EA 105 – Field Ornithology, Class III

Another informative class tonight, and I’m glad I missed out on last Saturday’s field trip, since it poured rain and they cut it short after only 90 minutes (Sarah & I were enjoying very pleasant weather on the Mendocino coast :).

My notes for the evening follow.

• Sexual dimorphism
  • this is when one gender of a species is notably larger than the other.
  • “reverse” sexual dimorphism is when the female is larger than the male (why exactly is this “reverse” since the usual condition for birds is that both sexes are about the same size?).
  • Raptors and owls are the most prominent dimorphic families
  • Why does this occur? Some theories:
    • Intraspecific nice exclusion (say that three times fast!): differing size results in different diets, allowing for less food competition. Appears to be disproven as a theory since diet examination shows no variance between the genders.
    • Another theory is that having a larger female allows her to sit on the nest for extended periods while the male hunts. Being larger also has better heat retention, perhaps helping egg incubation.
• Identifying Cooper’s Hawks (COHA) vs. Sharp-shinned Hawks (SSHA):
  • Size not very useful except at extremes
  • Both are “reverse sexually dimorphic” (the females are much larger than the males).
  • Female SSHA & male Cooper’s can be very similar in size
  • The Golden Gate Raptor Observatory, which conducts annual raptor counts just north of San Francisco at Hawk Hill, conducted a blind study where their trained observers were asked to identify (without talking among themselves) a bird that had just been banded and released. Their accuracy rate was about 75%, which seems better than it actually was, since they were nearly always right for male SSHA & female COHA, but were wrong fully half of the time for female SSHA and male COHA.
  • Tail shape has been shown to be a poor field mark in and of itself.
• Gulls:
  • Thayer’s Gull
    • Was initially a separate species, but research concluded it belonged in the same species as the Iceland Gull
    • This research was later revealed to be fraudulent, and Thayer’s became a species again
    • Current research suggests that it is actually a subspecies of the Iceland Gull
  • Male & female select their nest site together, with a series of gestures indicating the suggestion and acceptance of a potential site
  • Quite unlike the Oystercatchers, which trigger on the sight of an egg, and exhibit nothing but incubation behavior when presented with one, most gulls trigger on the nest itself, and will not leave it, even when the eggs are moved a few feet away. If the eggs are moved to a distance where the nesting gull can retrieve them without leaving the nest, they will do so.
  • Young are semi-precocial – they’re not born blind and naked, but do not leave the nest and are fed by the parents for some time
  • Gonys spot
    • This is the colored spot on the lower bill of many larger gulls
    • It serves as a target for chicks to peck at. Studies showed that 1) if a chick doesn’t peck at this spot, the parents will not regurgitate food for them, and 2) if there is no dot, the chick will not peck
    • Furthermore, in an experiment with cutouts of assorted shapes and dot locations, chicks preferred the highest-contrast patter (red on white), and ignored dot placement entirely – a dot placed on the “neck” of a cutout actually generated more chick interest, since red-on-white has higher contrast than red-on-yellow!
  • Gonydeal angle
    • Gonys: the place where the 2 parts (called the “rami”) of the mandible meet
    • The gonydeal angle is the angle of this junction, and is useful for ID’ing some gulls
  • Western Gull
    • Is the only breeding gull of the San Francisco Bay, other than the California Gull.
    • Breeds from Puget Sound in Washington south to Southern California
    • is strictly a west coast bird – nearly never seen inland or on the eastern seaboard. Also not found in deep water.
    • The largest Western Gull colony is on the Farallone Islands
    • They nest on the ground on marine terraces
    • Typical clutch size is 3
    • The parent who does most of the incubation develops “brood patches,” bare bits of skin where the eggs come into contact, so heat can transfer more efficiently.
    • Since WeGu typically have 3 eggs, they also have 3 brood patches – this is typical of larger birds
    • Chicks take ~4 hours to hatch, once started.
    • Each Western Gull chick has a unique pattern of spots on its downy head. Studies show that the parents recognize this partern and will reject or even sometimes kill others’ offspring that wander near their nests
    • Western Gulls love dumps, like nearly all large gulls worldwide. Because of this, large gulls are on the rise, worldwide (yay, garbage!).
    • Western Gulls USED to be on the rise in the SF Bay Area, but now are declining. Why? Because nearly all of the bay-side dumps have either been closed or are putting fresh earth on top of their garbage. This has lead to a decline in Short-eared Owls too, since they prey on the rats that frequent dumps.
  • Aging gulls:
    • Gulls have 2-4year plumage sequences before reaching adult plumage (2 for smaller gulls, 4 for larger ones)
    • juvenal plumage (their first full set of feathers) is often all-dark w/light feather tips
    • formative plumage is a partial molt during their first winter (they keep their juvenal flight feathers)
  • • 2nd-year gulls usually have a greyish back with brownish sides (in general)
    • 3rd-year gulls have black dots on their tail, while adults have an all-white tail.
• Synchronous hatching
  • when all of the eggs in a clutch hatch at exactly the same time. Asynchronous hatching is the opposite
  • most birds don’t start incubating their eggs until the clutch is laid, and eggs do not start developing until incubation begins, so most broods exhibit synchronous hatching
  • Western Gulls are partially asynchronous, since they start incubating after only 2 of their 3 eggs are laid.
• The notion that human scent on a bird nest will cause abandonment is totally without merit, especially since most birds (Turkey Vultures notwithstanding) have a poor sense of smell. Some birds will abandon their nests early-on if disturbed by anything, not just humans, and Scrub Jays have been observed to watch researchers and follow them to nesting sites to steal eggs (smart little things!).
• Alcidae (Auks, Murres, Puffins)
  • have rear-mounted feet
  • are the northern-hemisphere analogue to penguins, which exist only in the southern hemisphere
  • both penguins and alcids are diving birds that use their wings as flippers
  • most are a contrasty black and white
  • Penguins and Alcids are an example of what is called “convergent evolution,” a process where two species evolved to inhabit very similar niches, but took completely different evolutionary paths to get there (looks notwithstanding, Alcids and Penguins are not closely related)
  • A key difference between Penguins and Alcids is that the former are flightless, while the latter are not
  • Common Murre
    • breeds along the Northern California coast
    • is the most common alcid, fittingly
    • nests on cliff edges
    • egg is pear-shaped so it rolls in circles, rather than off of the cliff(!)
    • the Farallon Islands have one of the largest colonies of Common Murres
    • They can also sometimes be seen on the northern side of the Point Reyes Lighthouse
    • They nest on north-facing cliffs when possible so they can take off into the wind.
    • they have high wing loading, which is why their flight looks somewhat frantic
    • Hurricane Pt. in Monterey also has a large Murre colony
    • Baby Common Murres look a lot like Xantu’s Murrelet, but 1) are always seen next to an adult Common Murre, and 2) are fairly common, while Xantu’s Murrelets are extremely rare, except in deep water.
    • Murres feed w/their young in SF Bay in August
    • After hatching, chicks run the gauntlet through adjacent gull colonies on the Farallones, where many area eaten, then jump off of cliffs into the water. Their father then escorts them (swimming, since the young cannot fly yet) to feeding grounds