Roosting Behavior Of Recently Fledged Baby Purple Martins
From: Steve Kroenke, Tallahassee, Florida
Date: 6/24/00
Time: 12:17:28 PM
Comments:
Purple martins are one of the few species of birds that will bring their recently fledged young back to their nest to roost at night. Most hole nesting birds NEVER bring their young back once the babies have left the nest. In fact, most of the parent birds never return unless they return later to raise another brood or return the next year. Returning to the nest site with recently fledged young can be a dangerous proposition for both the parents and their young; the dangers from predators and other possible problems increase with more time spent at one location.
Recently Fledged Baby Martins "Grow Up" Fast
Recently fledged baby martins become independent relatively quickly compared to other hole nesting birds. These young martins are fairly good flyers on their first day out of the nest and their flying skills improve each day. Within a week to ten days of learning flying and hunting skills from their parents, many young martins have the speed and agility to capture their own food. After about two weeks out of the nest, most young martins appear to be fairly independent and pretty much on their on. The martin family unit is not as enduring as bluebirds, for example, and soon dissolves after the young become independent. Since martins are birds of the open skies and must fly long distances to hunt for flying insects, young martins develop flying and foraging skills rapidly. Martins do not have a defined foraging territory like most hole nesting birds. Martins are nomadic feeders and forage over many miles to find food. Becoming independent quickly is biologically adaptive to such nomadic species as purple martins.
Most Purple Martins Do It In My Colonies
Even though recently fledged baby martins become independent rather quickly, in all my martin colonies, the majority of parent martins would bring their young back to roost in their gourds/houses at least for a few nights. (Since all my gourds/houses are largely parasite free for the entire season, this no doubt contributes to this high roosting rate.) This roosting behavior by the young varied from as little as one night to as long as eight nights for the ones which I kept records. I believe that some parent martins brought their young back to roost for longer periods. This is a good example of the individual behavioral differences that can exist in martins. On some occasions, parent martins would skip the first night out and then bring their young back to roost for subsequent evenings.
Some Martin Pairs Do Not Bring Their Young Back To Roost
This is an interesting phenomenon and I suspect that these martin parents take their young to trees to roost, particularly for the first few nights. When the young are better flyers, these parents may take their babies to the local pre-migratory communal roost, which contains both adult and young martins. But again, why would they not bring their young back to roost when most other parent martins do? This may simply be a reflection of individual differences in martins. In some of those cases, I would observe both parents, without their young, return for a short time during the evening. They would often enter their nest, but never roosted. Then they would leave. Another possibility is that the young may have been lost to predators or other hazards on that first day out or their nest has been overrun with parasites. These may be some reasons why martin pairs never bring any of their young back to roost in their nests.
Parent Martins For The Most Part Do Not Roost In Their Nests After Their Young Have Fledged
While most of the parent martins in my colonies bring their young back to roost at the colony site, the vast majority of these parents do NOT remain with their young. These parents bring their young back and then before dark leave for the nearby pre-migratory communal roost. Some female martins may remain with their young in large gourds or compartments; I never saw any males remain.
The next morning the parents arrive to "collect" their young and take them to the family assembly sites. These sites are convenient places that parent martins assemble their young. They are usually in tops of dead trees, on exposed tree limbs or on utility lines. The parent martins keep their young at these sites for a short time and feed them here. I have seen these sites anywhere from a few hundred feet to as far as a mile or two from my colonies.
Dangers Of Returning Home To Roost
There are inherent dangers for any bird and its young in returning back to their nest site to roost and birds instinctively know this. The longer any parent birds and their young are at a particular area, the chances for predation naturally increase. Additionally, many nests may become infested with blood-sucking parasites like mites and soiled by fecal droppings; these are a definite threat to the health of any roosting birds. Plus, cavities are hotter later in the season and no doubt not as comfortable for roosting. So, there are biological reasons why most birds and their young would NEVER return back to their nest to continue roosting at night.
So Why Do Martins Do It?
There could be several possibilities why martins bring their recently fledged young back to roost in the nest at night. First, roosting in an enclosed cavity may be safer from certain predators for recently fledged babies than roosting on exposed tree limbs. Baby martins would not be as vulnerable to hunting owls unless the owls had been raiding a colony and were "skilled" at pulling martins out of gourds/house compartments. I have observed on many occasions barred owls swoop into my large colonies and snatch recently fledged baby martins that were roosting on exposed gourd crossbars or house porches; I had bright lights shinning on the colony. These young were completely helpless. The young that had entered the gourds/houses had somewhat more protection.
Second, bringing the young back to roost in the nest cavity may protect them from possible life threatening stormy weather and heavy rains. Recently fledged baby martins would no doubt have a more difficult time "riding out" heavy thunderstorms and the associated winds and rains, particularly during the night.
Third, if all the young are back together at the nest in one location to roost, then the adults would have an easier time perhaps to "collect" their babies in the mornings and take them to the family assembly sites. A centralized roosting site in a familiar territory like the colony/nesting area may help the parents keep track of their recently fledged young.
Fourth, and least likely, parent martins could possibly bring their young back to roost to establish site recognition for future nesting of their young. However, from a biological perspective this may be maladaptive and would encourage inbreeding and future competition for nest sites between the previous young and their parents.
Time Of Day For Returning Back To Roost
Most parent martins in my colonies bring their young back to roost about an hour before dark while there is still sufficient light. Some pairs will bring their young back "way before evening". These young will often be in their nests or in some other nests and the sun is still high. I have seen some parent martins bring their young back in the early afternoons. There are advantages to bringing the young back earlier, as there is less confusion and the parents have more time to try to get their babies in the "right" nest. The longer they wait, the darker it becomes and the more difficult it is for parents to successfully get their young in a nest. This delay increases the chances that the baby martins will roost on exposed gourd crossbars or house porches/roofs.
Bringing Their Young Back To Roost
From what I have observed, parent purple martins may learn to recognize their own young through a combination of sight and most importantly sound when the young leave the nest and there is an interchange of vocalizations between the parents and their flying young. This recognition becomes important at roosting time when many young are in the air at one time and the chances for commingling with different families are great. The parents must be able to lead THEIR young back to their nest and not become confused by all the other babies that may be in the air at the same time.
Parent purple martins go through a somewhat standardized behavioral ritual in bringing their young back to roost in their nests, though variations do occur. This largely involves both parents leading their young from the family assembly site, which could be as much as a mile or more from the colony, back to their nest. Occasionally, one parent will lead some of the young back to the nest, and the other parent will lead the others. The parents, with the male often in the lead, are calling to their baby, which is calling back and following them. If this is the first night to return, then the parents often bring just one baby back at a time, though not always. If the young have been out of the nest for more than a day, the entire family group or several of the young may arrive at the same time.
As the family group approaches the colony site, the parents, which are calling frequently, dive down toward the colony, and their baby, also calling, follows closely behind them. One of the parents will fly directly to their nest gourd/house compartment entrance hole and usually, but not always, enters quickly. If the baby has managed to successfully follow its parent, then the baby "hits" the entrance hole, and tries to enter. The baby usually manages to enter the nest, but not always. Other times, the parent martin will hang on to a gourd entrance hole or sit on the house porch, and the baby will have to navigate around the parent. This situation may cause the youngster to miss its nest and fly away from the colony site. The parents fly "after" their young, calling to it, and the baby calls back. Then both parents and young repeat the process over and over again as the parents try to get their babies back in the nest. Even after the parents manage to get all their young back, some youngsters decide to leave and fly out. Again, the parents fly with their youngster and repeat the process until the baby is back in the nest.
After the parents get one youngster in the nest, then they go back to retrieve their other babies. However, often times all the babies will arrive with the parents on the first trip back to the colony. This situation creates confusion when a large number of young are in the air and trying to follow their parents back to the nest. But the parent martins seem to "know" their own babies, perhaps through unique vocalizations, and they sort them out among the multitudes.
That first trip back to the nest is always the most difficult for the recently fledged young and their parents, particularly for those nesting in gourds, which do not have porches. Houses with porches are somewhat easier for the young to land on. These young have not yet developed the flying skills to maneuver that well, so they are rather inept in following their parents back to the nest on the first few returns.
Often times, the young get in the wrong nest and are scattered among several gourds/house compartments, or they miss the gourd cluster/house entirely and have to be brought back several times. There is usually no problem with this as the parent martins will "collect" their young the next morning and take them back to the family assembly site.
However, if the young have entered the nests of other martins that have eggs or small babies, then these other martins attack the youngsters and do their best to evict them or even try to prevent them from returning to the house/gourd cluster in the first place. If the youngsters land on the gourd crossbars or house porches/roofs, other nesting martins readily attack the babies and try to drive them away. Martins are colonial nesters but NOT one big family. Each martin family is independent of the other martins and "treats" other martins that "trespass" as possible competitors or intruders. These "other" young martins are potential nest competitors next year, so driving them away may be related to territory and intra specific (between martins) competition. Also, martins have no biological interest in young that are not their own.
There is another threat to the recently fledged young that are being brought back to their nests to roost: unmated sub adult (SY) males. These SY males may try to attack the babies and can sometimes force a baby martin to the ground or into thickets. The babies’ father ferociously attacks these SY males and so does the mother, though not at the same level of intensity. Unmated SY males without territory can be problems for established martins and their young. These SY males are seeking territory and mates and may view young martins as potential competitors. Some unmated SY males will readily remove recently hatched baby martins from their nests and try to evict well feathered young in an attempt to take over the nest and break the pair bond of the parent martins. The SY male is attempting to secure a female and territory.
As the young become more proficient flyers, then the parents have an easier time bringing their babies back to the nest to roost. These young learn to follow their parents and enter their nests more efficiently. Sometimes several young will follow their parents back at the same time and will land on the gourd crossbars or house porches. The parents will enter their nest or perhaps an adjacent unoccupied gourd/house compartment and call repeatedly to the youngsters. These young, with better flying skills, will fly to the gourd/house compartment and enter with their parents.
In big colonies, many times, large numbers of young, which have been out of the nest for several days, may start communal roosting on exposed gourd crossbars or house porches/roofs. This situation occurs as the young become more independent and the parents often just bring their young back "to the colony site" and not necessarily to their natal nests. At my two super colonies that I managed, huge numbers of young martins started roosting on exposed gourd crossbars/house porches. These young had been out of the nests for over a week and no longer returned to a specific nest to roost inside. The bright lights that I had shinning on one of my colonies, in a futile attempt to discourage barred owls, seemed to encourage the martins to roost together on the gourd clusters. I would have several hundred youngsters roosting side by side on the gourd crossbars; the bright lights turned night into day. This communal roosting behavior may continue for a week or two or until an owl attacks the martins. A family of barred owls raided one of my super colonies continuously as the young roosted on the exposed gourd crossbars. The owls would swoop in and grab baby after baby off the gourd crossbars. These attacks caused the more independent young to flee into the darkness for their lives. The less independent young remained on the crossbars and were caught in large numbers by the owls. After repeated owl attacks, I turned off the lights at night, and most of the martins quit roosting on the exposed gourd crossbars. It is interesting to note that NO adult martins roosted with these young martins on the gourd crossbars. The adults always left before dark and roosted at the "local" communal pre-migratory roost.
Some Statistics On Roosting Behavior
One year I kept roosting behavior records on the recently fledged young of 8 pairs of mature (ASY) martins that were nesting in gourds. I wanted to see how many nights each of these martin pairs would bring their young back to roost in their nests. Even though these martins were not banded, I feel sure they were the same birds as they returned to the same gourd each evening. There would be no reason to suspect that martins switched nest sites or another pair returned in the absence of pair that nested in the applicable gourd. None of the adults roosted with their young in the following cases.
Here are the results: pair 1, 5 young, 5 nights; pair 2, 4 young, 3 nights; pair 3, 6 young, 8 nights; pair 4, 3 young, 2 nights; pair 5, 5 young, 1 night; pair 6, 4 young, 4 nights; pair 7, 5 young, 6 nights; and pair 8, 6 young, 3 nights. It appears from this small sample and what I have generally observed over the years at my colonies the average time frame that most martin parents bring their young back to roost at night in their natal nests is between 3 and 6 nights. I still believe that some pairs may bring their young back for longer periods of time. More independent young, which no longer roost in their nests, may roost communally on exposed gourd crossbars for longer periods of time.
Purple martins are unique among many hole nesting birds since martins will often bring their recently fledged young back to roost in their nests for at least a few nights. This behavior by many parent martins may be done to help protect their young from nocturnal predators like owls or severe storms. Additionally, bringing their young back to a central location may help the parents keep track of their recently fledged babies. A martin landlord might be able to encourage this roosting behavior by keeping his/her gourds/houses free of parasites.
Steve
