Department of Biology
Birds lay eggs. Some species of mammals, fish, reptiles and amphibians lay eggs too, but in all these classes of animals there are species among them that give birth to young that develop inside an adult female, also known as “live” young. Of all the closely related animals with backbones, birds (and jawles fish) are the only group that never gives birth to live young.
So why do birds only lay eggs? One reason may be that it would be too energetically costly for a flying animal to carry her developing embryo all the way to birth. Egg-laying birds can ovulate and lay an egg within 24 hours; the female does not carry the egg inside her oviduct for long. If young birds developed to the size of a chick inside the female oviduct, she would have to fly for two. Picture a sparrow with a belly the size of a walnut – not very aerodynamic! However, other flying animals, like bats, do carry their developing embryos internally for up to seven months. Therefore, the cost of carrying developing offspring may not be the only reason birds lay eggs.
An alternative explanation for birds only laying eggs is that eggs allow male birds to participate (spend energy) in parental care. Females that bear live young spend a lot of energy providing nutrients for the young during development (e.g., pregnancy) and often after birth (e.g., nursing). During this process, males are “off the hook,” energetically speaking. Once a female bird lays an egg, care of the egg is no longer restricted to just the female; the male can incubate too. This incubation opens the door for increased parental care by the male, allowing the female some free time during the developmental process to take care of her own energy demands. For female birds, the increased energy that would be needed to fly with embryos and provide nutrients may be too high an energetic cost to bear alone. Thus, egg laying seems like a good option for a female bird.
For bird eggs to hatch, they must receive care from the adult birds. Without adult care, embryos inside the eggs will not develop. Adults typically incubate, or warm, the eggs with their body heat. Many birds develop brood patches an area of skin on the belly and breast where birds lose or pull out their feathers and where blood vessels swell. The bare, vascularized brood patch allows the adults to transfer body heat to the eggs. In hot and dry environments birds will squat above eggs to shade and cool them. Some birds will even soak their bellies in water and then cool the eggs with the water from their feathers. By warming and cooling eggs as needed, adult birds create the appropriate environment for embryo development.
Egg Care Can Be Dangerous
Parental care is not restricted to regulating egg temperature. Adults must also protect the eggs from predators. Raccoons, foxes, snakes and other birds will eat eggs if they find them. When nests are in danger, adults will protect themselves and the eggs in a variety of ways. For instance, many birds will attack the predator, even if it’s a mammal, by dive-bombing it and calling with alarm and shrill vocalizations. And although it might not sound very scary to be attacked by a little bird that weighs about as much as a dry kitchen sponge, believe me, it is! I know one homeowner who had a mailbox under a tree housing a gray kingbird nest. She would carry an open umbrella to pick up her mail and protect herself from the daily attack. Aside from a direct attack, birds will also use whatever they’ve got inside, aiming fecal material and vomit at the predator. This tactic can be very effective, especially when used by birds that feed on fish. As if fish don’t smell bad enough in the first place! But perhaps the most innovative nest defense maneuver is used by a species that nests here on Long Island. Piping Plover parents will feign an injury by dragging a perfectly good wing on the ground and leading the predator away from their nest. Then, when the predator is far enough away from the eggs, the adult flies away.
Avian parental care behaviors are complex and necessary for the successful hatching of eggs and the raising of young. These complex behaviors are affected by major physiological changes that take place within the bird as well as cues from the bird’s environment. Have you ever tried to catch a bird by hand? Unless you are fast, like Allen Iverson fast, you probably are not fast enough. The bird will fly away before you have taken your first step. However, an incubating bird is different. I have picked up incubating birds right from the nest. Biologists in Hawaii walk up to incubating Albatrosses, take a blood sample, mark the bird, and then put it back on the nest. The hormonal changes that convert a flighty, quick animal to a languid, immobile incubating parent are powerful. These hormonal changes are responses to environmental cues such as day-length, mate presence, and the feeling of eggs on the brood patch. However, human activities are changing the nesting environment of many bird species, and the physiological and environmental cues that drive parental care are being affected. I am interested in the mechanisms that stimulate parental care behavior in birds and how the changing environment may impact these mechanisms and, consequently, the ability of birds to successfully breed and sustain their populations.
Physiology and Parental Care
I became interested in avian parental care while I was studying the reproductive physiology of White Ibises in the Florida Everglades. White Ibises are large white birds with dramatic red faces and down-curved bills. Classified as a wading bird, it is a long-legged bird that feeds and nests in wetlands. The Florida Everglades are filled with many species of wading birds: Great Egrets, Snowy Egrets, Tri-colored Herons, Little Blue Herons, and the prehistoric looking, endangered Wood Storks. Although there are many species of wading birds in the Everglades, White Ibises are the most numerous and often respond most dramatically to changes in the ecosystem. For this reason, they are considered a sentinel species for the Everglades. In the early 1900s, White Ibises formed colonies of hundreds of thousands of birds, but like all the wading birds in the Everglades, their recent populations have been declining. Over the past 100 years there have been stretches of time during which a few hundred ibises would try to breed in the Everglades, but then there have been other years with no breeding ibises. It seems this erratic nesting behavior might be the result of a larger problem within the Everglades ecosystem.
In the hope of gaining insight as to which environmental factors were affecting their breeding, I studied the reproductive physiology of ibises. Aside from the declining numbers of nesting attempts, I observed that the ibises were abandoning their eggs, which is contrary to their normal behavior. In wild bird populations, individual birds must tolerate an incredibly wide range of environmental conditions. Wild birds are faced with frequent storms, constant harassment by predators, human disturbance of nesting areas, and the physiological challenge of finding enough food to care for themselves with enough energy left over to provide care to eggs or young. However, in the Everglades, ibises were uncharacteristically abandoning their nests every time there was a big rain storm. For those of you who have ever visited south Florida, you know that these storms can be torrential, but Florida ibises had withstood rainstorms for centuries, without giving up on nesting. Considering that in the past ibises have been seen trying to incubate floating eggs from nests that were destroyed after a hurricane, this seemed like an inappropriate and new change in parental behavior.
Mercury is a major health risk to animals (including humans), and it has been a problem in the Everglades for many years, showing up in alligators and fish. The major source of mercury in the Everglades ecosystem is trash incinerators across the Florida peninsula. When trash is burned, mercury is released into the atmosphere and then deposited in wetlands (wet lands) during rain storms. Bacteria in the wetland soil convert the rain-deposited mercury to a biologically available and toxic form of methyl-mercury. It is this form of mercury that is ingested by animals, such as insects, fishes, or White Ibises. Because the effects of methyl-mercury are magnified as it moves up the food chain, top predators, such as ibises (or humans that eat fish from the Everglades), receive a higher dose of mercury than insects or fish.
While conducting my research, I captured White Ibises in the Everglades to document their physiological changes during the nesting season. I drew small blood samples from individual birds to measure their hormone levels and clipped small bits of feather to see if birds were ingesting harmful pollutants. I found a correlation between high mercury levels in White Ibis feathers and low levels of progesterone, one of the important hormones that stimulates parental care. These results suggest that birds exposed to mercury no longer have the physiological mechanism (adequate levels of the hormone progesterone) in place to promote parental behavior (e.g., nest attendance). This hypothesis seemed to explain the high rate of nest abandonment observed in the Everglades. Moreover, our research team found additional corroborative evidence to suggest that high mercury levels were affecting White Ibis nesting in the Everglades. My collaborator from the University of Florida had collected feathers to measure wading bird mercury exposure in the Everglades over the past eight years. His results seemed to indicate that in years with high mercury concentrations in the Everglades, there were fewer ibis nests. However, this evidence was only for a small number of years, and in wild populations there are many factors other than mercury levels that could have affected numbers of nesting ibises.
Therefore, I developed a more controlled laboratory study with Zebra Finches to test my hypothesis that mercury exposure reduces progesterone levels, which decreases nest attendance. Some Zebra Finches were exposed to low levels of mercury similar to the exposure of wading birds in the Everglades, and others were not. The birds were allowed to breed, and I monitored their parental care with video cameras. As I predicted, the Zebra Finches that were exposed to mercury spent less time incubating eggs and had lowered hatching success. The male birds also had different progesterone levels compared to control males. These results were similar to the relationship between mercury and progesterone in wild ibises. This laboratory evidence supports the field findings and provides some interesting insight into how exposure to low levels of pollutants can impact bird populations by altering the physiology that sustains the behaviors necessary for successful reproduction. In other words, pollutants can have negative effects on bird populations without killing adults, but by interfering with the hormones that control bird behavior. The good news is that in the past few years, incinerators have put scrubbers on smoke stacks, and mercury levels in the Everglades have decreased. However, it will take years for the mercury that is already in the soil to be out of the system, and long-lived animals such as alligators, wood storks, and even White Ibises may still be affected.
Nesting Environments and Parental Care
One of the most interesting bird species on Long Island is the Piping Plover. Piping Plovers, like many Long Islanders, spend their summers at the beach. Plovers frequently build their nests on the edges of the dunes at the beach. Shortly after Piping Plover chicks hatch, they travel by foot with the adults to foraging areas near the nest. Even though Piping Plover chicks can’t fly, they can still move around enough to hide in beach vegetation or walk to muddy intertidal foraging areas. Female Piping Plovers lay four speckled eggs, and male and female birds take turns incubating. Nesting on the ground does not give much protection from predators, like foxes or raccoons, but these birds have evolved behaviors and cryptic coloration to avoid being detected. Piping Plover eggs, chicks and adults are all hard to see because they are colored like the sand. If a predator (or a human) does get too close, the adult bird will quickly run away from the nest and display the broken wing behavior described earlier. Piping Plovers may also benefit from the anti-predator parental behavior of another species that nests in colonies on the beach, the Least Tern. Piping Plovers frequently nest near Least Tern colonies. When a predator gets too close to a tern colony, all the nesting terns band together and dive at the predator, driving it away from the colony (and any nearby plover nests).
The number of predators on Piping Plover eggs and chicks has increased right along with the increase in human population. Raccoons and gull populations thrive because of food supplies provided by sloppy human trash disposal. However, the No. 1 human associated predator that affects Piping Plovers and all other birds is cats. Following habitat loss, predation by either domestic cats that people let outside or by feral cats that are fed by humans, is the No. 2 reason for bird declines in North America. To give Piping Plovers a fighting chance against this new (in evolutionary time) suite of predators, beach managers often place fence-like exclosures around nests. The exclosures are 2.5 meters high and 4 meters across and are made of wire mesh with a netted roof. Exclosures work because although plover adults can come and go by passing through the fencing holes, larger predators can’t get in. A common question people ask me while I am conducting my research on the beach is, “how do you get the plovers to nest inside of the cages?” But this isn’t the case. Once plovers begin nesting, crews of stewards walk along the beach and look for Piping Plover nests. This can be hard because as I already said the adults try to lead you away from their eggs! Once a nest has been found, the plover stewards set up the exclosure, and the adults quickly return to their nests. After that, their progress is monitored but steps are taken not to disturb the pairs. Many animals respond to humans as if they were predators, and this constant predator threat can cause reproductive failure. To further decrease human-plover interaction, beach managers put up string fences and post signs to prohibit driving on the beach.
The plover habitat is in high demand. More than one-half of the population in North America lives on the coast. It is estimated that by 2015 there will be 165 million people living on the coast. In addition, humans interrupt natural processes that occur in coastal habitats, such as by attempting to stabilize barrier islands that inherently move, or by attempting to prevent over-washing and the breeching of beaches to protect homes. Since 1965, the U.S. Army Corps of Engineers has conducted 800 nourishment projects on the East and Gulf Coast and has added 3,333 tons of dredged sand to keep beaches wide and to provide a place for humans to sunbathe and play Frisbee. Because so few coastal environments have been preserved, these managed beaches are also the only places where Piping Plovers can try to hatch eggs and raise their young.
My students and I have been studying the environmental factors that affect Piping Plover reproduction and parental care. Last summer, undergraduate students Ashley Clark and Brugisha Patel examined whether vegetation, weather and other physical characteristics affected plover nesting success. Brugisha found that plovers nesting in vegetation did not do as well as plovers that nested on the bare sand, and her results were similar to those from other plover studies. When we consider the strategies plovers use to protect their eggs, this result may not be surprising. Because plover eggs blend with the sand and the adults lead predators away with the broken-wing display, they may do best when they can see predators coming. Ashley found that male and female plovers switched incubation duties more often when the weather was cold. This strategy probably allows the adults to forage more often and maintain enough energy to keep active and incubate. She also confirmed that when plovers spend less time on the nest, because of foraging trips, disturbance from predators or humans, etc., they have lowered hatching success. Graduate student Annie McIntyre is exploring the effects of creating foraging habitat on plover nesting success. Habitat restoration projects that create suitable places for plovers to find food may help their reproductive efforts. For example, the closer the foraging area is to a nesting area, the less energy birds expend to get there, and their journey is less risky. This year graduate student Paul Doherty will be investigating how human and predator disturbance affects plover parental care. Because plover nests are in exclosures, predators are not able to access eggs, but they can pace around the outside of the exclosure. Adult plovers may respond to this predator activity with a physiological stress response (much like stress responses humans have) and, as a result, they may stop caring for their eggs.
Parental care is the primary behavior that ensures reproductivesuccess and population growth in birds. When birds cannot reproduce, their populations decline, and as populations decline, the threat of species extinction increases. Human activities can affect the hormonal and behavioral mechanisms that promote parental care via environmental contamination, habitat changes, predator introduction and/or competition for space. As we better understand the underlying mechanisms that control how birds care for their eggs and young, we also increase our understanding of how we can minimize our negative influence on these processes.