Food caching is highly specialized.
Food is stored differently depending on how easily it decomposes.
Serotinous cones release their seeds only after a fire. Non-serotinous cones release them at fixed periods.
Species that cache seeds (we’ll talk about birds) have specialized brains and spatial learning abilities to match storage behavior.
Function of seed caching.
Clark’s nutcracker caches 33,000 seeds, survives harsh winter.
Pinyohn jay caches 25,000 seeds, survives in harsh winter.
Scrub jay only caches 6000 seeds, survives mild winters. Cooperatively breed in Florida where offspring help parents raise next offspring.
All of the above are corvids and jays, but the Willow tit is not, caches 500,000 seeds, survives extremely harsh winter.
Clark’s nutcracker has a success rate of remembering 72% in the field.
This is measured by seeing where seed husks exist in location.
In the lab this is hard to test due to spatial scales. Might not replicate cues to store seeds found in the wild.
Vander Wall allowed 4 Clark’s nutcrackers to cache seeds in aviary, only 2 cached seeds. The 2 that cached got 70% of their caches.
Clark’s nutcracker and Pinyon Jays outperform other jays in spatial learning tests.
Using 8 arm radial maze.
Pinyon jays and Nutcrackers learn faster and retain info for longer.
Avian Hippocampal Formation (HF) is the neural basis for seed caching. It is similar to the mammalian hippocampus. There’s evidence for its role in spatial learning in comparative and manipulative studies.
HF for storing passerines have a positive residual size relative to body size. E.g. above average HF size relative to body weight.
Brown headed cowbird females need to remember many nests they are laying their eggs in, parasitizing other females. Their hippocampal volume is much larger than in males, despite volume size being evenly distributed in other species that are not parasitic.
If you lesion HF in black capped chickadees, they search and cache but do not find seeds successfully.
35 day old chickadees were given one trial a day.
Half were given 20 minutes with seeds and cache sites, held for 2 hours after this, then reintroduced in the aviary to find them.
Half were given same time in aviary, but not allowed to cache seeds.
Where’s the interaction here?
Some caching patterns help tree recruitment
- Scatter-hoarding: where no one area has a lot of seeds stored. Spread seeds around allowing them to germinate separately without competition.
Some caching patterns are costly to trees.
- Larders store lots of seeds in a central base. Bad for growth of seeds and bad for trees.
tradeoff between trees and squirrels.
Grey squirrels are scatter hoarders. Acorns want to germinate before being eaten.
- Some germinate in the fall before being dug up and establish a storage root and small seedling (white oak).
Grey squirrels “notch” more than half of the white oak acorns with their teeth to try to prevent germination.
Seed traits affect caching.
Large seeds favor scatter hoarding while small seeds are more likely to be consumed.
Seeds with greater handling time are more likely to be cached.
Thick shell - black oaks
Defensive chemicals like tannins. Red Oak acorns less likely to be eaten than white oaks by gray squirrels.
Spatially synchronized pattern of high seed production every few years.
Predator satiation - low seed years reduces seed predator population, which doesn’t catch up to mast years.
Wind-pollination: seed set is higher in synchronized flowering plants doing it in perfectly opportune times.
Seed-storage ultimately increased.
Vander Wall studied masting and found that squirrels were dispersing seeds further in high production years, and more survived to germination.
Scatter hoarders help recruitment
- Fenced enclosures had fewer Jeffries pine seedlings. Less dispersal by squirrels.
Maple Syrup and Masting
Rapp showed there’s a relationship between syrup and seed production. Need a lot of stored sugars for sap, but also for seeds.
If trees produce a lot of syrup in a given year, they’ll have a lot of sugar to make seeds, but if they produced a lot of seeds last year, they will have little syrup this year. Thus mast years correspond to high syrup years.
Plant defenses may underlie population cycles
Hare populations cycle on a 8-11 year period.
Why does it take hares so long to recover when their densities are low?
Plants may increase defensive compounds, further reducing population size during the crash.
- When pop is large, plants increase defensive compounds, and herbivores decrease in size, and the defensive compounds continue even while herbivore population is low, decreasing herbivore size even more until they are almost completely gone. Then plants decrease defenses and herbivores bounce back.
Alaska and Siberia have the most defended shrubs, along with the most tolerant hares.
Voles and winter grass
Voles feed on winter grass. Silica levels were measured following high and low vole density years.
Voles were fed high and low silica grass.
In highest silica content populations, population crashed. In lowest silica content populations, population increases. Growth rate of adult female voles is higher on low silica diet, and negative on high silica diet.
Plant and herbivore populations coevolve.
shrub defenses evolve in response to long term pressure from hares.
Hares from regions with highly defended plants are more tolerant.
Migrations Vs. Dispersal
Migration, not the same as dispersal. Defined in the following ways. Moving not necessarily to get away from other individuals.
Dispersal - incresae distance between individuals
Behavioral definitioin of migration.
Straightened out directed movement
Inhibition of responses to normal stopping cues.
John Kennedy studied aphids in flight chambers to discover this. Aphids wouldn’t land in response to juicy leaf as stopping cue until they satisfied their migration needs.
Hohenstaufen found that geese really love favorable winds, and will forgo food for them. Stopping cues inhibited.
Physiological definition of migration.
- Accumulation and mobilization of resources to fat for fuel.
Fruit is good fuel.
- More fat storage in fruity diet versus insect diet.
Costs of fat in sedge warblers
- Acceleration and velocity decrease with fat load.
Evolution of migration
Migratory versus Sedentary morphs.
- Garden fleahopper when sedentary doesn’t have wings.
Migratory syndrome (syndrome is collection of traits indicating something)
- Long wings and flight muscles.
- Motivation to fly versus finding a mate.
- Diapause, plasticity, body size, other life history
Polymorphism or polyphenism
Polymorphism is the presence of alternate forms under genetic control. Polyphenism is a difference in forms under environmental control.
Aphids are a good model for studying this.
- During summer most aphids have no wings and are asexual. If leaves are crowded, will produce winged asexual females, if uncrowded, wingless asexual females. In winter, there are almost no leaves, so in the fall they produce winged males and females that reproduce to make overwintering eggs.
What is the origin of migration?
Did birds evolve with homes in the north or south?
- More likely that songbirds are tropical and evolved a northern breeding ground. Northern home hypothesis would require that birds first expanded their range out of glaciers, more steps.
What limits migratory populations?
Factors on breeding range?
Fragmentatipon leading to parasitism
Factors on the wintering grounds?
Factors on the migration route
- Stopover sites lost
Breeding ground: American coot hunt harvests increase with the number of may ponds (temporary ponds where they breed). Pied flycatcher number of offspring per pair is related to increased breeding density.
Wintering ground: in swallow, overwinter survival is positively related to breeding density.
Studies showing an association between breeding ground and population tend to be in north america. Studies showing association with wintering ground tend to be in europe and old world.
Could be that there’s more studies in old world species than new world species.
Desertification in africa might have big effect for wintering grounds.
Another mechanism could be north america has more recent development and urbanization, affecting breeding grounds more.
Deforestation might be less impactful in neotropics because of savannah being good habitat.
Stopover site decrease might have an effect on neotropical organisms
It’s not just monarchs that are insect migrators
Wandering Glider migrates 14-18000 km.
Goes from india to east africa.
Just like birds, do not fly without favorable wind.
Green darner dragonfly goes through New Jersey.
What limits monarch butterflies?
When they leave for breeding grounds in the spring, feed on milkweeds in the gulf states
Overwinter on Fir trees in dense congregations.
Brower is known for viceroy study. Viceroys mimic the monarchs in appearance, protecting them from jays because monarchs taste bad from eating milkweed.
- Overwintering site lets you get an idea of world population of monarchs. It has declined despite protections.
What limits monarch populations?
There’s been a shift in focus from wintering grounds to breeding grounds. A lot of people thought Bt corn may be cause of decline. Probably not major cause of death.
Roundup ready crops have increased herbicides which have killed milkweed that is food for butterflies in their breeding grounds.
Positive correlation between egg production in midwest and size of overwintering population, suggesting breeeding ground is limit on population.
Bt could still have an effect, could be both Bt and roundup.