Island Birds, Flightlessness & Biogeography –

Vocabulary flashcards covering key terms, concepts, and findings related to island bird endemism, biogeography theory, morphological evolution toward flightlessness, and the impact of human-driven extinctions.

Island Endemism

The confinement of species to one or more islands, resulting in unique island-restricted taxa.

Aotearoa/New Zealand Endemics

Bird species found nowhere else but within the New Zealand archipelago, many restricted to offshore or ‘mainland-island’ reserves.

Island Biogeography Theory

MacArthur & Wilson’s model explaining species richness on islands as a balance between immigration and extinction, influenced by island area and distance from source.

Species–Area Relationship

The positive correlation whereby larger islands support more species than smaller ones, all else equal.

Distance Effect

The tendency for islands farther from a continental source to harbour fewer species because of reduced immigration rates.

Immigration Rate

The frequency at which new species colonise an island; highest for near islands and declines as the island species list grows.

Extinction Rate

The rate at which resident species disappear from an island; increases as island communities become saturated or when island area/climate lower carrying capacity.

Equilibrium Species Number (S*)

The point where immigration and extinction curves intersect, predicting the stable number of species on an island.

Gigantism (Island Rule)

Evolutionary trend where small mainland species grow larger on islands due to ecological release and reduced predation.

Flightlessness

Loss of sustained flight ability, common in island birds, often accompanied by morphological changes such as reduced wings and deepened legs.

Rails (Rallidae)

A bird family exceptionally prone to evolving flightlessness on islands, with hundreds of extinct flightless forms in Oceania.

Gallirallus

A genus group of Pacific rails that has produced numerous independent flightless species across islands.

Dryolimnas

Rail lineage on Aldabra showing repeated evolution of flightlessness after colonisation events following sea-level changes.

Simultaneous Wing Moult

Replacement of all primary feathers at once, rendering birds temporarily flightless; lineages with this strategy show higher rates of permanent flightlessness.

Sequential Wing Moult

Stepwise feather replacement that allows continuous flight; typical of most volant birds.

Morphological Convergence

Independent evolution of similar body shapes (e.g., reduced sternum, longer legs) in different flightless rail lineages.

Sternal Keel Depth

Vertical height of the breastbone ridge; shallower in flightless birds due to smaller flight muscles.

Forelimb-Hindlimb Index

Ratio summarising relative flight muscle mass (forelimb) to leg length (hindlimb); low values signal reduced flight capability.

Tarsometatarsus

Long bone of the lower leg; lengthened and widened in many island and flightless birds for enhanced terrestrial locomotion.

Pelvis Widening

Broadening of hip bones that accompanies enlarged leg musculature in flightless species.

Energy Expenditure Reduction

Lower daily metabolic costs observed in island flightless rails compared with volant relatives.

Basal Metabolic Rate (BMR)

Minimum energy requirement at rest; lower in flightless rallids, reflecting energy savings without flight.

Human-Induced Extinction

Species loss caused by hunting, habitat alteration, and introduced predators following human colonisation of islands.

Anthropogenic Extinctions Study (Sayol et al., 2020)

Research showing flightless birds are four times more numerous when extinct taxa are included, highlighting hidden evolutionary transitions.

Island Raptor Richness

Number of predatory bird species on an island; higher raptor richness selects for larger flight muscles in resident landbirds.

Mammalian Predator Presence

Occurrence of introduced mammals (rats, cats, etc.) which influences island bird morphology by maintaining selection for flight.

Pacific Colonisation by Rails

Multiple dispersal waves from Australasia/New Guinea leading to widespread rail radiation and repeated flightlessness across Oceania.

Endemic Species

Species native to and restricted within a defined geographic location, such as an island or archipelago.

Mainland Islands (NZ)

Large fenced reserves on New Zealand mainland that function ecologically like predator-free islands for conserving endemics.

Offshore Islands (NZ)

Small predator-free islands surrounding New Zealand, harbouring many of the nation’s island-restricted birds.

Cook Strait Barrier

Water gap between North and South Islands of New Zealand that separates many endemic bird populations.

Flight Muscle Reduction

Evolutionary decrease in pectoral muscle size as flying becomes unnecessary on islands with low predation.

Take-Off Compensation

Use of longer legs to generate initial lift when flight muscles are diminished, common in island birds with smaller keels.

Species Saturation

Measure of how completely an island’s potential species list is filled; low saturation occurs on remote or small islands.

Source Pool

Set of species capable of colonising an island; its size affects immigration rates and eventual species richness.

Oceania

Vast region of Pacific islands where rails have repeatedly evolved flightlessness and where avian extinctions are extensive.

Nonpasserines

Bird group comprising orders outside Passeriformes; in Oceania many nonpasserine lineages suffered heavy island extinctions.

Passerines

Perching birds; some island passerine groups show leg elongation and reduced keels on predator-poor islands.

Predation Pressure

Intensity of predator threats; reduced pressure on islands promotes evolution towards ground-dwelling and flightlessness.

Convergent Evolution

Independent evolution of similar traits in unrelated lineages facing comparable ecological conditions (e.g., island flightlessness).

Preadaptation

Existing trait (e.g., simultaneous wing moult) that facilitates evolution of a new trait (permanent flightlessness) under changed conditions.

MacArthur & Wilson Model Limitations

Island biogeography predictions based solely on surviving species underestimate true past diversity due to anthropogenic losses.

Species Pool Reduction

Loss of potential colonists (through extinction or barriers) that lowers immigration rates and island species richness.

Wing Moult Strategy

Pattern of feather replacement; simultaneous strategy is linked to higher propensity for evolution of flightlessness.

Hidden Flightless Diversity

Undocumented extinct flightless birds, particularly rails, that mask the true frequency of flightless evolution in avian history.