<script src='https://api.mapbox.com/mapbox.js/plugins/leaflet-markercluster/v1.0.0/leaflet.markercluster.js'></script>
<link href='https://api.mapbox.com/mapbox.js/plugins/leaflet-markercluster/v1.0.0/MarkerCluster.css' rel='stylesheet' />
<link href='https://api.mapbox.com/mapbox.js/plugins/leaflet-markercluster/v1.0.0/MarkerCluster.Default.css' rel='stylesheet' />

<div id='map'></div>

<script>
L.mapbox.accessToken = '<your access token here>';
// Here we don't use the second argument to map, since that would automatically
// load in non-clustered markers from the layer. Instead we add just the
// backing tileLayer, and then use the featureLayer only for its data.
var map = L.mapbox.map('map')
    .setView([40.73, -74.011], 13)
    .addLayer(L.mapbox.tileLayer('mapbox.streets'));

// Since featureLayer is an asynchronous method, we use the `.on('ready'`
// call to only use its marker data once we know it is actually loaded.
L.mapbox.featureLayer('examples.map-h61e8o8e').on('ready', function(e) {
    // The clusterGroup gets each marker in the group added to it
    // once loaded, and then is added to the map
    var clusterGroup = new L.MarkerClusterGroup({
      // The iconCreateFunction takes the cluster as an argument and returns
      // an icon that represents it. We use L.mapbox.marker.icon in this
      // example, but you could also use L.icon or L.divIcon.
      iconCreateFunction: function(cluster) {
        return L.mapbox.marker.icon({
          // show the number of markers in the cluster on the icon.
          'marker-symbol': cluster.getChildCount(),
          'marker-color': '#422'
        });
      }
    });
    e.target.eachLayer(function(layer) {
        clusterGroup.addLayer(layer);
    });
    map.addLayer(clusterGroup);
});
</script>