Osmoregulation in the Field by Salt-Marsh Savannah SparrowsPasserculus sandwichensis beldingi

We studied the osmoregulatory strategy of salt-marsh savannah sparrows (Passerculus sandwichensis beldingi) in the field in Baja California, Mexico, and we compared these birds with a subspecies of savannah sparrow found concurrently in the uplands surrounding the salt marsh. Passerculus sandwichensis beldingi had higher plasma osmolalities (349 vs. 339 mmol/kg), higher ureteral urine osmolalities (577 vs. 477 mmol/kg), higher urine sodium concentrations (126 vs. 74 meq/L), higher urine flow rates (7–10 vs. 1.7 μL/min), and larger kidneys (0.36vs. 0.21 g/pair of kidneys) than the upland sparrows. Other unusual findings in the salt-marsh birds were the presence of substantial volumes of hyperosmotic fluids (up to 811 mmol/kg) in the lower intestine and uric acid extending into the ileum. The rectal epithelium of P. s. beldingi was relatively smooth, with few low apical membrane extensions, whereas that of the upland sparrows had a dense apical microvillous brush border. The maximum urine concentration that we measured in freshly captured P. s. beldingi was 2.7 times plasma osmolality. Our findings suggest an osmoregulatory strategy that includes a high intake of solute, a copious urine flow to excrete this solute load, and, at least at times, substantial refluxing into the intestines of hyperosmotic urinary fluids. This unusual strategy presumably reflects the unusual circumstance of a bird without salt glands living in a salt marsh.