我如何解决apple mach-o链接器错误？ Xcode 6.4

Question

每次我插入这两行代码时，都会收到此错误消息，我根本不明白为什么。

   NSTimer *BirdMovement;
NSTimer *TunnelMovement;

这是整个脚本

#import <UIKit/UIKit.h>

int BirdFlight;
int RandomTopTunnelPosition;
int RandomBottomTunnelPosition;
int ScoreNumber;
NSInteger HighScoreNumber;

@interface Game : UIViewController
{

    IBOutlet UIImageView *Bird;
    IBOutlet UIButton *StartGame;
    IBOutlet UIImageView *TunnelUp;
    IBOutlet UIImageView *TunnelDown;
    IBOutlet UIImageView *Top;
    IBOutlet UIImageView *Bottom;
    IBOutlet UIButton *Exit;
    IBOutlet UILabel *ScoreLabel;


    NSTimer *BirdMovement;
    NSTimer *TunnelMovement;

}

-(IBAction)StartGame:(id)sender;
-(void)BirdMoving;
-(void)TunnelMoving;
-(void)PlaceTunnels;
-(void)Score;
-(void)GameOver;

@end

我不认为其他脚本是问题，如果有人有想法感谢。