Winter Predator Irruptions

The Snowy Owl is a species that is generally restricted to the far northern limits of the continent where they feed largely on rodents. In most years, they will remain in the far north even through the dark winter months when the sun never rises. They have been shown to move around during these dark months, with one radio-tagged female individual moving from the north slope of Alaska to the Kamchatka Peninsula and back. However, in particularly harsh years where storms are frequent and strong and food sources diminished, members of this species can be forced to move long distances to the south to find areas with suitable food supplies. Just such an irruption is exactly what has been happening across much of the eastern U.S.A. this winter. The polar vortex has made conditions at very high latitudes too difficult even for these hardy bird and they are showing up in unusually high numbers.

These irruptions are commonly viewed as quite unpredictable, but they are actually not as unpredictable as many people think, at least in the eastern parts of the continent. In eastern North America, Snowy Owl irruptions have been documented since the 1880s , and they occur at roughly 3-5 year intervals, mean = 3.9 years.  They are also often synchronized with the irruptions of other northern predatory birds such as Rough-legged Hawks and Northern Shrikes. All three of these species are commonly found in the most northern regions of North American and all three are to some extent specialist predators on small rodents, and so when the populations of small rodents are reduced or made unavailable by deep snow, all of these predators are effected. Irruptions being driven by food availability, and weather conditions often not being the same across North America, irruptions on the western or eastern halves of the continent are often not synchronized.

In fact, western irruptions are generally much less regular than eastern irruptions. This opens the possibility that some individuals could move along an east-west axis until they find favorable conditions. This would allow them to stay in the northern parts of their range. Some individuals probably do this, but what makes some individuals move east-west and other move north-south? No one knows. That is just one of the many questions surrounding large and irregular movements of birds that are irruptive or nomadic.

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Happy Darwin Day from ABirdingNaturalist!

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Abnormal Plumages

Yesterday, as I was driving along Harbor Blvd. in West Sacramento, I saw a small group of House Sparrows coming down to the street. They were feeding on something they were finding along the gutter. By itself, this would not be a particularly interesting sight. House Sparrows are everywhere in West Sacramento as they are in practically every urban area in North America, and seeing a small group at this particular location is pretty normals as well. What drew my eye on this occasion was the fact that one of the birds had a white head! As I drove closer, I saw that its head was not completely white, but instead had a bit of a dark line over its eye. It also had a white patch on each wing and a bit of white in the tail (maybe one feather on each side). These white feathers were, as far as I could tell, pure white, and the other feathers on the bird had normal House Sparrow coloration. This got me thinking about plumage variations and the associated causes and terminology.

Bird pigmentation, and the terms we use to describe that pigmentation, can get a little complicated, but it goes something like this. Albinos are individuals that have a genetic mutation that prevents them from depositing any melanin anywhere in their body. This results in individuals that have red eyes, and are generally white on their bodies. However, albinism only effects melanin production, so birds that use other molecules as pigments, such as carotinoids, may still have coloration. They either look yellow-ish, orange-ish, red-ish, or have parts of their body that have yellow, orange, or red feathers while the rest are white. In birds, there are two types of melanin: eumelanin and phaeomelanin. Albinism occurs when both types are completely absent. When only eumelanin is present birds will lack the warmer chestnut and buff tones and so will appear plainer, paler, and greyer. When only phaeomelanin is present birds will lack the color black, and also have generally paler wing and tail tips. In birds that only black, such as crows, this results in a completely white bird. When both types of melanin are present but only at low levels, birds will appear to have pale, washed out versions of their usual plumage. This is referred to as a dilute plumage.

Leucism is a genetic mutation that affects all the pigment producing cells. Fully leucistic individuals are completely white, often confused with albinos, but they still have dark eyes (pigments cells in the eye are derived from a different line of cells than pigments cells in the rest of the body). Partially leucitsic individuals have certain patches that have been effected by this mutation, and so are white, and other parts of the body that have not been effected, and so are normally colored. This ‘pied’ look is what I saw in that House Sparrow, so that bird is partially leucistic.

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Phylogenetic Vocabulary

As a graduate student, teaching biology labs is a regular part of my week and the lab that I teach most quarters is on phylogenetics and biodiversity.  Recently I have been having discussions about phylogenetic terms with my students, fellow TAs, and the staff and professors who are in charge of the class.  Terminology often gets confusing in phylogenetics, and words are sometimes used to mean different things by different people.  One of the facets that makes these terms extra confusing is that they are not mutually exclusive and depend on the groups being discussed, so for example, a trait can be both a synapomorphy and an autapomorphy (see below) depending on the groups being examined.  So, in an attempt to clear things up in my own mind, and so hopefully be able to teach them more effectively, here are some commonly used terms in cladistics with accompanying definitions and explanations.

Apomprphy – A derived character state.  This is anything that is an innovation along an evolutionary linage.  So anything that is different from the ancestral character state.  For example, within the phylum Chordata, the evolution of a vertebral column, which is something lineages that branched off earlier in Chordate evolution do not have and so is new in the Class Vertebrata, would be an apomorphy.

Synapomorphy – A shared, derived character state.  This is an apomorphy that two taxa share and that is assumed to have been present in the common ancestor of those two taxa.  An example would be feathers in birds.  All birds have feathers, and it is assumed that they have feathers because the common ancestor to all birds had feathers and passed that characteristic down through the generations.

Plesiomorphy – An ancestral character state.  This is any trait that was inherited from the ancestor of a group.  For example, reptiles are exothermic, they do not maintain a constant internal body temperature.  They have this characteristic because the ancestor of all reptiles was exothermic.  This differs from a synapomorphy because some descendants of the first reptiles are not exothermic (birds are endothermic).  In other words, this trait is ancestral, but is shared by some, but not all, of that ancestors; descendants.

Symplesiomorphy – A shared, ancestral character state.  This is any trait that was inherited from the ancestor of a group and has been passed on into more than one descendant lineage.  To carry on with the example for a plesiomorphy, the fact that crocodiles and turtles are both exothermic, but

Autapomorphy – A derived trait that is unique to a particular taxa.  These are not useful in determining how groups are related since only one group will have the particular trait.  However, these are extremely useful in identifying taxa.  For example, feathers only occur in birds.  This makes the character “feathers” and autapomorphy for class Aves.  The character “feathers” is also a synapomorphy for taxa within class Aves.  Raptors and songbirds both have feathers and they inherited them from a common ancestor.

Open circles = ancestral character state, filled circles = derived character state.

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