Digestive Adaptations of Phainopepla nitens Associated with the Eating of Mistletoe Berries

1. What they did

The Phainopepla is a desert bird of Mexico and the southwestern United States that lives closely with the Desert Mistletoe and feeds heavily on its small (3 to 5 mm) berries. The author asked how the bird’s gut is adapted to this unusual diet, how fast and how efficiently the bird processes the berries, and how well the House Finch, which eats the berries regularly but less, can live on them.

• Anatomy. Eleven Phainopeplas, collected while feeding on mistletoe and frozen in the field, were dissected, along with eight Palm Chats and five Cedar Waxwings for comparison. Gizzards were weighed and the position of berries in the gut was recorded.
• Marked berries. Captive Phainopeplas were fed one marked berry (nail polish on the skin, blue food colouring in the pulp) with 5 to 10 unmarked ones. The birds were watched through a one-way screen and timed, and the droppings were examined. This was repeated 17 times.
• Digestive efficiency. Four Phainopeplas ate only mistletoe berries and water for 72 hours, and all droppings from the last 24 hours were collected and analysed for energy, fat and nitrogen.
• House Finches. Groups of finches were fed berries only, berries and water, or berries and bird seed, and were weighed daily.

2. What they found

The gut

• Like other fruit eaters, the Phainopepla has a short, wide intestine. It has no true crop; food is stored in the upper oesophagus.
• The gizzard is small and thin-walled: 1.5% of body weight in ten Phainopeplas, against 2.9% in Cedar Waxwings.

How the gizzard works

• Berries in the oesophagus were all whole. The gizzard does not grind them. Instead, it squeezes the seed and sticky pulp out of the skin and passes them into the intestine, keeping the skins behind.
• After 8 to 16 berries, the packed skins are pushed into the intestine as one packet. The intestines held packets of 8 to 16 skins alternating with strings of 8 to 16 skinless berries.
• All 17 marked-berry trials matched this explanation: the string of skinless berries was passed first, then the packet of skins, in mirror-image order.
• A berry took 12 to 45 minutes (average 29 minutes) from swallowing to dropping.

Efficiency

• Each Phainopepla ate an average of 264 berries a day. Berries in the droppings could still be told apart as individual berries.
• Overall the birds used 49% of the berries’ energy, 62% of the fat and 3% of the nitrogen. The low nitrogen figure suggests the birds were close to nitrogen balance, which fits with their steady weight.
• 78% of berries were passed in strings with whitish urates. These berries had been digested more completely (56% of energy used) than those passed without urates (26%).
• From these figures, a bird’s daily energy use was 10.9 kilocalories (kcal), 90% of the 12.1 kcal predicted for a bird of its weight (26.7 g).

House Finches

• Finches given berries and bird seed held their weight. Finches given berries only, or berries and water, kept losing weight, to 92.3% and 89.2% of their starting weight after seven days.
• Finches digested the berries more efficiently (62%), but their 148 berries a day gave only 7.7 kcal, short of the 10.1 kcal they needed. The same berries gave far more water (7.46 g) than they needed (1.99 g).

3. What they concluded

• The Phainopepla’s stomach is highly adapted to mistletoe berries. The small gizzard does not grind but squeezes out seed and pulp while holding the skin back for a while.
• The sticky layer under the berry skin acts as a “slip zone” that lets the skin come off easily. This may be an adaptation of the mistletoe that helps the bird remove the skin. Without grinding, the seed stays whole and viable, and processing is faster.
• Other mistletoe specialists remove the skin in other ways: flowerpeckers and euphonias peel it off with the bill, and several species have a reduced or bypassed gizzard. Swallowing berries whole may save the Phainopepla feeding time and lets it use nutrients from the skin.
• Berries mixed with urates may be digested more fully because urine flows back from the cloaca into the rectum, keeping berries longer in the gut. How birds balance these two uses of the large intestine likely depends on their energy and water balance.
• House Finches cannot live on mistletoe alone, but can use it as a water source. The author found finches nesting in the Colorado desert 4.6 miles from the nearest drinking water, in areas with mistletoe; they left once the mistletoe stopped fruiting.

6. Other technologies, skills or innovations

• Marking single berries on the skin and in the pulp to follow their passage through the gut and match the parts in the droppings.
• Microbomb calorimeter for energy content, ether extraction for fat, and a nitrogen analyser.

7. Implications for conservation (from the paper)

• Mistletoe seeds are spread mainly by birds, and the birds and mistletoes show matching adaptations: the Phainopepla’s way of processing berries keeps the seed whole and viable.
• For House Finches in the Colorado desert, mistletoe may serve chiefly as a source of water.