experimenting with default interface implementation overrides in C#

Main.cs

using System;
					
public interface ISparrow
{
	double GetAirspeed() => 1.0;
}

public class AfricanSparrow : ISparrow
{
	public double GetAirspeed() => 2.0;
}

public class EuropeanSparrow : ISparrow
{
	public double GetAirspeed() => 3.0;
}

public class WestEuropeanSparrow : EuropeanSparrow
{
	public double GetAirspeed() => 4.0;
}

public class EastEuropeanSparrow : EuropeanSparrow
{
	public double GetAirspeed() => 5.0;
}

public interface IArcticPoleSparrow : ISparrow
{
	double GetAirspeed() => 6.0;
}

public class NorthPoleSparrow : IArcticPoleSparrow
{
	public double GetAirspeed() => 7.0;
}

public class SouthPoleSparrow : IArcticPoleSparrow
{
	public double GetAirspeed() => 8.0;
}
					
public class Program
{
	public static void Main()
	{
		var african = new AfricanSparrow();
		var western = new WestEuropeanSparrow();
		var eastern = new EastEuropeanSparrow();
		var northern = new NorthPoleSparrow();
		var southern = new SouthPoleSparrow();
		
		Console.WriteLine(african.GetAirspeed());
		Console.WriteLine(((ISparrow)african).GetAirspeed());
		
		Console.WriteLine(western.GetAirspeed());
		Console.WriteLine(((ISparrow)western).GetAirspeed());			
		
		Console.WriteLine(eastern.GetAirspeed());
		Console.WriteLine(((ISparrow)eastern).GetAirspeed());
		
		Console.WriteLine(northern.GetAirspeed());
		Console.WriteLine(((IArcticPoleSparrow)northern).GetAirspeed());		
		Console.WriteLine(((ISparrow)northern).GetAirspeed());
		
		Console.WriteLine(southern.GetAirspeed());
		Console.WriteLine(((IArcticPoleSparrow)southern).GetAirspeed());			
		Console.WriteLine(((ISparrow)southern).GetAirspeed());				
	}
}

/* output:

2
2
4
3
5
3
7
7
7
8
8
8

*/