Ask Audubon

We are happy to try to help identify wildlife and get many inquiries to do so. To clarify the process and give it the best chance for a successful identification, we ask that callers/e-mailers understand the following: You need to be as detailed as possible in your description of the animal. If you call and say, “there was a small brown bird in my yard”, it is unlikely we will be able to help you.

Here are some pointers:

-For birds, is it more closely the size of a sparrow, a robin, or a crow…or bigger?
-For birds, you should be able to describe the coloration on the head, chest, back, wings and tail. The more specific, the better.
-For birds, you should note if they have any notable markings – e.g. an eye stripe, wing bars, a crest on the head, chest markings, a forked tail etc.
-For birds, you should note the shape of the beak and its size in relationship to the head. – e.g. “the beak curves down at the end and is twice as long as the head is wide.”
-For birds, you should note the feet/leg color and size and how it walks – e.g. “the bird had short gray legs and hopped about…or the bird had long green legs and waded slowly in the water.”
-For any animal, you should note the habitat in which it was seen and time of day, if possible, and what it was doing, e.g. running along, climbing a tree etc.
-For mammals, insects, reptiles, or amphibians, be sure to note size, coloration, markings, sound (if heard) and time of day it was seen.

If you are unable to answer most of the above questions, then it is likely that we will not be able to identify the animal from your description, and you may need to wait and see if you can observe the animal again and gather more information.

A photo is truly worth a thousand words. A photo will dramatically increase the likelihood of a good I.D. Our experienced naturalists know what to look for and can often instantly discern a species, even from a blurry, hurried photo. A poor photo is better than none at all.  Send to audubon@asri.org.

Bird nests are quite variable and are VERY hard to I.D., so we cannot make any promises with nests.

Our naturalists and refuge managers teach and work out in the field quite a bit and are often not in their offices at opportune times of day to answer calls and may not pick up their voicemails every day. Emailing your description or photo is probably the quickest way to get a response, but whether you call or email, please allow several days for a response. If you do not get a phone response, it is usually because you did not leave enough information – like a name or phone number or it was not audible on the message. Remember to speak slowly and clearly so we can be sure to get the right name and number. Also, we have a lot of commonly asked questions answered right here on the website, with more being added over time. Before calling or emailing, check the website to see if the answer to your question is already there.

Nature is an amazing thing. Every part of it - animal, vegetable and mineral fits into its unique place. Instincts long-hardened into the brain become active when day length changes, when temperatures dip or when moisture dries up. Survival skills and inbred behaviors ensure that individuals survive the changes of the seasons.

There are two nature-based reasons behind decreased bird sightings.

Seasonal Breeding Behavior: During the spring and summer, birds are focused on breeding. They come out to find a mate. They come out to find nesting material. They come out to forage endlessly for all those mouths to feed. They are constantly taking risks by coming out in the open and exposing themselves to predators to get these things done. Once breeding is over for the season, they have some time before the winter comes to 'lay low' a bit, staying out of the view of predators and keeping out of the open areas. This behavior is normal and comes with the season.

Nutritional Needs: Many people are disappointed to learn the birdseed they lovingly put out for their feathered friends is not the birds' first choice. Studies have shown that the seed you put out, even in the cold of winter, is only a percentage of the birds' complete diet (up to only 25%). Bird feeding is a human hobby for human enjoyment. The birds take advantage of it, but do not require it (and will not starve if you go away on vacation, so don't worry). In late summer and early fall, the forests and fields are simply burgeoning with trees and shrubs that are full of ripe fruit, berries, seed pods, fat insects and other fresh, highly nutritious and easily obtainable food. The birds are foraging in these trees, thickets and meadows, in search of these treats. Whether a bird will migrate or will stay around and have to survive the cold of winter, this fall harvest helps them build up the necessary fat and muscle for the winter ahead. They take advantage of the food sources while they can. It will be soon enough that the winter will come and they will once again have to work harder to find food. You can attract birds by planting native plants and fruiting trees such as crabapple, winterberry, cedar, asters, clover, Joe-pye weed and so many more! In order to attract and maintain healthy habitat for insects , minimize the use pesticides, and keep your yard a little “messy” .

Biodiversity loss due to human activity is becoming an increasing concern and is exacerbated by climate change.

We receive reports of decreased bird sightings at all different times of the year and usually they can be attributed to the nature-based reasons above. However, recent developments in the scientific community are confirming that declining bird sighting are increasingly due to anthropogenic (human) causes.

In the fall of 2019, a report was released: over 2.9 billion birds have been lost since the ‘70s – that’s 1 in 4 birds, and it’s happening in every type of ecosystem. There are in fact many causes for the decline of birds. We are entering a period of time where many species of plants, animals and insects, even the common ones, are in trouble. Birds are "indicator species", meaning if one is to notice something is off with the bird populations, it is usually because there are greater issues in the ecosystem(s) where they reside. There’s a reason the subject of the phrase “canary in the coal mine” is a bird!

In May of 2019, the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services released a report on the current and future state of the globe's biodiversity. You can read it here.

From the above linked article: "To increase the policy-relevance of the Report, the assessment’s authors have ranked, for the first time at this scale and based on a thorough analysis of the available evidence, the five direct drivers of change in nature with the largest relative global impacts so far. These culprits are, in descending order: (1) changes in land and sea use; (2) direct exploitation of organisms; (3) climate change; (4) pollution and (5) invasive alien species."

The good news is that conservation programs work – Turkeys, Osprey and Bald Eagles are all testament to this in Rhode Island and beyond. Their species have been targets of dedicated conservation programs and their numbers are now stable or on a promising upswing. Sound policy is also very important and some of Audubon Society of Rhode Island’s main policy focuses in recent and future years will be on laws regarding pesticide use and land use (especially when it comes to large-scale solar panel siting).

Every spring and fall, here at Audubon, we get inundated with phone calls about woodpeckers pecking on shingles, siding and wooden fascia and even metal sheathing on houses. It is a very common behavior and one that can be very annoying for homeowners. While woodpeckers are very important parts of the natural ecosystems, they sometimes come into conflict with humans. To a woodpecker, a wood clad house is just a large, oddly shaped tree, and therefore they often choose those structures for drilling and drumming behavior. Most often, the houses they target are dark colored or natural or stained wood. There are four species of woodpeckers that have been known to drill and drum on houses: the downy woodpecker, hairy woodpecker, the pileated woodpecker and the northern flicker. Most often it is the small downy woodpecker.

Drumming behavior is when the birds use the resonant hollow sound of the wood to attract a mate. Although woodpeckers can and do vocalize, they use drumming as their mating call. Drumming behavior can produce small, shallow holes in the wood.
Drilling behavior is when they chip out larger chunks of wood in search of food or in attempting to excavate cavities. These holes may be the size of a quarter to a half dollar and fairly deep.

There are a few theories as to why woodpeckers drill on houses:

One theory is that as spring nesting season approaches and also as winter roosting season comes on, woodpeckers repeatedly drill in search of an adequate spot to excavate nesting or roosting holes. Most houses don’t allow for deep excavation, but they will drill repeated test holes all over the surface in search of a good spot. Here at Audubon, we get two surges of these phone calls each year – spring and fall, so this timing makes sense for excavation times.

Another theory is that you have inexperienced juvenile males drumming and drilling in the fall. Drumming may occur as the day length shortens and mimics the day length of early spring when normal breeding behavior should begin. The confused youngster thinks it’s time to call a mate. Drilling may occur as the young bird is stressed for food in the fall and begins to try out any hollow sounding wood as a source for food. Thankfully, years of observations have indicated that having a woodpecker drilling on your house is rarely a sign that you have termites, ants or other infestations.

Thirdly it is thought that woodpeckers may be attracted to the hum of electrical wires inside the walls of a house as it may mimic the sound of insects moving in wood, but it has never been shown that woodpeckers prefer areas of houses where there is wiring and it is often far from wiring (eaves, corner fascia etc.), so this is probably not likely.

There are a few things you can do to deter this behavior, but a lot of it depends on how much you are willing or able to undertake.

First and foremost, you should act to stop the behavior as soon as possible before the bird becomes too attached to the spot. Cover the affected areas. This can be done by tacking up plastic sheeting (or heavy duty trash bags) or metal flashing over the areas. Plastic can be attached at the top and sides and allowed to billow at the bottom to aid in scaring them away. Don’t use fabric, as you want something that is hard for them to get purchase on. You can also use scare tactics such as shiny streamers, mylar strips or aluminum foil pie pans over the area that blow in the wind unpredictably and scare them off. In hard-to-reach areas, you can extend shiny helium filled mylar balloons on long strings so that they float right in front of the affected area. A gentle squirt with the hose can also be a deterrent in the morning before they start drilling for the day. Don’t bother with store bought repellents or moth balls as birds have virtually no sense of smell. Also fake snakes and owls are a waste of money, as the birds quickly learn that they pose no threat to them. Preserving standing dead trees and snags in your landscape (and encouraging your neighbors to do so) is one way to prevent this behavior. If there are enough dead trees in the environment, cavity nesters will have places to forage and roost. Dead trees have many more things living in them than live ones, but unfortunately we often remove these from the landscape. Whatever your approach, be assured that this behavior is very seasonal and will stop after a few weeks.

This behavior can be very disturbing and annoying, but it is actually very common behavior. When breeding season comes around, some species will defend their territory quite ferociously. Male American robins are the usual suspect, but other species will do this too. If they see their own reflection in the glass of a window or the side mirrors of a car, they think they are seeing a rival male and will attempt to roust the offender out of their territory. They do not understand that they are seeing themselves and will often exhaust themselves or even hurt themselves in an attempt to be a ‘good daddy’ and defend their nesting area. They will call and attack the image, sometimes for hours on end. There are a few things you can do to try to mitigate this behavior. The following are the most effective, but there is nothing in nature that is 100% effective. Thankfully, the behavior is temporary and as breeding season progresses, he’ll be too busy feeding babies to continue attacking his false rival.

Cover the reflection: Covering the window/glass on the OUTSIDE (so that the glare and reflection are gone) will often stop the behavior. You can use a trash bag, brown paper on a roll, window soap, static window clings, or privacy decals (check your local hardware store) to block the window. Leaving it up for a few days will break the bird’s habit. If the bird is attacking your car mirrors, covering the mirrors with plastic bags, will often stop the behavior.
Scare them off: Hang up shiny streamers, mylar or aluminum pie pans on strings that will move unpredictably in the wind and scare them off. Avoid fake snakes, owl statues etc. They are a waste of money, as the birds wise up to them pretty quickly!
Move the offending image: If the bird is attacking your car windows or mirrors, often just moving the car a number of feet in one direction or another is enough to remove the mirrors/windows from the bird’s territory.

These are warnings you hear a great deal, even from some naturalists and biologists. There is, however, no hard evidence to support that claim and only one report, from 1961, that concluded that birds found dead after eating peanut butter must have choked on it.

Peanut butter is high in protein and fat and in some ways mimics the nutrition that insect-eating birds (like woodpeckers or nuthatches) might seek. The seed eating birds like the fat and protein too, but aren’t perhaps as used to the sticky nature of the peanut butter as a bird who eats insects with their gooey guts. Birds’ tongues are small and are not muscular and articulate like a human’s. Sticky foods are more of a chore for them, but there is no good evidence of them choking on it. Years back when this was a real controversy, we would mix peanut butter with corn meal to make it more textured, or corn starch to make it less sticky, when we would use it to make pinecone bird feeders.

Nowadays, we generally use Crisco or some other solid vegetable shortening when making the pine cone feeders with kids here at the nature centers. There are just so many kids with peanut allergies these days that it makes the ‘peanut butter for birds’ argument rather moot.

There are three grassland species that can greatly benefit from conscious timing of haying to avoid the destruction of nests: the eastern meadowlark, the bobolink and the savannah sparrow. These species have been greatly impacted by the destruction and fragmentation of large open tracts of grassland. These birds generally need 40 or more acres of intact grassland in order to nest. Hay can be taken before May 15 which is when these species would start nesting. After that, July 15 would be the date to assure the safe fledging of a first brood and August 15 would assure the safe fledging of a second brood.

Late mown hay quality can be poor, but certainly sold. If hay quality is paramount, then a good compromise would be having someone walk the fields at the end of May and mark off nests so they can be avoided in mowing and the birds can remain to raise their young.

Female mallard ducks will usually choose a place to nest that is under a shrub, in long grass or other vegetation and away from risk of predators and noise. Sometimes this area is ‘quiet and safe’ because it is an enclosure, and while the mother bird is able to fly in and out of this area, it will pose a problem for the ducklings when they hatch as they will not be able to fly for over two months after hatching.

HOW TO LIBERATE TRAPPED DUCKS (from Massachusetts Audubon’s ‘Living with Wildlife’) After the young have hatched and the mother is looking for a way out of the enclosed area, gather several people together and slowly and quietly herd the ducks out the exit.
If the only exit is through a building, place the ducklings in a shallow box (six inches high) and slowly carry the box out of the building. The female may follow if she can hear her offspring. Once you have it outside, place the box on the ground and, with the mother nearby, tip the box so the young can get out and join her. Should the mother fly off at any point during this process, place the box on the ground outside the building or enclosed area and wait (out of sight) for her to return. Previous to the hatching of the young, the female may have flown out of the enclosure in the same direction each time she left. If that is the case, place the box on that side of the building.

You may have heard the phrase, “If you care, leave them there.” Many young birds are unnecessarily “rescued” by well-meaning humans every year when they are just exhibiting normal growing-up behaviors.

When young birds are taken from the wild and cared for by untrained humans, they most often die, usually of stress and malnutrition. Even experienced, licensed wildlife rehabilitators are a very poor substitution for the parent birds.

Baby birds need to be fed every 15 minutes from dawn to dusk, and even when rehabbers undertake that extraordinary level of care, the babies may survive to fly away, but they have missed very important lessons learned from their parents about finding the right food and shelter as well as avoiding predators.

So, whenever possible, it is vitally important to avoid removing them from the wild if there is any chance the parents can care for them.

If You Find A Baby Bird

First you have to determine whether the bird needs help or not by asking the following important questions:

• Is the bird injured?
• How old is the bird?

If the bird is injured: If you notice any blood, cuts, punctures, air bubbles under the skin or any broken bones (misshapen legs or wings), then the baby should be taken to a licensed rehabilitator as soon as possible. The Audubon Society of Rhode Island is NOT a rehabilitator and does not have the licenses, staff or facilities to care for wildlife. Please contact the Wildlife Clinic of Rhode Island at 401-294-6363 or use ahnow.org to find your local wildlife rehabilitator.

Please understand that very few rehabbers will come and pick up wildlife. They do not receive any type of funding other than donations from people who bring them wildlife, and all their time and expenses are out of their own pocket. They are typically too busy caring for the animals they already have at their facilities (imagine having 20+ baby birds to feed every 15 minutes…and that is just the songbirds!) to leave to pick up another. Therefore, if you choose to intervene on the behalf of wildlife, you must be willing to get the animal to the rehabber.

For healthy babies, you must determine how old they are.

• Hatchling (0-3 days old): tiny wisps of down, eyes are closed, movement is shaky and uncoordinated
• Nestling (younger) (3-6 days old): eyes open, pin feathers start to emerge (look like bluish tubes piercing out of skin)
• Nestling (older) (7-10 days old): young start to respond to parents’ calls, primary feathers unsheathe and they start to stretch wings and legs and jostle around in nest.
• Pre-Fledgling (11 – 14 days old): They move about nest freely, can be seen sitting on edge of nest or nearby branches, they are fully feathered but have a short tail and primary feathers are not full length yet.
• Fledgling (14 – 28 days old): They have left the nest and do not return, but are still being cared for and support-fed by their parents in nearby trees or shrubs or on the ground. They are practicing flying and foraging, but may not have mastered these skills yet.

Any baby found out of the nest that is a healthy hatchling or nestling needs to be returned to the nest if at all possible to maximize chances for survival. Look in nearby trees and shrubs; many nests are in dense bushes. If the nest is not able to be found, is too high or is otherwise inaccessible, you can fashion a substitute nest out of a berry basket, woven basket or a deli tub with many holes punched in the bottom for drainage. You can line the bottom of the basket with grass. Fasten the basket to a sturdy branch in or close to the nest tree as high up and as close to the original nest as you can. Place the baby bird in the basket knowing that you have done all you can; trusting the rest to the parent birds and, ultimately, to nature.

If you discover that you have found a healthy fledgling, you need to leave it alone, or, at most, move it to a nearby shrub, so that it can learn to make its way in the world. The best way for you to help this youngster is to keep humans and pets away from it so that the parents can continue to raise it. Birds have little or no sense of smell, so unlike mammals, a parent bird will not detect your scent on a baby and will not reject it.

Wildlife should never be removed from the wild to protect them from pets or predators. Dogs and cats should be kept indoors until the babies can fly. If the pets belong to a neighbor, ask them to keep them indoors. If the pet owner is not known, you can spray the pet with water to encourage it to leave your yard. Predation by natural wild predators such as fox, crows, raptors, raccoons or other wildlife is normal and part of the natural cycles and processes in nature.

It is not uncommon for baby birds to leave the nest prematurely. Sometimes they are pushed out by rambunctious nest mates. Sometimes they are tossed out in a wind storm or taken out by a house cat or other predator. Other times they are ousted by their parents who have decided it’s time for flying lessons. The natural mortality rate for songbirds is very high at about 70%, and songbirds often have many young in the course of a year in the effort to assure that at least one or two survive. This is nature’s way of keeping populations in a balance that the natural environment can support.

This is a tricky one. In nature, the types of things that song birds feed on are often the same things that many mammals feed on, so it is understandable that many mammals will find your bird feeders very appealing. The best recommendation is to keep feeders over 6 feet off the ground and at least 10 feet from any surrounding structure, such as railings, decks, gazebos or other climbable structures. This is not always possible, so many people employ the use of other tools to keep squirrels, raccoons and other mammals out. Feeders mounted on poles can be mounted over a ‘baffle’ or structure that impedes the ability of a mammal to climb the pole and reach the feeder. Baffles can also be mounted over feeders that hang from hooks or wire or cable to keep invaders from climbing down from above. Today, there are many specialized feeders on the market that are specifically designed to deter mammals such as squirrels. There are no fool-proof methods and no such thing as a completely squirrel-proof feeder or baffle, but there are many ways to significantly lessen the pilfering of seed by mammals. If you are interested in finding out more about baffles or special feeders, please visit our nature shops in Bristol or Smithfield to see a selection. 

There are several species of hawks that will prey upon smaller birds. The two most common species that will frequent bird feeders in search of food are the sharp shinned hawk and the coopers hawk.    

Predation is a natural part of the lives of wildlife, and the natural mortality rate for song birds and small rodents is quite high. When you set up a feeding station, you are creating an artificial environment that brings large numbers of birds to one spot and makes it very appealing for a predator, whether it be a hawk or a house cat. A hawk can find an easy meal at a bird feeder. If the idea of witnessing predation at your feeder bothers you, you should take down your feeders for a few weeks to encourage the hawk to look elsewhere for food.

Usually, you can hang out your hummingbird feeders in late April or early May when the hummingbirds begin to arrive back from migration. Keeping humming bird feeders filled and cleaning out any old sugar water every few days is a great way to provide supplemental food for these tiny powerhouses. You are most likely to attract hummingbirds to a feeder if you have flowering plants in your landscape that they like. They are less likely to frequent a ‘sparsely planted’ landscape as it requires a lot of energy to forage for food, and they’ll prefer to set up territory in an area where food sources are plentiful. In the fall, you can keep hummingbird feeders up for as long as the birds are still visiting it and probably at least a week after they stop (in case of late movers). They will not ‘overstay’ or delay their migration due to the presence of food, so don’t worry. Keeping your feeder up until the birds stop visiting will allow them to keep packing on the fat for the long trip south for as long as they need.

It’s really your choice, but there is sometimes good reason to stop feeding during nesting season.

If your bird feeders are close to nesting boxes, you may find that invasive species, such as English house sparrows or European starlings will quickly become a problem. We stop feeding the birds here at RI Audubon headquarters in mid-March, because our feeding station is in close proximity to our bluebird nesting boxes, and we want to discourage the invasive English Sparrows from using those boxes. We remove the food source at the start of nesting season, and they move on. We resume feeding in August or September once breeding season is over. The native birds who use the nesting boxes are insect eaters and are not influenced by the presence or absence of seed.

Bird feeding and bird watching is one of the fastest growing hobbies in the United States. In all aspects of bird feeding, it’s really up to you when you start or stop. Bird feeding is a human-contrived hobby. The birds derive some benefit from our feeders, but they do not require it, and they do not become dependent upon it. You can start or stop feeding at will, go on vacation or feed only seasonally without unduly affecting birds. They are only eating a percentage (studies show up to only 25%) of their diet from bird feeders. In fact, they will often disappear from feeders for periods of time when there are richer or more abundant wild food sources to be had.

Water, on the other hand, is something from which the birds derive great benefit, and the presence of clean or moving water, especially in dry or winter conditions can make a big difference for birds. A nice, clean shallow bird bath can provide this for the birds.

Side Note: We stop feeding the birds here at RI Audubon headquarters in mid-March, but it is not because of the birds’ ability to find food. We do it because our feeding station is in close proximity to our bluebird nesting boxes, and we want to discourage the invasive English Sparrows from using those boxes. We remove the food source at the start of nesting season, and they move on. We resume feeding in August or September once breeding season is over. The native birds who use the nesting boxes are insect eaters and are not influenced by the presence or absence of seed.

Yes, we can, but here is a case where a photo or bringing a live snipped twig (wrapped in a wet paper towel so it doesn’t wilt) is a necessity. We need to be able to look at the leaves, leaf attachment, buds or flowers or fruit. Also, we need to know what sort of place/habitat it was growing in – e.g. a swamp, a suburban back yard, the edge of a meadow etc.

Dos:

Fresh birdseed such as:

-black oil sunflower seed
-niger/thistle seed
-any variety of ‘songbird’ mixes
-safflower seed
-cracked corn
-peanuts (although be careful, as they can go rancid)
-Fresh or packaged suet for insect eaters like woodpeckers
-Halves of oranges (for orioles)
-Sugar water (for nectar feeders like hummingbirds and orioles)
-Dried fruit (in winter, for overwintering insectivores)

Don’ts:

-Bread/crackers or any other baked goods
-Dairy/cheese (only young mammals can digest milk products!)
-Meats
-Pet food
-Old rancid birdseed (if it smells ‘tangy’, then it’s time to throw it out)

It is imperative that residents remain vigilant if feeding wild birds. Harmful pathogens and illnesses can be spread amongst birds throughout the year, especially during the winter, so cleaning bird feeders and baths should be a regular practice to keep wild bird communities healthy. If you notice any sign of sick bird, please take down your feeders immediately.

How to clean:
1.) SCRUB: Using very warm water, a liquid dish detergent, and a good scrub brush (we recommend a large bottle brush type for tube feeders – you can purchase one made just for bird feeders), scrub out all filmy build-up and any leftover seeds, taking care to get all the nooks and crannies. Many feeders are fairly easily disassembled with a screwdriver for those tough-to-reach spots. Rinse well.
2.) SANITIZE: Spray inside and out with a 10% bleach solution (1 part bleach to 10 parts water). Let the solution sit and sanitize for 10 minutes, then rinse extremely well.
3.) DRY: Allow everything to air dry completely.

It is always best to follow a feeder cleaning schedule of every two weeks, regardless of weather or wildlife issues. Clean more often in severely wet weather. Birdbaths should be carefully cleaned with a 10% bleach solution as well to remove any mold and bacteria build up, rinsed well and air dried completely. Make sure you regularly clear away seed hulls and debris from the ground below bird feeders, as these can also harbor pathogens and/or attract unwanted animals.

Yes, it is true. Bread or other cooked items are NOT a good food source for birds. They cannot digest it properly, and it provides little or no nutrition for them. They will eat it readily, but it means they are expending a lot of vital energy eating something that is not going to provide a good source of energy or nutrition. In Rhode Island, the feeding of waterfowl is illegal for the protection of the birds’ health and also for the safety of public health. When waterfowl are fed by the public, especially food items they cannot digest well, they produce copious droppings that can elevate the bacterial levels of rivers, ponds and lakes. 

Bird feeders are best placed:
    -in a spot where birds can escape quickly to the safety of a tree or shrub, within 20 feet or so.
    -in a spot where you can access them easily for filling and cleaning
    -in a spot where there is not a lot of human activity or movement (like away from a busy walkway or driveway)
    -in a spot where mammals cannot easily reach them (squirrels, raccoons, mice, chipmunks etc.)

Over the course of the autumn and winter, pool covers will often accumulate decaying leaves, branches and other natural debris as well as rain water and snow. This creates a temporary habitat for aquatic invertebrates as it mimics the edge of a pond. Ducks feed on invertebrates, algae and small pond plants. Ducks may seek out a pool cover because there is water and food to be had, or possibly because a female Mallard is nesting under a nearby shrub. It is illegal to disturb a nest or eggs but you can prevent ducks from using the pool cover as follows:

Remove the cover. Open water alone is not what the birds are seeking. Once you remove the food source, the pool becomes less appealing – just a barren body of water.

If this is not possible, clear the pool cover of water and debris, and keep emptying the water from the cover after it rains.

Ducks can sometimes be driven off by harassment, by clapping loudly or banging pots and pans together. This disturbance has to be repeated frequently and consistently before the ducks will give up and leave for good, and is unlikely to be effective if there is a nest in the area.

If ducks are using the pool itself there is no health risk, as pool chemicals will neutralize duck waste. Usually they will not stay long once they realize there is no food to be had.

To encourage the ducks to leave:
-Do not waste money on plastic owls or snakes; birds quickly learn that the predator is a fake.
-Purchase Mylar streamers, which are available in our gift shops or in the crepe paper section at most party stores. Place three-foot high stakes in the ground at each corner of the pool, then stretch the Mylar from stake to stake across the pool to form an “X.” The flashing, uneven movement of the streamers normally frightens the birds away.
-Place a lightweight cover, such as a solar cover, over the pool when it is not being used.
-Try the harassment techniques described above.

Ultimately, it becomes your choice as to how much effort you are willing to go through to discourage them (banging pots and pans etc). They can be stubborn sometimes. Remember, they are merely going about their day to day activities – finding food, shelter and safety – and are not purposely acting to annoy or bother you.  Hopefully one of these techniques will help.