a food source, and succeed at attaining a meal, they will return. Many martin colonies have been abandoned due to predation.
Unfortunately, martins also tolerate non-native cavity nesters in their colonies. Ultimately, these competitors, if left alone, will cause a gradual decline in the martin numbers, until the colony is vacant.
MIGRATORY
Being aerial insectivores, martins must migrate to warmer climates for food in winter months. Migration by nature is extremely taxing on physical needs, requiring extensive use of energy reserves. This tests the limits of their endurance, so many of them perish along their journey. Since martins migrate to Brazil, they have a 10-14,000 mile round trip to survive, including a possible trip across the Gulf of Mexico without a bite of food. It stands to reason the longer the trip, the greater the risk, hence, the higher mortality. It is accepted that adult martins have a yearly 50% mortality rate from one year to the next while the young of the season have 75% mortality. If left to all these predispositions, the Purple Martin could probably hold its own, but they face tough competition that sets back any natural gains they make.
NON-NATIVE COMPETITORS
Martins have immense competition with which they cannot compete. Two species of non-native cavity nesters were introduced by man at the turn of the century on to this continent with devastating results for many nesting birds.
The European Starling is one-third larger than an adult martin and is more aggressive and competitive. When it comes to selecting a nest site, starlings are not picky. They will take any cavity in any place and at any height. I have seen them nest in dryer vents, street lights, gas grills, garage eaves, exhaust pipes, any nook or cranny from 6" to 50' above ground. If the cavity is occupied, they will evict and/or kill the owner. Starling reproduction ability is extremely competitive. A clutch of 4-6 eggs will be easily replaced. Unlike martins, the starlings can produce a second brood potentially doubling their reproductive capability. Starlings can nest in loose colonies, or as individual pairs. This reduces the chance that predators could single-handedly knock out an entire colony.
These birds are also extremely aggressive. They will steal, drive off, or kill competitors for a nest site. They intentionally kill and destroy nestlings and eggs of competitors, even if they are not nesting in that particular cavity. Many martin nests are destroyed in this manner. Starlings have no equal when it comes to acquiring a nest site. They prove this by their sheer numbers. Starlings are typically non-migratory, moving only slightly from the coldest areas of their range. They only have to survive the rigors of winter to breed again . They acquire cavities months before migrant cavity nesters return.
Their diet is one key to their survival. They will eat anything. In winter they frequent garbage cans of restaurants, fast food places, and bakeries. On residential garbage days, they peck through plastic bags for table scraps. Misguided individuals toss bread and table scraps to them, helping them to survive the toughest winters. They glean farm fields, pick insects from your lawn, catch insects on the wing like fly-catchers, and even pick through droppings of horses and cattle. This kind of adaptability allows starlings to thrive anywhere in large numbers, even in the toughest environments. The starling is a generalist, preferring nothing special in any phase of their life. This is just what is needed for a quickly changing environment. The English Sparrow or House Sparrow, a.k.a. Weaver Finch is another cavity nester introduced by misguided bird lovers of the last century. These little marauders were greatly responsible for the decline of the Eastern Bluebird. They do not look like much of a threat. With their tiny size and innocent appearance, they delight many who fill bird feeders year round. Do not let their appearance fool you. This species is an aggressive competitor. They can make a nest almost anywhere and do so quickly. They will nest in any cavity available; drain pipes, abandoned hornets' nests, abandoned Robins nests, evergreen trees, even porches of a closed up martin house. Their reproductive capabilities are impressive. They lay a clutch of 4-6 eggs, 3-4 times a season. There is even reason to believe a female from the first clutch can possibly breed by the end of the same calendar season. This gives the House Sparrow an incredibly fast way to increase its numbers. They will eat just about anything. The seed heads from your unkempt front lawn, insects, table scraps, and bird seed placed by well meaning bird lovers. This diet allows them to overcome the most severe winters with little loss of life. If uncontrolled, their numbers grow at an alarming rate and they slowly replace most of the native birds at bird feeding stations. If allowed to co-exist with martins, they will intentionally destroy their eggs and young while the parents are out foraging. Any observant person will notice this, but many do not or refuse to accept this fact.
Purple Martins have many challenges facing them throughout their life cycle. Many of them are natural and Purple Martins have instinctive ways of coping with them, but these challenges combined with a human-altered environment will eventually spell disaster. Sparrows and starlings thrive in close proximity to Man. They go about their life cycle without human intervention and succeed in producing many more young than Purple Martins ever will. Ironically, human intervention on the martins’ behalf will only help them, but, it takes extremely dedicated individuals to accomplish this. So as time goes by, fewer and fewer of these individuals can be found. Just ask anybody if they have seen a Purple Martin lately. They won't have a clue. Ask that same question 10-40 years ago and somebody would know. Remember the Passenger Pigeon! They numbered in billions, yes, billions at one time. They were known for blocking out the sun for hours. They, too, were specialized birds that required large tracts of mature stands of oaks for food and colony nesting. The forests were felled and the birds hunted. They never recovered. The last one died alone in the Cincinnati Zoo in 1914. Many people today possess only their parents’ or grandparents’ memory of the Passenger Pigeon.
Let's do what we can to prevent this from happening to Purple Martins.
Rick Cruz is the Vice President of The Purple Martin Society, NA and is a featured columnist for the Society newsprogram, The Scout Report Online.
A paper by Rick Cruz
The Purple Martin is probably one of the most unique birds in the New World. Their continuous vocalizations, graceful flight, and colonial nesting even attracted them to Native Americans long before Europeans arrived. Today they are still the most sought after bird for backyard bird lovers.
Yet the Purple Martin has a major Achilles heel, or two, or three.
In my opinion, it is too specialized.
In this day and age specialization can lead to extinction. I know this sounds alarmist, but, history has shown modern species that are too specialized can not adapt fast enough to a changing environment. This eventually interrupts their reproductive capabilities until their numbers dwindle beyond the hope of recovery. I feel that The Purple Martin stands at this threshold.
What makes a martin so specialized? The Purple Martin’s entire life cycle. From start to finish this bird’s life faces a list of difficulties to overcome in a modern world.
DEPENDENCY ON MAN
Before the arrival of Europeans in North America, Native Americans began supplying nesting gourds that encouraged martins to nest in close proximity with Man. Europeans built bird houses and within the next centuries all Purple Martins, East of the Rocky Mountains, now nest only in human supplied housing. Abandoning their former cavities in trees would have been necessary as forests were cleared for farming. This dependency for nest sites leaves them at the whims of Man. Sheer ignorance of their housing needs and passive management leads to increased predation. Over time, fewer people provide housing and their numbers dropped.
NESTBOX PLACEMENT SELECTIVITY
Purple Martin Housing that is placed in an open area of at least 40 feet in diameter and placed within 100 feet of a human dwelling is considered very important martin attraction requirements. This almost excludes any human home within a standard city or suburban lot or yard. People on the outskirts of towns and cities seem to have a better chance of attracting them.
SPECIALIZED DIET
Martins feed exclusively on flying insects. If this is not specialization enough, they prefer to feed at a higher altitudes than other swallows, taking larger insect prey than their cousins. This feeding technique requires martins to spend more time away from their nests, leaving their eggs and young more vulnerable to predation. This diet is also susceptible to inclement weather. If temperatures drop below 45 degrees Fahrenheit or persistent rain continues for more than three days, martins will die of starvation and hypothermia. So, if a prolonged cold snap occurs at the height of nesting season, wide-pread loss of life and nest abandonment can be expected.
REPRODUCTIVE CAPABILITY
They may not be able to overcome natural attrition. The goal of any creature is to replace itself, so over the course of a martin’s life span, if two offspring survive, a martin couple has done their job. There are obstacles to this goal. Martins only lay one clutch of 2-7 eggs per season. If this clutch is lost for some reason or another, an attempt at another might not pan out. Due to a breeding cycle of 70 days, there usually is not much time left before migration nears. This long nesting cycle also provides that the young remain in the nest longer, leaving them more vulnerable to hazards. So, one pair of martins only gets one chance per season to succeed. In comparison, American Robins and Eastern Bluebirds fledge 2-3 clutches in a season.
COLONY BREEDING SPECIES
The martins’ nesting and breeding cycle concentrates and isolates the breeding population, making it more vulnerable to predators and reproductive failures. A large single concentration of birds staying in one location for an extended period of time, attracts predators much the same way a waterhole does on the African plains. Once predators are attracted to a food source, and succeed at attaining a meal, they will return. Many martin colonies have been abandoned due to predation.
Unfortunately, martins also tolerate non-native cavity nesters in their colonies. Ultimately, these competitors, if left alone, will cause a gradual decline in the martin numbers, until the colony is vacant.
MIGRATORY
Being aerial insectivores, martins must migrate to warmer climates for food in winter months. Migration by nature is extremely taxing on physical needs, requiring extensive use of energy reserves. This tests the limits of their endurance, so many of them perish along their journey. Since martins migrate to Brazil, they have a 10-14,000 mile round trip to survive, including a possible trip across the Gulf of Mexico without a bite of food. It stands to reason the longer the trip, the greater the risk, hence, the higher mortality. It is accepted that adult martins have a yearly 50% mortality rate from one year to the next while the young of the season have 75% mortality. If left to all these predispositions, the Purple Martin could probably hold its own, but they face tough competition that sets back any natural gains they make.
NON-NATIVE COMPETITORS
Martins have immense competition with which they cannot compete. Two species of non-native cavity nesters were introduced by man at the turn of the century on to this continent with devastating results for many nesting birds.
The European Starling is one-third larger than an adult martin and is more aggressive and competitive. When it comes to selecting a nest site, starlings are not picky. They will take any cavity in any place and at any height. I have seen them nest in dryer vents, street lights, gas grills, garage eaves, exhaust pipes, any nook or cranny from 6" to 50' above ground. If the cavity is occupied, they will evict and/or kill the owner. Starling reproduction ability is extremely competitive. A clutch of 4-6 eggs will be easily replaced. Unlike martins, the starlings can produce a second brood potentially doubling their reproductive capability. Starlings can nest in loose colonies, or as individual pairs. This reduces the chance that predators could single-handedly knock out an entire colony.
These birds are also extremely aggressive. They will steal, drive off, or kill competitors for a nest site. They intentionally kill and destroy nestlings and eggs of competitors, even if they are not nesting in that particular cavity. Many martin nests are destroyed in this manner. Starlings have no equal when it comes to acquiring a nest site. They prove this by their sheer numbers. Starlings are typically non-migratory, moving only slightly from the coldest areas of their range. They only have to survive the rigors of winter to breed again . They acquire cavities months before migrant cavity nesters return.
Their diet is one key to their survival. They will eat anything. In winter they frequent garbage cans of restaurants, fast food places, and bakeries. On residential garbage days, they peck through plastic bags for table scraps. Misguided individuals toss bread and table scraps to them, helping them to survive the toughest winters. They glean farm fields, pick insects from your lawn, catch insects on the wing like fly-catchers, and even pick through droppings of horses and cattle. This kind of adaptability allows starlings to thrive anywhere in large numbers, even in the toughest environments. The starling is a generalist, preferring nothing special in any phase of their life. This is just what is needed for a quickly changing environment. The English Sparrow or House Sparrow, a.k.a. Weaver Finch is another cavity nester introduced by misguided bird lovers of the last century. These little marauders were greatly responsible for the decline of the Eastern Bluebird. They do not look like much of a threat. With their tiny size and innocent appearance, they delight many who fill bird feeders year round. Do not let their appearance fool you. This species is an aggressive competitor. They can make a nest almost anywhere and do so quickly. They will nest in any cavity available; drain pipes, abandoned hornets' nests, abandoned Robins nests, evergreen trees, even porches of a closed up martin house. Their reproductive capabilities are impressive. They lay a clutch of 4-6 eggs, 3-4 times a season. There is even reason to believe a female from the first clutch can possibly breed by the end of the same calendar season. This gives the House Sparrow an incredibly fast way to increase its numbers. They will eat just about anything. The seed heads from your unkempt front lawn, insects, table scraps, and bird seed placed by well meaning bird lovers. This diet allows them to overcome the most severe winters with little loss of life. If uncontrolled, their numbers grow at an alarming rate and they slowly replace most of the native birds at bird feeding stations. If allowed to co-exist with martins, they will intentionally destroy their eggs and young while the parents are out foraging. Any observant person will notice this, but many do not or refuse to accept this fact.
Purple Martins have many challenges facing them throughout their life cycle. Many of them are natural and Purple Martins have instinctive ways of coping with them, but these challenges combined with a human-altered environment will eventually spell disaster. Sparrows and starlings thrive in close proximity to Man. They go about their life cycle without human intervention and succeed in producing many more young than Purple Martins ever will. Ironically, human intervention on the martins’ behalf will only help them, but, it takes extremely dedicated individuals to accomplish this. So as time goes by, fewer and fewer of these individuals can be found. Just ask anybody if they have seen a Purple Martin lately. They won't have a clue. Ask that same question 10-40 years ago and somebody would know. Remember the Passenger Pigeon! They numbered in billions, yes, billions at one time. They were known for blocking out the sun for hours. They, too, were specialized birds that required large tracts of mature stands of oaks for food and colony nesting. The forests were felled and the birds hunted. They never recovered. The last one died alone in the Cincinnati Zoo in 1914. Many people today possess only their parents’ or grandparents’ memory of the Passenger Pigeon.
Let's do what we can to prevent this from happening to Purple Martins.
Rick Cruz is the Vice President of The Purple Martin Society, NA and is a featured columnist for the Society newsprogram, The Scout Report Online.