August 2008

THE SNAIL KITE 
Why it is a 'Critically Endangered Species'

Male snail kite carrying an apple snail

INTRODUCTION

Because the Florida population of the snail kite continues to decline, it is exciting that a small colony is nesting in one of our Refuge impoundments (see Bulletin Board / In the News).

We will learn that the reasons for the kites' current crisis are centered on its feeding specialization. First, the kite (Rostrhamus  sociabilis) is a specialized hawk that eats only apple snails (Pomacea paludosa). And second, the ability of the kite to find and catch snails requires just the right water conditions; it must have open marshes that are not too wet, not too dry, and not too heavily vegetated.

We will see that drainage of The Everglades has reduced snail habitat and breeding success, so it is not surprising that the kite is also in trouble. Clearly we need to proceed quickly to complete Everglades restoration to help ensure the future survival of the snail kite.

A LITTLE GOOD NEWS AND LOTS OF BAD NEWS

The 12 adult kites with four nests on The Refuge this year is good news, but the recent precipitous decline in overall population numbers is very bad news. We do not know how low the kite numbers dropped with the progressive loss and fragmentation of open marsh habitat in the early 1900s, but by the 1960s less than 100 kites remained. Perhaps because most years had average or above average rainfall in the 70s and 80s, kite numbers increased to about 3,000 in the 1990s. But since 2001 there have been seven years of nearly consecutive dry and drought years and kite numbers have dropped to about 800.

We know that these population estimates are accurate because census techniques are now refined and extensively used. Dr. Wiley Kitchen's students and colleagues continue to use air searches of the entire Greater Everglades to locate hundreds of birds equipped with individually coded radio transmitters. And they also search selected areas with air boats to locate all non-banded birds and banded birds with colored bands coded to place of banding. Since the early 90s they have spent many thousands of hours censusing both ways. They have analyzed the resulting data and can estimate survival, reproduction, and movements of the kites. So they can now make statistically sophisticated estimates of total numbers.

HYPOTHESES FOR KITE DECLINE IN NUMBERS

All hypotheses that might explain the continuing decline of snail kites relate to its extreme feeding specialization. We will see how getting enough to snails to eat depends on the life history and habitat of apple snails. And we will see how this in turn affects the kites reproductive success.

THE AVAILABILITY OF SNAILS TO KITES DEPENDS ON WATER CONDITIONS

Apple snail -- about 2 inches (50 mm)
Apple snails may do well if water levels are high but kites cannot find or capture them easily. As water levels rise from August to January, apple snails get harder to see and catch. The reason is that more oxygen is dissolved in deeper and cooler water so snails can use their inefficient gills and need not come to the surface as often to breathe oxygen-rich air with their efficient lung. The availability of snails to kites is even less if water managers keep water levels too high when levels should be decreasing from winter to spring.

Apple snail floating -- easier for kite to catch

High nutrient water from agricultural pollution also makes it hard for kites to find snails. High phosphorus levels favor massive growth of alien floating plants, like water lettuce and water hyacinth, and thick growth of native emergent plants, like sawgrass and cattail. The high density of vegetation makes it impossible for kites to find and catch apple snails. Where floating or emergent vegetation is dense kites do not even attempt to forage.

During extremes of low water snails reproduce very little, so kites do not get sufficient food. Fast and extensive spring dry-downs, in droughts and with human water level manipulation, suppress snail egg-laying and may eliminate it for a year. Adult snails can burrow in the mud and greatly decrease their metabolic rates for many months but will not start to lay eggs again until the next spring. Since it takes nearly a year to grow to adult size this means that snails are not available for kites for a whole year.

KITES ARE MORE SENSITIVE TO PERIODS OF DROUGHT THAN TO PERIODS OF FLOODING

Throughout Florida, annual adult kite survival stayed high among six wet years (92-96 percent) but juvenile survival was much lower and variable (35-65 percent).

With single year droughts, such as in 1992/1993, annual adult kite survival did not change and juvenile survival dropped only 10-20 percent and increased again over the next four wet years. But from 2001 to today, conditions were bad again. Annual juvenile survival dropped and has remained at less than 20 percent. Even adult survival dropped to as low as 70 percent after the 2001 drought. By 2008 total kite numbers had dropped to an alarming low of 800. Without rapid improvement in Everglades restoration the snail kite faces a risk of extinction in about 40 years.

WHAT CAN AND SHOULD WE DO TO SAVE THE SNAIL KITE?

Trapping a sample of snails in a good habitat

We need to follow the mantra of "getting the water right" in quality, quantity, distribution, and timing to help the apple snail and to maximize the availability of the snail to the kite.

To help we must accelerate those parts of Everglades restoration that most help the ecology.  The Refuge is in excellent condition but is too small an area to support enough kites to ensure their long-term survival. 

The problem is that the area of the Greater Everglades has been reduced by half and the remaining marshes have been fragmented by canals, levees, water control structures, urban growth, and agriculture. To correct the problem, flow must be restored and all parts of the Greater Everglades must be reconnected. This will require full Everglades restoration and take a long time.

Beneficial short-term management changes include increasing the heterogeneity of water levels in local areas like the lakes in the Kissimmee basin and in our Refuge. By keeping one impoundment at an optimal water level for snails and kite foraging we have attracted a small breeding colony of kites this year (see Bulletin Board and In the News). By managing adjacent areas at different water levels and with different timings, nesting kites should be able to find snails in different areas at all times and in all years.

With accelerated restoration and wise water management, the incredible adaptations of snail kites should allow them to rebound.

FEEDING SPECIALIZATIONS OF SNAIL KITES

Kite talons gripping an apple snail.
Note the leg band.

Just as bobcats are specialized mammalian predators with stabbing canines plus shearing back teeth and sharp claws, hawks are specialized bird predators with hooked ripping beaks and sharp talons. Most hawks, like our Refuge red-shouldered in dry areas or harrier in grassy marshes, must eat many kinds of prey to survive. But snail kites live in open water marshes and are feeding specialists. More than 99 percent of their diet and food for their young are native apple snails 25 - 65 mm in size.

Snail kites have long and thin curved talons and long legs that help them to catch adult apple snails without getting wet. They mostly hunt by flying slowly over shallow open marshes with their heads down. They are looking for snails at or just below the surface. When they see one they hover and drop to the water surface with their feet extended and pick up the snail with their talons.

Once a snail is caught, the kite carries it to a preferred feeding perch or their nest to extract the snail from its shell. They hold the snail with both pairs of talons and peck at the snail body that is deep inside its coiled shell. Its thin and sharply curved beak allows a kite to reach deep inside the snail shell, pick at the snail body until it gets a good grip, and then slowly stretch the snail's body out until its attachment to the inner shell breaks. Once the soft snail is extracted the kite rips off pieces and eats them or feeds them to its young. The whole operation can take several minutes. Click here to watch a video (For a full-screen experience, click on the double rectangle at the bottom right corner of the video.)

KITES HAVE ADAPTATIONS THAT MAY HELP WITH UNPREDICTABLE AVAILABILITY OF APPLE SNAILS

Generalized adaptations, also seen in other hawks, include two ways of avoiding complete reproductive failure. By laying 3-4 eggs some young will survive. By incubating eggs as they are laid, the first young to hatch is biggest, begs loudest, and gets food first. It is the one least likely to starve if food is scarce but all survive if food is abundant. Kites also have a 6 - 12 year reproductive lifespan and so at least some young are likely to survive the many nesting attempts.

Possible specialized adaptations, not seen in other hawks, include a very long nesting season, colonial nesting, and a lack of territoriality and a lack of sexual size difference. Kites can start nesting any time, between December and July, when adult apple snails become easy to find. Colonial nesting of Rostrhamus  sociabilis may help it to somehow communicate where the best feeding is locally. Lack of territoriality may be explained by the fact that apple snails are rare and widely dispersed and so are not a defendable resource. In addition the sexes are the same size, unlike many other hawks where the larger female can catch different and larger prey than the male. The explanation is that kites eat the same prey of the same sizes.

Both sexes incubate eggs and feed young, so we do not know why the male plumage is so conspicuous. Unlike the relatively drab female, males have dark grey bodies, with bright red eyes, legs, and at the base of their black bill.

To compensate for decreased snail food availability several years in a row, experts hypothesized that adult kites could be nomadic. The Wiley Kitchen team data show that adults do move if apple snails become scarce. However the chances of movement to a better area decreases with distance between suitable patches of habitat. Within a large and continuous area like the Refuge, successful movement to find a better feeding area is high. But movement from the Refuge to Lake Okeechobee is more infrequent since these areas are separated by uninhabitable habitat. And movement from the Everglades to the upper Chain of Lakes south of Orlando is even more infrequent. Despite this, during the 2001 drought a few adults did move from the drier Everglades to the wetter Chain of Lakes where some areas had many apple snails.

JUVENILE SNAIL KITES ARE ESPECIALLY SENSITIVE TO SCARCITY OF APPLE SNAILS

Juvenile kites are at especially high risk of death due to starvation. First, they take two years to learn to efficiently hunt, catch, and eat native apple snails. Second, juveniles tend to stay near where they are raised even if snail prey are scarce. Adults are 4-5 times as likely as juveniles to move when local conditions get bad. For all these reasons, juveniles have much higher mortality rates than adults. On average, in good times adult annual mortality only varies from 6 to 10 percent while juvenile mortality varies from 37 to 65 percent