The Nutritional Ecology of Bird Migration

 

I attended a lecture by Dr. Susan Smith on her research related to migrating birds and berry nutrition. Migratory birds spend up to four months a year in the process of migration. These migrations are to wintering grounds and breeding grounds. During these trips, physiological demands are much greater. These trips require very large energy reserves. 79 percent of these reserves come from fats.

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These stores of energy are depleted and then restored at migratory stopovers. At these sites, refueling occurs. This must be rapidly done due to time constraints (must get to wintering ground before winter). Bird’s diets change during migration to be able to rapidly refuel. Birds eat large amounts of fruit. The benefits of eating fruit in place of their normal diet are fruit is high in fat and fruit is easier to hunt. There are limits birds have to eating fruit. One type of limitations are digestive. Bird’s digestive systems can only handle a certain amount of seed load. Also, some fruits contain a certain amount of toxins and the system can handle only so much. Other limitations are nutritional.

Fruit varies in energy and protein content. Some contain up to 40 percent fat, most of these include native species. As fat content increases, energy density also increases. A hermit thrush (Catharus guttatus) (that on average weighs 31.2 grams) would have to eat 18.8 grams of bayberry (Myrica sp.) (high in fat) or 90.7 grams of pokeweed (Phytolacca decandra) to fill the daily energy needs. Eating this much pokeweed is clearly not possible.

 

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A hermit thrush (Catharus guttatus)

 

One of Dr. Smith’s studies tried to see if birds prefer certain fruits during autumn migration time. Nets were placed around berry bushes and the amount that natural falls off was measured. The nets were removed and the amount of berries eaten during different times of the year was measured. The amount of arrowwood (Viburnum dentatum) (high in fat) eaten increased during migration, were as the amount of chokeberry (Aronia sp.) (low in fat) eaten stayed the same.

Another study was done by Dr. Smith using the plasma metabolic profile (blood samples). These samples provide information on metabolic fuel use and energetic condition. Lipid metabolites indicate how mass has changed over the last several hours (showing refueling). Other metabolites provide more information. Plasma was also sampled at two different stopover sites to compare them.

These sits were the Braddock Bay Bird Observatory and Rochester Institute of Technology Bird Observatory. At Bradock Bay Bird Observatory there was 200% more ripe fruit and this was 83% native berries (tend to be higher energy). At Rochester Institute of Technology, there were only 10% native berries. Birds were sampled the same day and same time at these locations. It was found that at Braddock Bay Bird Observatory those birds had higher triglyceride levels. This proves that fruit is helpful in birds refueling.

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Dr. Smith hopes to continue in researching the effects of fruit on birds. One study she wants to do is compare seasonal and site differences in fruits nutritional content. She also wants to look at if the bushes that provide fruit provide a year round value (such as a place to nest in or find insects). She wants to develop fruit fingers prints (light that comes off fruit when you shine a fluorescent light on it). This can be used to quickly tell if a fruit has high or low nutrients. Lastly, Dr. Smith wants to see if invasive species are so integrated into the system that removing then would cause harm to bird by removing a food source.

Passenger pigeon genetics

Unlike other species, passenger pigeons (Ectopistes migratorius) faced a recent extinction. This extinction took place at the turn of the 20th century, the last individual died in captivity in 1914. Based on morphology it was assumed that they belonged in the genus Zenaida (Fulton 2012). This genus consists of mourning doves. When the mitochondrial DNA was analyzed it was found that they are more closely related to Patagioenas which are the New World pigeons. They used mitochondrial DNA because there is a high number of copies of this DNA in the cells when you compare this to the 2 copies of DNA that are in the nucleus. For this reason, it is common to use this for the analysis of ancient DNA.

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Brown-headed Cowbird (Molothrus ater)

The Brown Headed cowbird  (Molothrus ater) is a parasitic bird of North America. They used to follow the bison around and consume the insects associated with them. With the expansion of cattle in North American, the brown-headed cowbird populations have also expanded which has raised some interesting questions. Female cowbirds lay their eggs in the nests of other birds and allow those birds to raise their young. This type of parasitism is called brood parasitism. Because the cowbird is typically larger than these birds their young die in the process of raising the cowbird. Because the cowbird was not tied to a nest this allowed them to follow the bison which was a food rich source. It is unknown if they evolve this strategy to follow bison or were able to follow bison because of this strategy. They also are able to produce more eggs in a season than a typical bird, up to 3 dozen. Bird populations of the birds they parasitize have suffered as a result of this and efforts have been made to try to reduce this. Using feed designed for small birds, not spreading seed on the ground, and avoid feeding sunflower seeds, cracked corn, and millet will reduce the number of cowbirds in your area. It is important to note that the birds are covered by the Migratory Bird Act so it is illegal to remove their eggs from a nest or harm them in any other way without a permit. In some states, if they threaten an endangered species you can obtain a permit to trap the birds.

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Avian eggshell formation

Many birds, more specifically passerines (order Passeriformes), rely on different sources of calcium for skeletal growth and store calcium in their bodies, a necessity for their ability to form strong eggs during the eggshell formation process. Near human settlements birds and in areas with high qualities soils, birds tend to have more access to the calcium that they need. Because of the importance of calcium passerines often ingest snail shells in areas in which the soil is calcium deficient. The most common source of calcium is snail shells followed by calcareous grit, bones, eggshells, and other sources.  Soil can be calcium deficient can be caused by acid precipitation which removes calcium from the soil. Most passerines are unable to ingest their much-needed amounts of calcium due to the scare amounts of nutrients in the habitats that they live in. This results in the development of thin fragile eggs that are unable to hatch thus affecting the bird population in various habitats.

 

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 Burgundy snail (Helix pomatia): a common European species

 

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Brookhaven Birds

Recently I had the joy of going birding with a local group in a small park in Brookhaven, Georgia. We saw over 33 species.

I was surprised that members of the group were excited to see double-crested cormorant (Phalacrocorax auritus). There is a common misconception about them that I have heard mostly due to the increase in their population, particularly on Lake Champlain in Vermont, where I grew up. When DDT was banned in 1972, it opened the door for the cormorant population to expand and it did. On Lake Champlain, the populations went from one breeding pair in 1981 to over 4,500 breeding pairs. Cormorants are piscivorous, meaning that they consume fish. Their population growth raised concerns with fisherman, one of which who told me that cormorants are an invasive species. To further this belief, the state of Vermont in 1999 began to oil the eggs of the birds. This prevents gas exchange and kills the birds in the eggs. The birds prefer to eat fish that are between 3 to 6 inches. On Lake Champlain, it has been shown that most of their diet is yellow perch (Perca flavescens) and alewives (Alosa pseudoharengus). Rarely are trout and salmon found in their stomachs, except when the fish are released annually into the lake from fish stocking facilities.

For more information about Cormorants

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We also saw 17 tufted titmouse (Baeolophus bicolor), which I learned to identify by their classic peter-peter-peter call. They are year-round residents of this area and in most of the Eastern United States. They often flock with chickadees, nuthatches, and woodpeckers. Flocking means that predators are less likely to capture one as more birds are on the lookout for them. Food gathering efficiency is also increased by flocking behavior. In addition, there is reduced inter-species aggression which reduces competition. If you would like to attract tufted titmouse to your property they visit bird feeders and also benefit from nesting boxes. It is important to place a guard around nesting boxes to protect from predators and to place the boxes up well before breeding season.

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Forest Owlet- an endangered bird

Forest Owlet (Athene blewitti) is an endangered bird that is endemic to Central India. They are small and stocky, just 9 inches tall. For comparison, the barred owl (Strix varia) is 19 inches tall. Forest Owlets eat mostly lizards (60 percent of their diet) but will also eat rodents, birds, invertebrates, and frogs. They mate in between October and May in which they are most easily found as they respond to playback calls. Unlike most other owls they hunt during the day. This is likely because their main prey are lizards which come out during the day to bask in the sun which is when they would be most vulnerable to predation. The owlet is however only a morning person as they are not that active after 10 am. They have a few different calls including a hissing call. Their songs are short and mellow which is unlike other owls.

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The bird was first described in 1873 and then not see after 1884 but then rediscovered in 1997. Before rediscovery knowledge of the bird came from 4 collected specimens from very different areas of India. After its recovery, several populations were discovered and appeared to have significant populations, which causes the bird to go from critically endangered to just endangered. Populations are very fragmented and face pressure from shrinking forest sizes. It requires old growth forest, (according to one source and young forest according to another source) which are some of the forests that are most at risk for logging since they have the largest trees. It needs these forests because they contain tree cavities which it uses to nest in. In 2000 5,000 hectares of Forest Owlet habitat was cleared I order to house people who had been displaced by the creation of a dam. Illegal logging is also a problem as people use the area for agriculture. Overgrazing by cattle removes the ground vegetation which in term reduces the populations of the owl’s prey.

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Scientists think there might be 250 or fewer birds left but there is only solid evidence for there being 25 birds. In 2000 a survey using callback recordings found 25 birds including 3 pairs at Taloda Forest Range and 7 pairs at Toranmal Forest Range. In 2004 another survey found 12 adults and 7 fledglings in Toranmal but no birds in Taloda. In 2009 it was determined that one 2 of the original 7 territories. In 2003 it was discovered that the Satpura Range (Maharashtra) is also home to 9 birds, which is about 300 km away from the other known areas. Thankfully Melghat Tiger Reserve contains over 100 individuals. The owl has always occurred in low density, thus putting it at risk for extinction. In addition males will eat their own chicks. It is protected under Schedule I of the Wildlife (Protection) Act 1972 meaning hunting and trapping of the owl are illegal. It is also Included Appendix I of the Convention on International Trade in Endangered Species (CITES), which means that international trade of the bird is illegal.

 

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Forest Owlet (Athene blewitti) range map

 

Effects of piscicides on American Dippers Lecture by Dr. Roarke Donnelly

I recently attended a lecture by Dr. Roarke Donnelly on the effects of piscicides on American Dippers (Cinclus mexicanus). A piscicide is a chemical that can be added to a water body to kill fish and is typically used to remove non-native fish. They are used all over the world in a variety of systems. The American Dipper looks like a songbird but hunts for its prey underwater, they are the only aquatic songbird in North America. They are considered a symbol of western birding and are quite interesting to watch! It consumes benthic macroinvertebrates.

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American Dipper swimming Cornell Lab of Ornithology American Dipper Page

Dr. Donnelly is a professor at Oglethorpe University and recently became a member of the board of the Atlanta Audubon Society. Dr. Donnelly opened his lecture by discussing western trout management. Currently, trout are in need of management for a variety of reasons; habitat loss due to damns, invasives, disease, and pollution. Invasives include Brook Trout (Salvelinus fontinalis) and Rainbow Trout (Oncorhynchus mykiss). While these species are native to Georgia, in Bozeman, MT where this study took place they are considered an invasive. Whirling disease is another challenge faced by native trout and can lead to deformed skeletons.

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