diff --git a/google/auth/_default.py b/google/auth/_default.py
index 5410a4c..d63dcee 100644
--- a/google/auth/_default.py
+++ b/google/auth/_default.py
@@ -129,12 +129,6 @@
     if not project_id:
         project_id = _cloud_sdk.get_project_id()
 
-    if not project_id:
-        _LOGGER.warning(
-            'No project ID could be determined from the Cloud SDK '
-            'configuration. Consider running `gcloud config set project` or '
-            'setting the %s environment variable', environment_vars.PROJECT)
-
     return credentials, project_id
 
 
@@ -147,12 +141,6 @@
         credentials, project_id = _load_credentials_from_file(
             os.environ[environment_vars.CREDENTIALS])
 
-        if not project_id:
-            _LOGGER.warning(
-                'No project ID could be determined from the credentials at %s '
-                'Consider setting the %s environment variable',
-                environment_vars.CREDENTIALS, environment_vars.PROJECT)
-
         return credentials, project_id
 
     else:
@@ -188,10 +176,6 @@
         try:
             project_id = _metadata.get_project_id(request=request)
         except exceptions.TransportError:
-            _LOGGER.warning(
-                'No project ID could be determined from the Compute Engine '
-                'metadata service. Consider setting the %s environment '
-                'variable.', environment_vars.PROJECT)
             project_id = None
 
         return compute_engine.Credentials(), project_id
@@ -287,6 +271,13 @@
         credentials, project_id = checker()
         if credentials is not None:
             credentials = with_scopes_if_required(credentials, scopes)
-            return credentials, explicit_project_id or project_id
+            effective_project_id = explicit_project_id or project_id
+            if not effective_project_id:
+                _LOGGER.warning(
+                    'No project ID could be determined. Consider running '
+                    '`gcloud config set project` or setting the %s '
+                    'environment variable',
+                    environment_vars.PROJECT)
+            return credentials, effective_project_id
 
     raise exceptions.DefaultCredentialsError(_HELP_MESSAGE)
diff --git a/tests/test__default.py b/tests/test__default.py
index 2df8a44..68c4fb0 100644
--- a/tests/test__default.py
+++ b/tests/test__default.py
@@ -302,6 +302,28 @@
 
 
 @mock.patch(
+    'logging.Logger.warning',
+    autospec=True)
+@mock.patch(
+    'google.auth._default._get_explicit_environ_credentials',
+    return_value=(mock.sentinel.credentials, None), autospec=True)
+@mock.patch(
+    'google.auth._default._get_gcloud_sdk_credentials',
+    return_value=(mock.sentinel.credentials, None), autospec=True)
+@mock.patch(
+    'google.auth._default._get_gae_credentials',
+    return_value=(mock.sentinel.credentials, None), autospec=True)
+@mock.patch(
+    'google.auth._default._get_gce_credentials',
+    return_value=(mock.sentinel.credentials, None), autospec=True)
+def test_default_without_project_id(
+        unused_gce, unused_gae, unused_sdk, unused_explicit, logger_warning):
+    assert _default.default() == (
+        mock.sentinel.credentials, None)
+    logger_warning.assert_called_with(mock.ANY, mock.ANY, mock.ANY)
+
+
+@mock.patch(
     'google.auth._default._get_explicit_environ_credentials',
     return_value=(None, None), autospec=True)
 @mock.patch(
@@ -324,7 +346,7 @@
     autospec=True)
 @mock.patch(
     'google.auth.credentials.with_scopes_if_required', autospec=True)
-def test_default_scoped(with_scopes, get):
+def test_default_scoped(with_scopes, unused_get):
     scopes = ['one', 'two']
 
     credentials, project_id = _default.default(scopes=scopes)