Lecture+9-+Annual+cycles+and+migration

Annual Cycles

• Birds exhibit changes in appearance, physiology, and behavior throughout the year.

  • Related to resource availability and environmental conditions.

  • Energy investments focused on survival, reproduction, molt, and migration.

• Circadian Rhythm: Biological clock based on photoperiod (day length).

Simple Annual Cycle: Case Study of Pigeons

• Good food + mate present: birds remain in breeding.

• Bad conditions (no mate or mate not in breeding state): birds stay in molt (non-breeding).

• Mate lost or young are independent: initiation of molt.

Molting

• Good food + mate present: birds initiate breeding.

• Insufficient food for breeding: birds remain in molt.

White-crowned Sparrow

• Annual Cycle Events:

  • Autumnal migration.

  • Molting phase.

  • Overwintering strategies.

  • Environmental cues influence transitions to breeding.

  • Vernal (spring) migration likewise influenced by conditions.

Nonannual Cycles

• Some bird species exhibit nonannual cycles.

  • Certain birds delay breeding for several years due to prolonged incubation & rearing, e.g., penguins.

  • Tropical and island birds can nest at varying times throughout the year.

Circadian Rhythm

• Pineal Gland: The location of the biological clock in birds located at the top of the brain.

  • Contains photosensitive cells and controls melatonin release.

• Coordinates with photosensitive cells in the hypothalamus and retina, which also influence melatonin levels.

• Rhythm Frequency: Generally adheres to a 23-24 hour cycle.

• Synchronized by daylight changes:

  • Longer days induce gonadal development.

  • Spring triggers pre-alternate molt and migration.

  • Shorter days signal pre-basic molt.

Hormonal Regulation

• Hormones: Table summarizes types, sources, and roles in birds.

• Corticosterone: Produced by adrenal glands, serves as a stress indicator.

  • Important for ornithological stress assessment in different environments.

Breeding Seasons

• Determined by both proximate and ultimate factors:

  • Ultimate Factors include evolutionary aspects like food timing, nest site availability, and climate.

  • Proximate Factors: Environmental cues that trigger reproduction such as temperature, habitat adequacy, food abundance, and social stimulation.

  • Example: Pinyon Jay breeding is triggered by the presence of pinyon pine seeds.

Breeding Variations: Tropical vs Temperate

• Tropical species breed mainly coinciding with rainy seasons, lasting up to 10 months.

• Temperate species breed around seasonal changes, generally limited to about 4 months.

Migration Overview

• Historical uncertainty among early naturalists about whether birds migrated or hibernated.

• Migration statistics:

  • Approx. 5 billion land birds migrate from Eurasia to Africa.

  • Another 5 billion migrate from North America to Central and South America.

  • Example: Veracruz sees millions of raptors during migration.

Boreal Forest Migration

• Boreal Bird Migration: Annually, 3-5 billion birds move from the boreal forest of Canada to the Americas.

Migration Costs and Challenges

• Migration entails substantial costs and risks:

  • High mortality rates, barriers, inclement weather, and predators.

  • Energetically demanding with potential for delayed breeding.

Types of Migration

• Classified based on food predictability and seasonal variability:

  • Obligate Annual Migrants: e.g., Flycatchers, Hummingbirds, Wood Warblers.

  • Irruptive Migrants: e.g., Pine Siskin, Redpolls.

  • Nodal/Partial Migrants: Some Blackcaps; capacity varies based on environmental conditions.

Migration Strategies

1. Acquiring Energy: Essential for sustaining the journey.

2. Flight Considerations:

   • Depends on phylogeny, habitat range, available diet, and timing of migration.

Decision-Making Factors During Migration

• Influences include:

  • Energy availability

  • Weather conditions

  • Habitat quality at stopovers

• Decisions influenced by species, age, experience, sex, timing, and local conditions.

Migration Observation Methods

1. Direct Observation: Timing and estimating numbers, including observing lighthouse kills.

2. Mark-Recapture Techniques (Banding): Tracking at a population level.

3. Radio and Satellite Telemetry: For collecting position data during migration.

4. Geolocators: For detailed tracking of specific migratory patterns.

Migration Adaptation Strategies

• Flight Types:

  • Diurnal Soaring: energy-efficient for certain raptors.

  • Powered Flight: used by ducks, shorebirds, and other species.

Orientation and Navigation

• Birds utilize multiple cues for navigation:

  • Visual landmarks (rivers, mountains).

  • Stellar positions, sunlight, and Earth's magnetic fields.

• Zugunruhe: Migratory restlessness allowing birds to orient correctly when displaced.