Не рисуется изображение в canvas

    window.onload = function() {
    var button = document.getElementById("previewButton");
    button.onclick = previewHandler;
}
 function previewHandler() {
    var canvas = document.getElementById("tshirtCanvas");
    var context = canvas.getContext("2d");
    fillBackgroundColor(canvas, context);
    var selectObj = document.getElementById("shape");
    var index = selectObj.selectedIndex;
    var shape = selectObj[index].value;
    if (shape == "squares") {
        for (var squares = 0; squares < 20; squares++) {
            drawSquare(canvas, context);
        }
    }
    if (shape == "circles") {
        for (var circles = 0; circles < 20; circles++) {
            drawCircle(canvas, context);
        }
    }
    drawText(canvas, context);
    drawBird(canvas, context);
}
function drawBird(canvas, context) {
    var twitterBird = new Image();
    twitterBird.src = 'twitterBird.png';
    context.drawImage(twitterBird, 50, 150, 70, 70);
    twitterBird.onload = function() {
        context.drawImage(twitterBird, 50, 150, 70, 70);
    };
}