Purple Martins Are Predators, Too
By Rick Cruz
Featured Columnist / The Scout Report Online
I do not think there is a boy alive that does not have some strange connection to bugs. Who canít remember seeing a little boy terrorizing some poor little girl with some hapless grasshopper or cricket on schools play ground? For so many children it seems like some right of passage.
As a little boy I was no different. I had 10 gallon fish tanks with breeding colonies of field crickets. I had several ant colonies I started from a single queen that lived for 10 years. As a 36-year-old man I still have a fascination and greater appreciation for insects. To this day not a summer goes by with out a pair of Preying Mantids in separate tanks on my windowsill.
It seems that the sole purpose of the insect world is to feed every other phylum in the animal kingdom. To us martin lovers nothing is more obvious. Purple martins feed exclusively on insects and in numbers we can never really understand. But how many landlords actually notice the variety of insects that martins actually eat? The only glimpse of a martins' diet humans can get is by watching them feed their young. To most of us various species of dragonflies seem to be the special of the day. These large predatory insects find themselves being prey to adult martins actively feeding their brood. But in the greater scheme of things martins are preying on all sorts of flying insects.
Managing a large active colony of martins has been the highlight of all my avian adventures. Getting up close and personal with these birds is a privilege and an honor. This past season I established such a rapor with this bird that I was able to observe several pairs feeding their young with in 5 feet of my face. It started simply as a way to take better photographs but it ended up as crash coarse in entomology. It is hard not to notice the beakfulls of insects at this short distance.
You never know what an adult will bring back to its brood. One Trip may produce a large Green Darner dragonfly, but the next a Japanese beetle. In between single large meals a parent may appear with a bolus of smaller insects that were too crushed up to identify. Still there were many occasions where the masses of insects in the martinsí beak were alive and well. For several days in mid July the martins were bringing in hordes of bean and cucumber beetles. So many that I often observed live beetles escape the grip of the beak only to be shaken off the annoyed martins head. The sheer number of these agricultural pests started to surprise me. Considering this colony is located in wet land area miles away from any large agriculture. It was at this time that plague like numbers of these pests were washing up live on the beaches of lake Michigan along the Chicago lakefront. It was reported that unusual winds from the NorthEast were blowing record numbers of these insects into the lake from Michigan. Once over the lake they would fall into the water and eventually they would blow across the surface in huge writhing masses till they reached land. Many a beach was abandoned due to the sheer volume of their numbers. These beetles eventually spread miles inland and became fodder for many a martin.
Annual cicadas are often referred to Dog day cicada. These large flying insects are yearly appearing versions of what are usually called 17-year locusts. They are more commonly heard buzzing in the trees during the noonday heat of mid to late summer. I saw martins bringing these large cellophane winged insects to their young several times. I found this rather interesting. These insects tend to lay hidden in the upper most branches of large trees. Taking wing is rather risky since birds do relish them so. I canít help but wonder if the martins just came across them, or if they were actively hunting them, by flushing them out of their hiding places in the foliage.
Dragon and Damselfly made up the majority of the martins prey items. Considering the type of location this colony was in it was not a big surprise. Several species were observed. . Amongst the Dragonflies, Green Darner, Clear wing, White Tail and 12 spot skimmers were well represented. The only damselflies I could identify were Ruby Spots and Violet Tails. It was not uncommon to have several different species of these Damselflies in one beak but trying to identify the odd balls was a little too much. These insects reach the height of their annual population just at the time martins hatch their young. It is at this time that active landlords begin to notice the sheer size and amount of insects martin parents are actually feeding to their young.
A dirty little secret that not many of us talk about is that martins do actively hunt butterflies and moths. No insect is more loved then butterflies. They come in such colorful varieties that they are actually collected and mounted like a trophy. No matter how many people love them this family of insects can cause great agricultural damage. Typically butterflies are active during the hottest part of a sunny day. They do not usually fly at the height martins prefer to feed but on occasion some stray into their feeding zone. I witnessed Red admirals, White Admirals, Morning Cloaks, Least Skippers, and European Cabbage butterflies being brought to martin nests. Butterflies are rather hard to identify when in the beak of a martin. They either look like fresh or dried leaves. The European Cabbage butterfly is a serious pest introduced to North America in the 1800s. It was more common to see these being fed to a martin brood.
With such a variety of insects being fed one has to wonder about the nutritional aspects that these insects are providing. I know for a fact that some insects are more nutritional then others. The larva of the Darkling beetle known as mealworms, is higher in nutrition than crickets. It has to do with the amount of soft tissue verses exoskeleton plus the type food the insects eat. Large hard shell beetles like June Bugs tend to be heavily armored. This armor or exoskeleton is made up of a substance called chitin. This is the same substance that makes a lobsters shell so tough. All insects have this but some have more then others. Mayflies for example are quite soft and fragile compared to a Japanese beetle. Chitin is indigestible and is passed out in the birdsí droppings. This is what gives martins dropping a crystallized look to them at times. It may seem strange but what kind of insects martins are eating may play a role in how successful they are at laying eggs and rearing young.
Martins that live near large bodies of water tend to be more successful at reproducing. This may simply be do to a greater variety of insects providing a greater range of nutritional requirements. The diversity of insects is often effected on the diversity of plants, the habitat they are in and the amount of pollution in that area. Mayflies are a wonderful example of a healthy ecosystem. They are often called an indicator species. They need to have clean water free from chemical pollutants. When they flee the water as adults their sheer numbers feed just about every bug-eating creature for miles. On the other end of the spectrum Midges can tolerate the most polluted water one could imagine. Actually water born insects are used to determine the health of a body of water, simply by their ability to tolerate pollution. In the greater scheme of things martins are what they eat. In a highly urbanized setting away from a variety of plants or water you may expect their reproductive efforts to be less then ones in open farm country. This also depends on the type of climate in such settings also. Orlando Florida is going to be a much better habitat for insects then Chicago.
I bring this point up because many landlords feel that they may be doing something wrong since they may notice low productivity with their colony. It may have nothing to do with any of their management or care. It may be something as simple as a low diversity in insects. All creatures need proper nutrition according to their species. And to my knowledge there is nothing known of the Purple martinsí. Their ability to lay viable eggs, and fledge strong offspring, is directly related to their consumption of insect prey. If you happen to live in an area infested with Japanese beetles, the martins maybe consuming more of them then any other insect. For all we know that may be the equivalent of us eating Big Macs everyday. They may fill you up but they are not the best things you could be eating.
Paying closer attention to what your martins are preying upon may help you determine the health of your ecosystem. It may also help you determine the health of your colony. Any way you look at it, will add a new dimension to watching your colony. Get a good field guide for insects and a good pair of binoculars. Practice getting close to you martins till they pay no attention to your presence. Carefully look at the insects they bring in and look them up. It may just help you become a better landlord, or at least give you a better appreciation for your birds.